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TDX (TanDEM-X: TerraSAR-X add-on for Digital Elevation Measurement)

Spacecraft     Launch    Mission Status     Sensor Complement    Ground Segment    References

TSX/TanDEM-X is a high-resolution interferometric SAR mission of DLR (German Aerospace Center), together with the partners EADS Astrium GmbH and Infoterra GmbH in a PPP (Public Private Partnership) consortium. The mission concept is based on a second TerraSAR-X (TSX) radar satellite flying in close formation to achieve the desired interferometric baselines in a highly reconfigurable constellation. A contract to build the TanDEM-X spacecraft was signed in September 2006 between DLR and EADS Astrium.

The primary goal of the innovative TanDEM-X/TerraSAR-X constellation is the generation of a global, consistent, timely and high-precision DEM (Digital Elevation Model), corresponding to the HRTE-3 (High Resolution Terrain Elevation, level-3) model specifications (12 m posting, 2 m relative height accuracy for flat terrain). The HRTE-3/HRTI-3 models were defined by NGA (National Geospatial-Intelligence Agency), Washington, D. C. 1) 2) 3) 4) 5) 6) 7) 8) 9)

The achievable DEM height accuracy has been confirmed in Phase A by a detailed performance analysis taking into account all major system and scene parameters like the finite radiometric sensitivity of the individual radar sensors, co-registration and processing errors, range and azimuth ambiguities, baseline and Doppler decorrelation, the strength and orientation of surface and vegetation scattering, quantization errors, temporal and volume decorrelation, baseline estimation errors and the chosen independent post-spacing (horizontal resolution). 10) 11)

For generating the global DEM, roughly 300 TByte of raw data will be acquired using a network of ground receiving stations. Processing to DEM products requires advanced multi-baseline techniques and involves mosaicking and a sophisticated calibration scheme on a continental scale.

Beyond its primary mission objective of generating a global HRTI-3 DEM, TanDEM-X provides a configurable SAR interferometry platform for demonstrating new SAR techniques and applications, such as digital beamforming, single-pass polarimetric SAR interferometry, ATI (Along-Track Interferometry) with varying baseline, or super resolution. Close formation flight collision avoidance becomes a major issue and a new orbit concept based on a double helix formation has been developed to ensure a safe orbit separation.

 

Background: In the time frame 2007, the global coverage with topographic data at sufficiently high spatial resolution is inadequate or simply not available for scientific and governmental use. The first step to meet the requirements of the scientific community for a homogenous, highly reliable DEM with DTED-2 specifications was SRTM (Shuttle Radar Topography Mission), launch Feb. 11, 2000. SRTM, representing the first spaceborne single-pass interferometer, was built by supplementing the Shuttle Imaging Radar-C/X-Synthetic Aperture Radar system by second receive antennas mounted at the tip of a 60 m deployable mast structure. Within a ten day mission, SRTM collected interferometric data for a near global DTED-2 (Digital Terrain Elevation Data Level 2) land surface coverage. DTED-2 is the current basic high resolution elevation data source for all military activities and civil systems that require landform, slope, elevation, and/or terrain roughness in a digital format. DTED-2 is a uniform gridded matrix of terrain elevation values with post spacing of one arc second (approximately 30 m). SRTM mapped the Earth between 60 N and 56 S; however, there are still wide gaps, in particular at the lower latitudes.

The TanDEM-X/TerraSAR-X (TDX/TSX) constellation has the potential to close these gaps, to fulfil the requirements of a global homogeneous and high-resolution coverage of all land areas thereby providing the vital information for a variety of applications. The high-precision DEM models are of utmost interest for the civil and military communities, representing the basis for all modern navigation applications.

Parameter

Specification

HRTI-3 definition

DTED-2

Relative vertical accuracy

90% linear point-to-point error over a 1º x 1º cell

2 m (slope ≤ 20%)
4 m (slope ≥ 20%)

12 m (slope < 20%)
15 m (slope > 20%)

Absolute vertical accuracy

90% linear error

10 m

18 m

Relative horizontal accuracy

90% circular error

3 m

15 m

Horizontal accuracy

90% circular error

10 m

23 m

Spatial resolution

Independent pixels

12 m (1 arcsec)

30 m (1 arcsec)

Table 1: DEM specification for HRTE/HRTI level 3 standard - and comparison with DTED-2 model

Figure 1 gives an overview of DEM-level coverage estimates of various observation technologies in the different HRTI classes. It should be noted that a surface area of 150 x 106 km2 represents a global coverage of Terra Firma (i.e., all land areas).

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Figure 1: DEM-level versus coverage indicating the uniqueness of the global TanDEM-X HRTI-3 product (image credit: DLR)

 


 

Mission concept:

The TanDEM-X mission concept is based on an extension the TerraSAR-X mission by a second almost identical satellite, namely TanDEM-X. Flying the two satellites in a close formation with typical cross-track distances of 300-500 m provide a flexible single-pass SAR interferometer configuration, where the baseline can be selected according to the specific needs of the application. 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23)

The SAR (Synthetic Aperture Radar) instruments of TerraSAR-X and TanDEM-X are fully compatible, both offer transmit and receive capabilities along with polarimetry. These features provide a maximum of flexibility in supporting operational services (acquisition of highly accurate cross-track and along-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry) and in data product quality. The following basic interferometric SAR (InSAR) observational modes are available (Figures 2 and -3):

1) Bistatic mode where the SAR instruments of both spacecraft look into a common footprint thus providing different views of the observed target area (Note: bistatic InSAR is characterized by the simultaneous measurement of the same scene and overlapping Doppler spectra with 2 receivers, avoiding temporal decorrelation; PRF synchronization and relative phase referencing between the satellites are mandatory). - One satellite serves as a transmitter and both satellites record the scattered signal simultaneously. In this tandem configuration, both spacecraft fly in a close orbit formation. The baseline of this configuration can be selected according to the specific needs of the application. This enables the acquisition of highly accurate single-pass cross-track and/or along-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry due to temporal decorrelation and atmospheric disturbances.

2) Pursuit monostatic mode where both satellites are operated independently avoiding the need for synchronization; hence, both SAR instruments look acquire data from the same swath with a short time difference of a few seconds corresponding to an along-track distance of 30-50 km. Different to conventional repeat-pass (i.e., two‐pass or multi‐pass) InSAR observations, the temporal decorrelation is still small for most terrain types with the exception of ocean surfaces and vegetation in the case of moderate to high wind speeds.

3) Alternating bistatic mode is similar to bistatic mode, but the transmitter is switched from pulse to pulse between the two satellites.

The baseline for operational DEM generation is the bistatic mode which minimizes temporal decorrelation and uses efficiently the transmit power. This mode uses either TSX or TDX as a transmitter to illuminate a common radar footprint on the Earth's surface. The scattered signal is then recorded by both satellites simultaneously. This simultaneous data acquisition makes dual use of the available transmit power and is mandatory to avoid possible errors from temporal decorrelation and atmospheric disturbances.

The alternating bistatic mode can be used for phase synchronization, system calibration, and to acquire interferograms with two different phase to height sensitivities; the simultaneously acquired monostatic interferogram has a higher susceptibility to ambiguities especially at high incident angles.

A mission concept has been developed which enables the acquisition/generation of a global DEM within three years. This concept includes multiple data takes with different baselines, different incidence angles, and data takes from ascending and descending orbits to deal with difficult terrain like mountains, valleys, tall vegetation, etc.

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Figure 2: Concept of TanDEM-X InSAR observations in bistatic (left) and monostatic (right) modes (image credit: DLR)

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Figure 3: Schematic view of the alternating bistatic mode (image credit: DLR)

The TanDEM-X mission concept allocates also sufficient acquisition time and satellite resources to secondary mission objectives which cover the following application spectrum:

• Moving target indication with a distributed four aperture displaced phase centre system

• The measurement of ocean currents and the detection of ice drift by along-track interferometry

• High resolution SAR imaging based on a baseline-induced shift of the Doppler and range spectra (super-resolution)

• The derivation of vegetation parameters with polarimetric SAR interferometry

• Large baseline bistatic SAR imaging for improved scene classification, as well as localized very high-resolution DEM generation based on spotlight interferometry.

• Demonstration of high resolution wide-swath SAR imaging with four-phase-center digital beamforming.

In short, the TanDEM-X mission concept encompasses enabling technologies in a number of ways, including the first demonstration of a bistatic interferometric satellite formation in space, as well as the first close formation flight in operational mode. Several new SAR techniques will also be demonstrated for the first time, such as digital beamforming (DBF) with two satellites, single-pass polarimetric SAR interferometry, as well as single-pass along-track interferometry with varying baseline. 24)

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Figure 4: Artist's view of bistatic observation by the TanDEM-X configuration (image credit: EADS Astrium)

 

TanDEM orbits:

Close formation flight of TerraSAR-X and TanDEM-X. The TerraSAR-X spacecraft remains its sun-synchronous dawn-dusk orbit with the following parameters: mean altitude of 515 km, inclination = 97.44º, local equatorial crossing time at 18 hours on the ascending node, nominal revisit period of 11 days (167 orbits in the repeat, 15 2/11 orbits/day. 25) 26) 27) 28) 29) 30) 31)

For setting up the effective baseline, TanDEM-X is separated from TerraSAR-X in the right ascension of the ascending node. This will span a horizontal baseline, which will be adjusted between 200 m and 3000 m to achieve the effective baselines required for DEM-acquisition at different latitudes. An additional vertical separation at the northern and southern turns is achieved by a relative shift of the eccentricity vectors of the satellites. The result is a complete separation of the two satellite orbits called Helix-formation, which enables a safe operation of close formations with minimum collision risk. Such a Helix formation with an offset in eccentricity vectors and a separation in the right ascension of the ascending node is shown in Figure 2.

The TanDEM-X operational scenario requires a coordinated operation of two satellites flying in close formation. Several options have been investigated and the Helix satellite formation has finally been selected. The helix configuration allows maintaining a relatively small distance between both satellites while at the same time avoiding the collision risk at the poles. This formation combines an out-of-plane orbital displacement (e.g. by different ascending nodes) with a radial (vertical) separation (e.g. by different eccentricity vectors) resulting in a helix-like relative movement of the satellites along the orbit. Since there exists no crossing of the satellite orbits, it is now possible to arbitrarily shift the satellites along their orbits, e.g. to adjust very small along-track baselines at predefined latitudes and to allow safe spacecraft operation without autonomous control.

The Helix orbit for close formation flight, involving the maintenance of baselines of a cluster of spacecraft in orbit for cross-track and along-track interferometric observations, has been patented by DLR. The inventors are: Alberto Moreira, Gerhard Krieger, and Josef Mittermayer.

1) European Patent Office, Patent No: EP 1 273 518 A2 of Jan. 8, 2003. Title: "Satellitenkonfiguration zur interferometrischen und/oder tomographischen Abbildung der Erdoberfläche mittels Radar mit synthetischer Apertur."

2) US Patent No: US 6,677,884 B2 of Jan. 13, 2004. Title: "Satellite Configuration for Interferometric and/or Tomographic Remote Sensing by Means of Synthetic Aperture Radar (SAR)."

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Figure 5: Illustration of the Helix orbit configuration of both spacecraft (image credit: DLR)

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Figure 6: Helical shape of interferometric baseline during one orbit (image credit: DLR)

The HELIX formation enables a complete coverage of the Earth with a stable height of ambiguity by using a small number of formations (e.g. ΔΩ ={300 m, 400 m, 500 m} and Δe ={300 m, 500 m}, where `Ω' is the right ascension of the ascending node, and `e' is the eccentricity. Baseline fine tuning can be achieved by taking advantage of the natural rotation of the eccentricity vectors due to secular disturbances and fixing the eccentricity vectors at different relative phasings. Since there exists no crossing of the satellite orbits, it is possible to arbitrarily shift the satellites along their orbits, e.g. to adjust very small along-track baselines at predefined latitudes and to allow safe spacecraft operation without autonomous control.

An appropriate reference scenario has been derived which enables one complete coverage of the Earth with baselines corresponding to a height of ambiguity of ca. 35 m within 1 year assuming a bistatic acquisition in stripmap mode with an average acquisition time of 140 s per orbit.

Both high precision orbit determination (POD) and interferometric baseline vector determination of the tandem configuration will be accomplished by means of the GPS-based TOR (Tracking, Occultation and Ranging) device, a dual-frequency receiver, which will be provided by GFZ as for TerraSAR-X.

Coarse orbit control and maintenance of the tandem configuration will be done as part of the regular maintenance maneuvers using thrusters. Fine-tuning of the Helix of the TanDEM-X satellite will be performed using additional cold gas thrusters.

TanDEM-X formation flight: The Helix formation geometry implies maximum out-of-plane (cross-track) orbit separation at the equator crossings and maximum radial separation at the poles. This is realized by small ascending node differences and by slightly different eccentricity vectors, respectively, as depicted in Figure 7. This concept of relative eccentricity / inclination vector separation results in a Helix-like relative motion of the satellites along the orbit and provides a maximum level of passive safety in case of a vanishing along-track separation. 32)

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Figure 7: Formation building with relative eccentricity / inclination vector separation (image credit: DLR)

Legend to Figure 7: From left to right: (1) identical orbits, (2) maximum horizontal separation at equator crossings by a small offset in the ascending node (green arrow), (3) a small eccentricity offset causes different heights of perigee / apogee and hence yields a maximum radial separation at the poles. (4) Optional rotation of the argument of perigee to achieve larger baselines at high latitude regions.

 


 

Spacecraft:

During the development phase of the TerraSAR-X spacecraft, the TanDEM-X mission concept became a vision. However, a realization of the vision of two SAR missions in orbit could only have a chance with a necessary minimum extension of the SAR design on TerraSAR-X to support the synchronized operation of both radars. 'Minimum' meant that the TerraSAR-X schedule was not endangered and was further constrained to allow a cost-effective 1:1 rebuild approach for the SAR on TanDEM-X. 33)

For the spacecraft bus, the approach was constrained by only allowing software changes on TerraSAR-X. The bus design on TanDEM-X was extended to allow formation flight of both satellites - with TanDEM-X as the 'Master of the Constellation.' Particularly the bus hardware extensions were constrained by the tight schedule leading to strong orientation on existing hardware designs. The software changes are being verified during the TanDEM-X on-ground tests and will be uplinked to TerraSAR-X in preparation for the constellation flight.

Like the TerraSAR-X (TSX) satellite, the TanDEM-X (TDX) satellite is based on a mission-tailored AstroBus service module and a radar instrument developed according to the AstroSAR concept. Main differences to the TerraSAR-X satellite are the more sophisticated cold gas propulsion system to allow for constellation control, the additional S-band receiver to enable for reception of status and GPS position information broadcasted by TerraSAR-X, and the X-band intersatellite link for phase referencing between the TSX and TDX radars (the required modifications on the TSX spacecraft have already been implemented).

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Figure 8: Artist's view of the TanDEM-X spacecraft (image credit: DLR)

The outer shape of the spacecraft is mainly driven by the accommodation of the X-band radar instrument, the body mounted solar array and the geometrical limitations given by the Dnepr-1 launcher fairing. A standard S-band TT&C system with full spherical coverage in uplink and downlink is used for satellite command reception and telemetry transmission.

An additional intersatellite S-band receiver, operating at the TerraSAR-X downlink frequency, will allow for the reception of status and GPS position information broadcasted by TerraSAR-X. It provides a 1-way link with which TanDEM-X can receive real-time position and velocity data from TerraSAR-X from its nominal 1-frequency GPS receiver. The TanDEM-X OBC (On-Board Computer) software uses such data from both satellites to generate a collision warning flag. Furthermore, this data is used by the TAFF (TanDEM-X Autonomous Formation Flying) algorithm running on the OBC (see Ref. 37).

Nominally, formation flying will be under ground control. The TAFF algorithm will be tested open-loop during the commissioning phase and could then become the standard approach for the constellation phases. The ISLR (Intersatellite Link Receiver) is laid out to receive the TerraSAR-X S-band transmissions in low power mode. It is cold redundant with each receiver/decoder cross-strapped to two patch antennas. This layout keeps contact gaps to less than 15 minutes in addition to an interruption imposed by the nominal high rate S-band contact with the ground station.

The OBC is a fully redundant unit that aims at performing the onboard data handling and the attitude and control functions on the satellites. The processor module in based on the ERC32, clocked at 40 MHz, and ensures an execution of software with a processing capability of more than 10 MIPS. The internal RAM memory comprises 6 MByte, with 4 MByte used nominally and 2 MByte reserved for the implementation of a cold redundancy.

The TanDEM-X attitude control system is based on reaction wheels for fine-pointing with magnet torquers for wheel de-saturation. A combined hydrazine/cold-gas propulsion system allows for orbit maintenance and rapid rate damping during initial acquisition. Attitude and orbit measurement is performed with a GPS/Star Tracker system during nominal operation and a CESS (Coarse Earth and Sun Sensor) in safe mode situations and during the initial acquisition. A combination of laser gyro and magnetometer allows for rate measurements in all mission phases.

CGS (Cold Gas Propulsion System): The CGS on TanDEM-X is of CryoSat-2 heritage and uses a high pressure tank of nitrogen gas. This provides small thruster impulses fitting the needs for constellation flight. There are 2 redundant branches each culminating in 2 redundant pairs of thrusters mounted on the satellite in each of the ± flight directions. A formation flight maneuver involves operation of a pair of thrusters in one of these directions.

The TanDEM-X spacecraft has a launch mass of about 1340 kg (payload mass of 400 kg); the nominal design life is five years after the end of the commissioning phase (estimated to be 3 months); the satellite consumables will last for 6.5 years after commissioning.

The Public-Private Partnership (PPP) between DLR and EADS Astrium has been extended to cover the design, build, launch, commissioning and operation of the TanDEM-X spacecraft. Like TerraSAR-X, TanDEM-X is a dual-purpose (scientific and commercial) Earth observation mission, providing its data services to the science (DLR) and to the non-science communities (Infoterra). This shared approach makes the program affordable to all parties of interest.

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Figure 9: TanDEM-X in the satellite integration center at IABG (image credit: DLR, EADS Astrium)

 

Launch: The TanDEM-X spacecraft was launched successfully on June 21, 2010 on a Dnepr-1 launch vehicle with a 1.5 m long fairing extension. The launch provider is ISC Kosmotras, the launch site is the Baikonur Cosmodrome, Kazakhstan. 34) 35)

RF communications: A standard S-band TT&C system with 360º coverage in uplink and downlink is used for satellite command reception and housekeeping telemetry transmission. The uplink path is encrypted. Generated payload (SAR) data are stored onboard in a SSMM (Solid State Mass Memory) unit of 768 Gbit EOL capacity prior to transmission via the XDA (X-band Downlink Assembly) at a data rate of 300 Mbit/s. The X-band downlink is encrypted.

The on-board SAR raw data are compressed using the BAQ (Block Adaptive Quantization) algorithm, a standard SAR procedure. The compression factor is selectable between 8/6, 8/4, 8/3 or 8/2 (more efficient techniques can only be applied to processed SAR imagery). Both communication links are designed according to the ESA CCSDS Packet Telemetry Standard.

Spacecraft

Rebuild of the TerraSAR-X satellite which was based on the Astrium Flexbus concept and extensive heritage from the CHAMP and GRACE missions

Features

- X-band downlink horn antenna is mounted at the tip of a 3.3 m long boom
- SSMM (Solid State Mass Memory) data storage with a capacity of 768 Gbit (EOL)
- High-pressure nitrogen gas propulsion system for formation flying

Spacecraft launch mass

1340 kg (spacecraft: 1220 kg, fuel: 120 kg)

Spacecraft size

5 m length, 2.4 m diameter (hexagonal cross section)

Spacecraft design life

5 years nominal (after the end of the commissioning phase)

RF communications

- X-band of 300 Mbit/s link of payload data downlink with DQPSK modulation;
- S-band uplink of 4 kbit/s (2025-2110 MHz), BPSK modulation; S-band downlink of 32 kbit/s to 1 Mbit/s (2200-2400 MHz), BPSK modulation

Primary payload

Secondary payloads

- TDX-SAR instrument is identical to the TSX-SAR (TerraSAR-X SAR instrument)
in layout, operational performance and support modes.
- TOR (Tracking, Occultation and Ranging)
- LCT (Laser Communication Terminal)
- LRR (Laser Retroreflector)

Table 2: Overview of the TanDEM-X spacecraft parameters 36)

TAFF (TanDEM-X Autonomous Formation Flying)

TAFF is navigation and formation flying software package developed at DLR/GSOC. The overall objective of TAFF is to ease the ground and space operations. Its accurate orbit control performance facilitates the synchronization of the two SAR systems via dedicated horns. In fact the positions of the satellites will be known with a good precision well in advance of real operations. TAFF will enable a safe and robust formation control with minimum collision risk. 37) 38) 39)

On top of ensuring a stable and more precise baseline for SAR interferometry, TAFF will enhance the exploitation of along-track interferometry techniques. Along-track interferometry is enabled by a special configuration of the formation which provides dedicated osculating along-track separations at desired locations along the orbit. This method improves the detection, localization and the signal ambiguity resolution for ground moving targets and can be used for traffic monitoring applications. Furthermore real-time collision risk assessments will be performed by TAFF on a routine basis in order to support automated FDIR (Fault Detection Isolation and Recovery) tasks.

Two GPS receivers are installed on each spacecraft. The dual-frequency IGOR GPS receiver of BroadReach Inc., which serves exclusively scientific purposes, and the single frequency MosaicGNSS receiver of EADS Astrium, whose navigation data are used by TAFF.

A one-way intersatellite link (ISL) is being implemented between the two satellites, using the existing S-band downlink system on TSX and an additional receiver on TDX. The link is designed to function properly up to distances of a few km (ca. 2-5 km).

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Figure 10: Overview of the ground and space segments and their interface to TAFF (image credit: DLR)

The TAFF software package resides in the OBC of the TDX spacecraft. TAFF gets as inputs the GPS data provided by the GPS receiver onboard TDX and, through the ISL, also from the GPS receiver data onboard the TSX. TAFF uses the CGS (Cold Gas Propulsion System) to control the formation and performs in-plane control maneuvers in the flight and anti-flight directions only.

The in-flight performance validation of the experimental autonomous formation keeping system embarked by the German TanDEM-X formation has been performed during a 12-day-long closed-loop campaign conducted in June 2012. Relative control performance better than 10 m was achieved, demonstrating that a significant gain of performance can be achieved when the control of the formation is done autonomously on-board instead of on-ground. Furthermore, the formation keeping system was shown to be operationally robust, easy to operate and fully predictable, i.e. fully suited for routine mission operations. This campaign concludes successfully a series of validation activities, opening new doors to future innovative scientific TanDEM-X experiments for which enhanced formation control is required.

TAFF is the first onboard autonomous formation keeping system ever employed on a high-cost scientific formation flying mission with routine data acquisition. As such, it has to face inherent natural fears and reluctance to rely on onboard autonomy for critical activities like formation maintenance. TAFF aims at making evolving the minds by proving that a proper design of the formation (passively safe) as well as a smart implementation of the onboard navigation software (robust navigation and control, internal safety mechanisms) can guarantee simple, accurate and safe formation keeping.

Table 3: Inflight performance test of TAFF 40)

 

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Figure 11: Illustration of the spaceborne DGPS tracking scheme (image credit: DLR)

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Figure 12: Photo of the MosaicGNSS (left) and IGOR (right) devices (image credit: DLR)

Parameter / Instrument

MosaicGNSS (EADS Astrium)

IGOR (BroadReach Inc.)

GPS tracking capability

8 channels L1

16 x 3 channels L1/L2

Raw data
Accuracy

C/A: 5 m
L1: 3 mm

C/A, P(Y) 0.2 m
L1, L2: 1 mm

Power consumption

10 W

15 W

Radiation tolerance

35 krad

12 krad

Table 4: Key parameters of the onboard GPS receivers

 

Formation Flight and Safety measures:

The requirement of a configurable close formation between TSX and TDX arises from the need for a SAR interferometer in space. The satellites fly in almost identical orbits whereby the position of TDX describes a helix around the trajectory of TSX. This is achieved by separation of the relative eccentricity and inclination vector. The maximal radial separation is reached over the poles (vertical baseline typically between 200 - 500 m) and the maximum separation in normal direction occurs at the equator (horizontal baseline typically 200 – 500 m; see Figure 5). In this way, it can be assured that the radial and normal separation never become zero at the same time. The shape of the helix depends upon the mission phase. The formation with the smallest baseline had a minimum separation of 150 m. Orbit correction maneuvers are carried out with the hydrazine propulsion system simultaneously on both spacecraft with exactly the same ΔV. Additionally formation keeping maneuvers are needed to compensate the drift of the relative e-vector that arises from the J2-perturbation (Ref. 31). These maneuvers are made only on TDX with the cold gas system. 41) 42)

Thrusters were originally planned to be the prime actuators during non-nominal situations in AOCS safe mode. The experience with TSX showed, however, that the design with the thrusters mounted at the back of the satellite is far from ideal for flight in close formation. Analyses showed a collision risk of 1/500 due to orbit changes in case of a drop to the thruster based safe mode. 43)The reason is that just a minor part of the thrust is available for attitude control, whereas the major part is changing the orbit in an unpredictable way. - Hence, a second type of safe mode was implemented with the intention to control the attitude without changing the orbit. The so-called ASM-MTQ (Acquisition and Safe Mode-Magnetorquer) only uses the magnetic torque rods as actuators, whereas it still relies on CESS, magnetometer and IMU as sensors, just like the original ASM-RCS (Acquisition and Safe Mode-Reaction Control System).

However, the damping of the rotation rates and the recovery of the attitude takes longer in ASM-MTQ than in ASM-RCS due to the weakness of the magnetic field at 514 km altitude. The maximum overall body rate that can be handled are 0.5º/s due to the concept that the torque rods and the magnetometers are operated in alternation to allow disturbance free measurements of the Earth's magnetic field.

The new FDIR (Fault detection, Isolation and Recovery) design intends to always use the magnetorquer based safe mode first when a severe anomaly has been detected. There are performance limitations in ASM-MTQ as mentioned above, and it might still become necessary to make use of the conventional but more powerful ASM-RCS. The latter will only be used if the continuation of the mission is seriously endangered. A possible scenario would be the battery voltage dropping below a certain value, a star tracker getting too hot or non-convergence of the attitude after three orbits. The thruster on-time is limited at first instance to make sure that the generated ΔV cannot lead to a collision of the satellites. A reboot of the on-board computer will follow in the worst case scenario when despite of limited use of the thrusters, no convergence was reached. The spacecraft will come up after the reboot in ASM-MTQ again, but this time with wider power/thermal limits. However, the described sequence will be tried only once. If there is still no convergence or the power/thermal limits are yet violated, the spacecraft will be sent by FDIR to ASM-RCS once more, but this time without limitations to the thruster on-time. 44)

The ISL (Inter-Satellite Link) is also used for surveillance, but is subject to some limitations. In the first place, the link only works in one direction and in the second, the connection is interrupted anytime the transmitter of TSX or TDX is switched to high-rate for ground station contacts. Therefore it is seen more as an extra safety rather than the part to rely on completely. The ISL is used to transmit some essential parameters of TSX (including GPS position and velocity) to TDX in order to feed TAFF algorithms (Tandem Autonomous Formation Flight).

AOCS surveillance: The most vital AOCS parameters, such as sensor performance, attitude errors, actuator commands, etc. are monitored on-board. In case of severe anomalies FDIR can react immediately and switch to the redundant hardware. During ground station contacts, a large number of parameters are checked in the mission control system against pre-defined limit settings and violations are indicated by yellow or red flags. The dump files (data covering also the time span in between ground station contacts) are screened with the same limit settings, and violations are reported by email. The events will subsequently be analyzed and it is then decided if they can be disregarded or if a threat to the satellite is developing.

Minimize Mission Status

Mission status:

• July 2017: After the launch in June 2010 and the subsequent commissioning phase, global DEM acquisitions started in December 2010. Parallel to the first month of operational data acquisition the team concentrated its efforts on the calibration of the bistatic interferometer. Correction of differential delays between TSX and TDX was necessary to facilitate the utilization of radargrammetry for resolving the 2π-ambiguity band. Phase, delay and baseline calibration have reached such an accuracy level, that more than 90% of all so-called Raw DEMs (long data takes are processed to scene based DEMs of 50 km by 30 km extension) are within ±10 m of DEMs derived from SRTM/ICESat data already before the final calibration step using ICESat data as reference heights. More than 500,000 Raw DEMs have been generated in a fully automated process employing multibaseline interferometric techniques. 45)

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Figure 13: The global TanDEM-X DEM is a consistent data set covering all land surfaces at unprecedented absolute height accuracy of about 1m at a horizontal sampling of 12 m by 12 m. Between 2011 and 2014 at least two acquisitions have been collected by the bistatic TanDEM-X SAR interferometer, mountainous areas have covered up to six times (image credit: DLR)

- The first and second global coverages (except Antarctica) were completed in January 2012 and March 2013, respectively. After some gap-filling, Antarctica was mapped for the first time under local winter conditions. In early August 2013 the satellite helix formation was changed to allow imaging of mountainous areas from the opposite viewing geometry. Due to a low SNR, desert areas had to be re-acquired as well, but at steeper incidence angles. Afterwards the satellites were maneuvered back to the original formation and Antarctica was covered again at larger baselines. The primary data acquisition program was concluded mid-2014.

- A comprehensive system has been established for continuous performance monitoring and verification, including feedback to the TanDEM-X acquisition planning for additional acquisitions. Since the end of 2013 the final calibration and mosaicking chain has been fully operational and completed the global DEM consisting of more than 19,000 1º by 1º (lat/long) tiles in September 2016.

- Global DEM Performance: The quality of the final DEMs is well within the expected performance for the global DEM. Figure 14 shows as an example the absolute height accuracy (90% linear error) per tile derived from the comparison of the TanDEM-X heights against ICESat validation points. The cumulated absolute height error over the complete data set totals 3.5 m. If the forested and ice-covered areas are excluded, where the X-band reflective surface deviates from the laser surface due to different penetration, we end up with an outstanding 0.9 m global absolute height error that is one order of magnitude below the 10-meter requirement.

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Figure 14: TanDEM-X DEM absolute height accuracy (90% linear error) per 1º by 1º DEM tile; the cumulated absolute height error for ice-free and non-forested areas is with 0.9 m one order of magnitude below the 10-m requirement (image credit: DLR)

- As the system is very well calibrated and tilts and trends are negligible, the relative height accuracy is well described solely by the random errors in the system. It can be calculated from the interferometric coherence and the resulting phase error. It is specified as the point-to-point error within a 1° by 1° tile. Again, excluding ice and forest areas, where additional volume decorrelation deteriorates the coherence and in consequence the relative height error, 97.8 % of all DEM tiles fulfil the relative height error specification of 2 m (4 m) for flat (steep) terrain.

- Finally, compared to SRTM the TanDEM-X DEM features a much lower percentage of void areas (global count of 0.1 %), especially in desert areas, a result of the reacquisition at steeper incidence angles and hence better SNR. Further details on the TanDEM-X DEM quality can be found in Ref. 46)

- The above mentioned performance monitoring used the interferometric coherence as the key quantity to control the data acquisition, processing and mosaicking. In case of volume scatterers the coherence is mainly determined by the volume decorrelation effect, which in turn can be used to discriminate forested and non-forested areas as shown in Figure 15. As a byproduct of the global DEM a global forest map at a resolution of 50 m x 50 m is currently being generated and is planned to be made freely available for scientific users. 47)

- As both satellites are still working very well and have plenty of resources left, an agreement to continue the mission was concluded between DLR and AIRBUS Defence & Space. Key objectives for this extra mission phase are the acquisition of interferometric data for improvements of the global DEM and the generation of a global change layer, that can be considered as a demonstration for the future climate research and environmental monitoring mission Tandem-L. If the baseline geometries are suitable, further scientific experiments will be included in the timeline as well.

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Figure 15: Forest map of an area in Rondonia/Brazil derived from TanDEM-X coherence quick looks (50 m x 50 m resolution), image credit: DLR

- In summary, the generation of the global DEM, the primary mission objective, has been successfully completed. Quality and coverage of the data are outstanding. A science phase dedicated to demonstrating applications based on along-track interferometry and new SAR techniques, has been finished last year. A continuation of the mission was approved with the main objective to use bistatic interferometry in close formation flight to generate a global 3D information change layer.

- TanDEM-X has demonstrated the feasibility of an interferometric radar mission with close formation flight and delivers an important contribution for the conception and design of future SAR missions. One example is Tandem-L, a mission for monitoring dynamic processes on the Earth surface with unprecedented accuracy

• July 6, 2017: Airbus DS has again expanded its WorldDEM portfolio with the launch of WorldDEM4Ortho. Tailored for orthorectification of high and very high resolution optical and radar satellite data, WorldDEM4Ortho will enable corrections of all distortions induced by the topographical variations of the Earth's surface and satellite orientation when acquiring an image. Covering the Earth's entire land surface, WorldDEM4Ortho is the most consistent and accurate elevation model for orthorectification on a global scale. 48)

- Without these geometrical corrections, satellite images cannot be used in GIS (Geographical Information Systems) or for any mapping related applications. With the huge development of new geolocated applications like business analytics, location-based services or tourism, the needs for such a consistent and precise elevation model are exploding.

- WorldDEM4Ortho is based on the global WorldDEM dataset. It is produced via a fully automated process and features a vertical accuracy of four meters in a 24 m raster. Identified disturbing terrain artefacts are removed. Bodies of water like lakes or sea are flattened. Rivers are stepped with a flow that follows the surrounding shorelines. Adaptive smoothing processes are also applied to different landscapes and land-use such as urban areas to avoid distortions in the orthorectified image.

• May 19, 2017: Look at a mangrove and it's clear that it is unlike any other forest type. Roots rise above ground in looping, arching shapes—an adaptation to the low-oxygen soils of subtropical coastal areas. But other aspects of these forests are less obvious, starting with their importance for storing carbon dioxide and keeping it out of the atmosphere.

- Research has shown that mangroves account for only 3% of global forest cover. However, they happen to be the most carbon-rich type of forest in the tropics. This means that mangrove loss can have a large effect; up to 10 % of global carbon emissions from deforestation has been attributed to mangroves. (When trees are harvested and die, whether they are burned or eventually rot, their stored carbon is released to the atmosphere.) Because of their importance to the carbon cycle and climate, researchers have been investigating the structure of these forests from the ground, air, and space. 49) 50) 51) 52)

- The maps of Figures 16 and 17 are the result of one such effort, which uses satellite radar data to model the height of mangrove canopies in Africa. This map, based on a model developed by SeungKuk Lee of NASA's Goddard Space Flight Center, shows tree canopy heights for 2015 in the vicinity of the Akanda and Pongara national parks in Gabon. The parks span 540 and 929 km2, respectively, and together account for 25 percent of Africa's protected mangrove area.

- Dark greens represent areas where mangrove trees are the tallest. The darkest greens appear in Pongara National Park, where trees tower up to 60 meters in places—some of the tallest mangroves in the world.

- Information on the height of mangrove canopies can help scientists estimate things like the total amount of biomass in a forest. That information, in turn, can be used to get more precise estimates of how much carbon is locked up in a mangrove, and how land cover changes are affecting where that carbon ends up.

- But mangrove forests are not the same everywhere. Forests across Africa and around the world contain mangroves of various species and structures, with various capacities for storing carbon. Air- and ground-based data are important for helping researchers estimate total mangrove ecosystem carbon stocks in Gabon and around the planet.

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Figure 16: Overview of the mangrove parks in Gabon, Africa, acquired in 2015 by TanDEM-X of DLR (image credit: NASA Earth Observatory, maps by Joshua Stevens, using canopy height data courtesy of SeungKuk Lee/NASA GSFC/NASA Carbon Monitoring Systems. Story by Kathryn Hansen)

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Figure 17: Detail map of the mangrove parks in Gabon, acquired in 2015 by TanDEM-X of DLR (image credit: NASA Earth Observatory, maps by Joshua Stevens, using canopy height data courtesy of SeungKuk Lee/NASA GSFC/NASA Carbon Monitoring Systems. Story by Kathryn Hansen)

• January 13, 2017: The satellite duo, TerraSAR-X and TanDEM-X, continue to orbit in close formation to make bistatic observations for scientific applications. Both satellites have used their consumables frugally; each spacecraft spent so far about half a tank volume of hydrazine; the batteries are in good operational conditions, and the quality of the radar imagery remains excellent since the start of the missions. The project expects a continuation of operational services of the two missions to about 2020 — these predictions depend of course on the assumption that no unanticipated events occur. After all, in June 2017, TerraSAR-X will be 10 years on orbit. 53)

• October 17, 2016: The German satellite duo TerraSAR-X and TanDEM-X have consistently delivered one-of-a-kind Earth observation data since 2007 and 2010, hence shaping the international research landscape. Now, scientific users from across the globe have gathered for the TerraSAR-X and TanDEM-X Science Meeting at DLR (German Aerospace Center) in Oberpfaffenhofen, where they will discuss the results obtained from the data and define requirements for future remote sensing technology. 54)

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Figure 18: TSX/TDX terrain image of Mount Erebus in Antarctica (image credit: DLR)

Legend to Figure 18: Within the framework of the German TSX/TDX radar mission, the polar regions were surveyed for the first time in a comprehensive and highly accurate manner, which is of vital interest to climate research. The terrain model of Figure 18 shows a region of the Antacrtic around the 3794 m Mount Erebus (upper left), an active volcano covered by glacier ice.

• October 4, 2016: The new three-dimensional map of Earth has been completed. Mountain peaks and valley floors across the globe can now be seen with an accuracy of just one meter. The global elevation model was created as part of the TanDEM-X satellite mission; it offers unprecedented accuracy compared with other global datasets and is based on a uniform database. The approximately 150 million km2 of land surface were scanned from space by radar sensors. 55)

- The quality of the global elevation model has surpassed all expectations. Exceeding the required 10 meter accuracy, the topographic map has an elevation accuracy of a single meter. This is a result of excellent system calibration. The distance between the two satellites in formation flight, for example, is determined with millimeter precision. The global coverage achieved by TanDEM-X is also unparalleled – all land surfaces were scanned multiple times and the data was then processed to create elevation models. In this process, DLR's remote sensing specialists created a digital world map consisting of more than 450,000 individual models with pixel by pixel height detail – creating a special kind of three-dimensional mosaic.

- This mission broke new ground in many areas. The close formation flight of the two satellites at a minimum distance of 120 m has become as routine as the various maneuvers required to continuously change the formation and adapt it to the requirements of the imaging geometry. A similar situation applies to bistatic radar operation; simultaneous data acquisition using two radar satellites was initially a major challenge, but was a necessity to ensure the high accuracy of the elevation models. DLR is now a world leader for this pioneering technology.

- TerraSAR-X and TanDEM-X have long exceeded their specified service lives and continue operating faultlessly and in such an efficient way that they still have enough propellant for several more years. Completion of the 3D world map does not signify the end of the mission. Due to the special nature of the formation flight, further scientific experiments are scheduled. Alberto Moreira points out: "Earth as a system is highly dynamic, which is also reflected in its topography. Through frequent updates, we could capture such dynamic processes systematically in the future. This is the primary goal of the Tandem-L mission that we have proposed."

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Figure 19: Crater landscape of the Nevada Test Site (image credit: DLR)

Legend to Figure 19: The 'Nevada Test Site' was, from 1951, the area for numerous nuclear tests. The desert area, 100 km northwest of Las Vegas, is dotted with explosion craters.

• July 2016: The TSX and TDX satellites were designed for a nominal lifetime of 5.5 years. Predictions based on the current status of system resources indicate a lifetime for both satellites and a joint operation until at least 2020 and 2018, respectively. 56)

• June 2016: HDEMs (High-resolution DEMs): After the great achievement in terms of DEM performance of the TanDEM-X mission and the demonstration of the high potentiality of the different experimental modes, the plan of the mission is to continue the acquisitions. In particular, acquisitions for HDEMs and acquisitions for further improvement of the DEM will be carried on, together with the always ongoing science activities. 57)

- As a next milestone, the TanDEM-X mission will carry on a new goal: so-called "high-resolution" DEMs (HDEMs) will be produced for selected areas with an independent posting of 0.2 arcsec (< 6m) by maintaining and even reducing the relative height error (for TanDEM-X DEM 2 m, for HDEM goal 0.8 m for specific areas). The data set will be available on special user-request. Next to the acquisition strategy the following paper highlights the proposed HDEM processing chain. 58)

- The performance on the first HDEM acquisitions performed during the science phase in 2015 is currently under analysis. To support further acquisitions of HDEMs and in order to optimize the SAR acquisition parameters, studies on the relative height accuracy have been performed (Ref. 57).

- Figure 20 shows the relative height accuracy as a function of the range bandwidth for a flat soil-rock terrain acquired with an incidence angle of about 41º, for different backscattering coefficients σ0. Different colors represent the relative height accuracy resulting from different combinations of height of ambiguity (HoA) values. The HoA is defined as the height difference equivalent to a complete 2π cycle of the interferometric phase. It depends on the imaging incidence angle and is inversely proportional to the baseline length. It is a direct scaling factor that relates the interferometric phase error to the relative height accuracy.

- From the figure, one can see the improvement in the height accuracy when increasing the bandwidth for σ0 larger than -15 dB. From the plot, one can also notice the need to combine together several acquisitions with smaller HoA in order to achieve the goal of 0.8m in the relative height accuracy (the black line in Figure 20).

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Figure 20: Relative height accuracy versus range bandwidth for different combinations of acquisitions and σ0 values. The black horizontal line indicates the required height accuracy of 0.8 m (image credit: DLR)

- The HDEM regions of interest are shown in Figure 21. These regions include "preparation-areas" (in red, excluding Antarctica) which have good coherence and relatively flat terrains, but exclude forested areas where volume decorrelation effects are strong. In addition, some smaller "demo-areas" (the ones in blue in Figure 21) will be acquired three to four times in order to improve the relative height accuracy, to demonstrate the high resolution DEM performance.

- The new acquisitions are planned with a higher bandwidth of 150 MHz, compared to 100MHz of the nominal DEM acquisitions. This is required to ensure sufficient multi-looking even with the higher resolution of 6m x 6 m. In addition, higher BAQ (Block Adaptive Quantization) of 4 bits/sample (compared to 3 bits/sample of the DEM) is applied where possible to further improve the height accuracy.

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Figure 21: Regions of interest for high resolution acquisitions from 2016. In red the "preparation-areas" which are planned only once, in blue the "demo-areas" which will be acquired three to four times (image credit: DLR)

- The Helix formation has been optimized as well in order to reach the required height accuracy (with HoAs ranging from 15 m to 25 m.) at all the latitudes and for all the incidence angles. In particular, as it is shown in Figure 22, the horizontal distance between the two satellites increases from 600 m to 1200 m over 8 months, while the vertical distance remain constant for most of the time, with a small increase in the last one and a half month in order to allow acquisitions at higher latitudes.

- Additional acquisitions are also planned over the Antarctica coast (also shown in Figure 21) in order to improve performance for future versions of the DEM (Ref. 57).

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Figure 22: Formation parameters evolution over time for the HDEM acquisitions (image credit: DLR)

• June 2016: Digital Elevation Models (DEMs) are raster-based digital datasets representing the topography of a planetary body and are of fundamental importance for a wide range of scientific and commercial applications. Within the ±60º latitude band, up to now data from the SRTM (Shuttle Radar Topography Mission) has been the primary source of elevation information. Since 2010 the DLR (German Aerospace Center) has been operating Germany's first two formation flying SAR (Synthetic Aperture Radar) satellites, TerraSAR-X and TanDEM-X, with the objective to generate an updated global DEM which exceeds the presently available global data sets in terms of resolution, coverage, and quality by orders of magnitude.

The primary mission of TanDEM-X is the generation of a world-wide, consistent, current, and high-precision DEM, with a spatial resolution of 0.4 arcseconds (12 m at the equator) and according to the height accuracy and data coverage shown in Table 5.

The TanDEM-X global DEM acquisition started in December 2010 and the first global coverage (except Antarctica) was completed in January 2012. By the end of 2014, the Earth's entire land mass had been mapped at least twice (four times in the case of difficult terrain) with varying baselines. Of the nearly 20,000 final DEM geocells expected to be produced, over 15,000 geocells or nearly 80% are available as of February 2016. Delivery of DEM products commenced in 2014 and the complete global DEM is expected to be available in September 2016. 59)

Parameter

Accuracy

Requirement

Absolute height accuracy

90% linear error – globally

≤ 10 meter

Relative height accuracy

90% linear point-to-point error in 1º x 1 º geocell

≤ 2 meters (slope ≤ 20%)
≤ 4 meters (slope > 20%)

Data coverage

97% of all global land mass

Table 5: TanDEM-X performance requirements

The DEM relative height accuracy is well described solely by system error due to random noise contributions, and can be calculated after correcting the systematic errors and anomalies. For this, the TanDEM-X products contain an additional layer with pixel-wise in-formation on the estimated relative height accuracy. 60)

Of the 15,154 available TanDEM-X geocells, 14,008 have a relative height accuracy above the required 90% confidence level for the specified 2 m (4 m) in flat (steep) terrain. Of those 1,146 products that are below 90%, 205 geocells are not considered to be reliable due to too few data points (e.g. small islands) or to sea ice coverage. An additional 690 geocells are dominated by highly forested areas. In these areas, the coherence estimation is artificially deteriorated due to volume decorrelation and consequently the relative height accuracy is also deteriorated. Hence, up to now only 191 geocells, or 1.32% of the available geocells, do not meet the relative height accuracy specification.

The absolute height accuracy of the TanDEM-X data will be globally validated using the ICESat points that have not already been utilized in the calibration process to evaluate the difference between ICESat and TanDEM-X data. When evaluating the absolute height accuracy on a global scale, only the first 1,000 points with the lowest height variation between DEM pixels within an ICESat footprint are considered. Hence, geo-cells with fewer validation points (e.g. coastal regions) are evaluated with similar weight as geocells with more copious validation points.

The most current height statistics, as of February 2016, of the available DEMs is shown in Table 6. The system specification of an absolute global height accuracy of at most 10 meters with a 90% linear error is met and far exceeded with an accuracy of 1.318 m.

Number of DEM geocells

15,154

Accumulated Number of Validation Points

12,490,957

Mean Height Deviation of Validation Points (m)

0.046

Accumulated Absolute Height Accuracy of 10 m (linear error)

99.66%

Accumulated Absolute Height Accuracy with 90% linear error (m)

1.318

Table 6: Absolute Height Accuracy Statistics for available TanDEM-X geocells

In addition to the global specification, the absolute height accuracy is also monitored on a geocell basis for all validation points in the geocell. This is shown in Figure 23. Out of 15,154 geocells, only 194 tiles or 1.2% have an absolute height accuracy greater than 10 m. The vast majority of the geocells, over 10,117 or 67%, have an absolute height accuracy of less than 2 m.

As the ICESat data is laser-based, there can be an offset to the radar-based TanDEM-X measured height, especially over vegetation or snow/ice where the signal penetration of the two systems can differ. In Greenland, this difference in penetration depth is so large that a special calibration case is utilized whereby calibration using ICESat is only performed on the rocky edges of the land mass and extended into the middle. Thus one sees, as expected, that the absolute height error in Greenland significantly increases and that the mean absolute height error per geocell, shown in Figure 24, drops down to less than -5 m, both due to the TanDEM-X radar system penetrating much deeper into the ice sheet than the Laser based ICESat system. It is expected that the quality of DEMs over other icy as well as mountainous terrain will impact the global statistics.

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Figure 23: TanDEM-X absolute height accuracy per geocell for available geocells as of February 2016. "N/A" represents tiles where no validation points are available (image credit: DLR)

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Figure 24: Status of TanDEM-X data coverage analysis per geocell as of February 2016. Further geocells are still in process (image credit: DLR)

Data coverage: Voids, i.e. invalid pixels, in DEM data arise when a pixel's height value cannot be determined during processing and can occur in a SAR system for various reasons, including phase unwrapping anomalies, low return signal power, or shadow/layover effects. The TanDEM-X final DEM is specified to the data coverage requirement shown in Table 5 and therefore the global data set can have up to 3% invalid data points (voids) over land (VOL).

The TanDEM-X DEM void pixels over land and water pixels are both flagged with the same invalid flag. In order to separate the voids over land from voids over water, a land/water body mask is needed. Between 56° south and 60° north latitudes the SRTM Water Body Data (SWBD) is utilized for the majority of the geocells. The SWBD does not contain data for a few geocells in the desert regions and small islands, thus a second land mask will be needed in these areas. For the remainder of the globe, including Antarctica, and for the missing areas in the SWBD, the ESA (European Space Agency) Climate Change Initiative Land Cover (CCI - LC) data set will be utilized. The CCI-LC mask represents open and permanent water bodies at a 300 m spatial resolution on a global scale.

The data coverage statistics are being processed separately from the final DEM processing and currently the data coverage statistics for 11,038 geocells are available. Of these geocells, voids over land account for only 0.0766% of the entire data set. In other words the data coverage is currently better than 99.9%. Figure 24 shows the current status of the global data coverage on a geocell level. Over 62% of the geocells shown in this figure contain zero invalid data pixels. Furthermore, only 685 or 6.20 % of the geocells contain more than 1% of invalid pixels over land. Bringing our focus onto the many geocells containing islands with void percentage greater than 3% (red in the plot); though these geocells have a high percentage of VOL, the contribution to the final statistic is minimal due to the small land size in each geocell (Ref. 59).

• May 2016: The TerraSAR-X and TanDEM-X mission are in routine operations since January 2008 and January 2011 respectively. The TanDEM-X science phase, which started in September 2014, posed a major challenge to the operations team because concepts established since many years needed to be modified. Furthermore, many components of the ground software system had to be upgraded. The enhanced versions needed to be thoroughly tested and integrated into a fully operational ground system with a minimum of maintenance outages. 61) 62)

- Validation campaigns for each new constellation were undertaken to prove compliance of the system with respect to the requirements of the TerraSAR-X and TanDEM-X mission. The handling of the ground station network for payload data downlink was subject to a number of studies within the mission planning system. It was not trivial to assign the payload data downlink opportunities to the satellites TDX and TSX in a way that the overall downlink capacity was somewhat equally shared and all boundary conditions were fulfilled. Therefore the payload ground station handling was subject to many configuration adaptations within the TerraSAR-X/TanDEM-X mission planning system.

- Especially, the transitions from one constellation to the following were challenging. The maneuver sequence had to be planned, safety aspects needed to be assessed and mission planning had to adapt to the evolving constellations. The transitions were carefully planned; aspects concerning flight dynamics, safety, on-board modifications and mission planning were assessed and findings were documented prior to the transition. In conclusion the TanDEM-X science phase was successfully handled and all requirements of the science segment could be fulfilled. The satellite operations ran very smoothly without any significant problems.

- Only the conclusion of Ref. 61) is provided. The interested reader is referred to Ref. 61) and to Ref. 62) for a better overview of the science mission details.

- From the Mission Planning point of view, the TanDEM-X Science phase can be seen as three consecutive periods of different formation geometries, in parallel to the introduction of the Dual Receive Antenna configuration. The formation before the initiation of this phase was defined as Bistatic Close formation, where the inter satellite distance between the two satellites was up to a few hundred meters. The chronological milestones of the TanDEM-X science phase are illustrated in Figure 25. The corresponding scientific applications were:

1) In September 2014, the formation geometry of the satellites switched to Pursuit Monostatic Far formation. The TDX satellite was flying on the same ground track as the TSX satellite, following the latter with an along-track separation of 76 km. This formation was kept until March 2015. The main application of this phase was the measurement of sea ice and glaciers.

2) In November 2014, a new acquisition mode was introduced. The Dual Receive Antenna configuration was enabled on both satellites, giving the opportunity for quad-polarization acquisitions. This configuration was kept throughout the TanDEM-X science phase. Its main applications are the ground moving target indication (i.e. traffic) as well as vegetation monitoring.

3) In March 2015, the formation geometry was changed from Pursuit Monostatic Far formation to Bistatic Close formation with Large Perpendicular Baseline. The TSX satellite remained in its orbit, while the TDX satellite was close to its partner over the poles, but with a cross-track separation of 3600 m over the equator. This formation remained for six months, performing acquisitions of the full growing cycle of vegetation as well as high resolution DEMs.

4) In September 2015, the TDX satellite slowly approached the TSX satellite's orbit, creating a pure Bistatic Close formation after one month. Four months after the end of this transition phase, performing acquisitions for research on forests and ocean-currents, the TanDEM-X Science phase was officially terminated, in February 2016.

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Figure 25: The TanDEM-X science phase timeline (image credit: DLR)

 

• Nov. 24, 2015: Airbus Defence and Space and the German Ministry of Defence have signed a contract for the utilization of TanDEM-X mission data, to update the Digital Elevation Model (DEM) of the Bundeswehr. The agreement includes both licences for the utilization of the global elevation data set covering the 150 Million km2 of the Earth's landmass, and support services for data-management and data-editing, helping to access, edit, store and disseminate the impressive volume of data. Additional support and training will also be provided under the agreement, so users can reap the full benefits of this unique and homogenous global dataset. 63)

- "We are proud of the trust shown by the German Ministry of Defence, which now becomes the first user of the global information provided through WorldDEM" says Evert Dudok, Executive Vice President of the CIS (Communications, Intelligence and Security) business line at Airbus Defence and Space.

- The 3D nature of the data provides an ideal visualization tool indispensable for surveillance, reconnaissance and mission planning. The TanDEM-X Digital Elevation Model facilitates the interpretation of landscapes with exceptional detail, which is essential for military engineering projects and operational planning incl. mapping of obstacles, line of sight estimation and flight path/possible landing site planning.

• October 2015: Mission outlook for 2016 64)

- Close bistatic formation with nominal formation parameters

- TerraSAR-X and TanDEM-X: continuing operations with both satellites

- TanDEM-X: global DEM expected to finish in autumn 2016

- TanDEM-X: further science and HDEM acquisitions

- Agreement between DLR and AIRBUS DS for mission continuation beyond 2016

- Based on available onboard resources operation up to 2020 predicted for both satellites.

• Sept. 2015: While the WorldDEMcore database is steadily growing with data ready for ordering, the Airbus DS database of fully edited and off-the-shelf WorldDEM™ is increasing as well. More and more countries and even entire continents are now complete: Europe, Australia – and the World's second-largest continent: Africa. Africa is richly endowed with mineral reserves and ranks first or second in quantity of world reserves of cobalt, platinum, gold, chromium and uranium. Gold mining is Africa's main mining resource. In North Africa, the oil and gas production is an important industrial sector. 65)

- WorldDEM delivers reliable terrain information to assess potential exploration sites, support seismic planning, develop scouting activities, facilitate geophysical surveying and evaluate potential environmental impact of activities

- Seamless high-quality DEMs even for remote and difficult-to-access locations for geological prospecting and feasibility studies.

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Figure 26: Sutherland, South Africa: The topography is characterized by the Roggeveld Mountain range which is part of the Large Karoo. In 2005 the SALT (Southern African Large Telescope), the largest single optical telescope in the southern hemisphere, was installed here (image credit: Airbus DS)

• June 12, 2015: DEMs (Digital Elevation Models) of the world's impact craters. There are just 188 known meteorite or asteroid craters worldwide. Some span a mere 10 m in diameter, while others extend 160 km across and are significantly more impressive. They all share a common history – an object from outer space must have hit the Earth, travelling at least 11 km/s, or 39,000 km/hr, to leave behind an impact crater. "They can all look very differently. Frequently they are aged, or even contain subterranean lakes," says Manfred Gottwald of the Earth Observation Center at DLR (German Aerospace Center). He has seen almost all of them – not in person, but through the eyes of the two German radar satellites TerraSAR-X and TanDEM-X. DLR uses their observation data to produce 3D DEMs in previously unseen precision. The craters presented include the Aorounga structure in the Sahara Desert of Chad, as well as the Tin Bider crater in Algeria, the Shunak impact in Kazakhstan and the Ries Crater in Germany. 66)

- Sculptures in sunlight: "First of all, the project team wants to learn what already known meteorite craters will look like in the 3D DEMs produced by the DLR radar satellites", explains Gottwald. From their orbital altitude of ~500 km above Earth's surface, even the most remote craters can be detected. The two radar satellites TerraSAR-X and TanDEM-X, which have orbited the Earth in formation flight since 2010, also have the advantage that – unlike optical satellites – they can map the surface of the Earth irrespective of cloud coverage or sun illumination. Therefore, craters dispersed around the world can, for the first time, be tracked down and compared as part of a uniform, global elevation model. Artificial lighting, in which the scientists define the solar elevation and view angle, is added to depict the various impact craters in a particularly sculptural form. Then, the shadows cast, in combination with the elevation model, permit very clear outlines of the crater edges, faults and erosion phenomena to be displayed.

- The image acquisitions, required for the new 3D DEMs of Earth's surface are now complete, which means that the two radar satellites are currently being used for a broad variety of scientific analyses. This phase of the project will run until the end of the year, focussing primarily on research in the fields of geology, hydrology, glaciology, agrarian science, forestry and urban land use. It will also involve the testing of new radar technology.

- So far, over 450,000 individual SAR images were processed within the DLR project to produce the 3D DEMs with a vertical precision of better than 2m. Sixty-five percent of the Earth's land mass has already been rendered in 3D. According to Gottwald, "This new elevation model provides a wealth of information on the impact craters, for instance their exact size and state of preservation. It also allows mapping of entire craters, even if parts of their visible sections are hidden beneath vegetation."

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Figure 27: This image, acquired by the TanDEM-X satellite, shows the Aorounga impact crater in Chad, North Africa. It is ~345 million years old, thus very heavily weathered. From space, both the inner and outer rings of the crater are visible (about 17 km in diameter), formed as part of a multiple impact event. Strong winds have acted as builders, carving parallel structures into the ground; they consists of wind-resistant rocky proturberances, referred to as yardangs, through which the sand dunes meander, carried by the wind (image credit: DLR)

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Figure 28: The Shunak meteorite impact crater structure in Kazakhstan stands out when viewed from space in the TanDEM-X DEM. Its high crater edge of ~400 m stands out in this image. With a diameter of 2.8 km, scientists estimate that the Shunak crater was formed about 45 million years ago (image credit: DLR)

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Figure 29: The Ries crater, on the border between the Swabian and Franconian Jura regions in southwest Germany (Nördlingen is the main city and county seat of the Ries), was formed when an asteroid, measuring 1 km across, crashed onto Earth some 14.8 million years ago. What remains of the crater can be seen in the digital elevation model created using observation data from the DLR radar satellites TerraSAR-X and TanDEM-X. The Ries crater has a diameter of 24 km (image credit: DLR)

• May 2015: TanDEM-X is an innovative SAR (Synthetic Aperture Radar) mission with the main goal to generate a global and homogeneous DEM (Digital Elevation Model) of the Earth's land masses. The final DEM product will reach a new dimension of detail with respect to resolution and quality. The absolute horizontal and vertical accuracy shall each be <10 m in a 90% confidence interval at a pixel spacing of 12 m. The relative vertical accuracy specification for the TanDEM-X mission foresees a 90% point-to-point error of 2 m (4 m) for areas with predominant terrain slopes smaller than 20% (greater than 20%) within a 1º longitude by 1 latitude cell. The global DEM is derived from interferometric SAR acquisitions performed by two radar satellites flying in close orbit formation. Interferometric performance parameters like the coherence between the two radar images have been monitored and evaluated throughout the mission. In a further step, over 500,000 single SAR scenes are interferometrically processed, calibrated, and mosaicked into a global DEM product which will be completely available in the second half of 2016. 67)

- Systematic data acquisition: The systematic planning of interferometric acquisitions has to consider the limited resources of the two satellites as well as the capacity of the ground segment downlink stations. The acquisition concept foresees a first global coverage at larger heights of ambiguity for a robust data basis and a second global acquisition at smaller heights of ambiguity to improve the height accuracy and resolve phase unwrapping problems by dual-baseline processing. Likewise, multiple acquisitions can be combined to achieve low and homogeneous relative height errors over larger regions.

- In the first two years of operations two global coverages of the Earth's land masses, excluding Antarctica, have been acquired. All the acquisitions have been carried out in the nominal right-looking observation mode, during ascending orbits in the northern hemisphere and during descending orbits in the southern hemisphere. Some difficult terrain such as forests affected by strong volume decorrelation, mountainous regions affected by shadow and layover, and deserts with low return signal needed to be acquired multiple times with different constraints . The regions are given in Figure 30. Acquisitions over mountainous regions, which are characterized by rugged topography, are strongly affected by geometrical distortions. Therefore, these areas have been reacquired twice, between August 2013 and April 2014, from the opposite viewing geometry: in descending orbits in the northern hemisphere and in ascending orbits in the southern hemisphere. In order to enable acquisitions from the opposite viewing geometry with a good height of ambiguity, the orbit formation was changed so that the rotation direction of the TanDEM-X satellite around TerraSAR-X, looking in the flight direction, was reversed.

- Antarctica has been acquired in separate phases during the local winter which is between May and September on the southern hemisphere. This is especially necessary for the outer regions of Antarctica near the ocean as the backscatter is significantly low during the summer period when the snow is partially melted. In the inner part of Antarctica, dedicated satellite left-looking acquisitions are required due to the inclination of the orbit. The first Antarctica coverage took place in April and May 2013, the second coverage was performed in April and May 2014.

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Figure 30: Regions affected by shadow and layover effects (in red) have been identified from terrain slope calculations. Deserts regions (in orange) have been derived from TanDEM-X coherence data of the first and second year acquisitions (image credit: DLR)

- Interferometric quality assessment: During the acquisition and data processing process, the single interferometric scenes are analyzed for their quality in terms of interferometric calibration and performance. Within this paper, the team presents an overview of over 458,000 interferometric SAR scenes and corresponding input DEMs.

- Interferometric calibration of single acquisitions: Interferometric calibration of the complete bistatic SAR system is a pre-requisite for generating useful DEM scenes. Each single DEM needs to be as close as possible to its real height to allow an accurate geocoding and to facilitate the final mosaicking and calibration process, which handles tilts and offsets in larger blocks of neighboring scenes. In the case of TanDEM-X, systematic baseline errors as well as phase and timing offsets have been measured and analyzed in order to calibrate the single input scenes. - The stability of the line of sight error of the satellite distance vector (the baseline) contributes to a height offset of the DEM. A baseline estimation error of 1 mm roughly corresponds to a height error of 1 m for a typical height of ambiguity. Hence, the bias in the baseline must be continuously monitored over all mission phases and acquisition settings. The long-term evolution of the baseline bias is shown in Figure 31 with the dashed lines representing the implemented system offsets. The measurements show a standard deviation of less than 1.4 mm.

- The two further aspects of interferometric calibration cover the measurements of radargrammetric shifts and the absolute radar phase. Internal time delays need to be compensated so that the correct height of ambiguity band can be derived from the radargrammetric shifts, in order to provide a rough absolute height information. For the radar phase, instrument systematics inside and between the satellites have to be calibrated. After interferometric calibration of the above mentioned aspects, the absolute height offset of single DEM scenes can be estimated from reference data such as SRTM or ICESat. 87% of all single input DEMs are already better than the ±10 m specification. The detailed statistics of the mean absolute height offset in the input DEMs for different acquisition coverages is listed in Table 7. For most of the outlying 13% of the scenes, the height of ambiguity band was not correctly resolved in the first interferometric processing attempt and will be corrected before the final mosaicking.

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Figure 31: Long-term evolution of the estimated baseline bias: radial and cross-track components in red and green, respectively. The solid lines show the fitted mean values (image credit: DLR)

 

Mean coherence >0.6

Relative height error confidence

Mean height offset < 10 m

1st Global Coverage

84.1%

88.0% (flat)

91.1% (steep)

93.6%

2nd Global Coverage

86.6%

90.0% (flat)

89.9% (steep)

92.0%

Additional coverage

88.5%

68.6% (flat)

82.9% (steep)

85.2%

Desert acquisitions

96.4%

90.8% (flat)

89.8% (steep)

89.2%

Difficult terrain
(opposite viewing direction)

93.3%

81.6% (flat)

92.3% (steep)

74.0%

Combined quality

89.3%

96.8% (flat)

98.8% (steep)

87.6%

Table 7: Quality of over 458,000 DEM scenes from single coverages processed as of February 2015. 64% of these scenes are from the two global coverages. Percentages for coherence and absolute height offset are derived from the mean value per scene of all respective acquisitions. The relative height error analysis gives the confidence level for achieving the 2 m (flat) and 4 m (steep) specification. Relative height error data is based on quicklook map evaluation with a resolution of 300 m by 300 m.

- Interferometric performance of single acquisitions: The key parameter for the evaluation of the interferometric performance is the coherence, which gives a measure for the amount of noise in the interferogram. Up to February 2015, more than 458,000 scenes have been processed and the mean coherence value of more than 89% of all scenes (with water bodies being filtered out) is higher than 0.6, where 0.6 is considered as a reliable reference value for interferometric processing (Figure 32).

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Figure 32: Histogram of mean coherence of land per scene. The red and the green dotted line give the relative occurrence of coherence over 0.6 and 0.8, respectively (February 2015), image credit: DLR

- Quality summary: The TanDEM-X mission is generating a high-resolution and very accurate digital elevation model (DEM) using single-pass SAR interferometry. The Earth's land masses are systematically mapped multiple times, where difficult terrain, like steep mountainous slopes, is covered at least four times. The systematic data acquisition phase has been competed in August 2014. Continuous data monitoring provided a quick performance feedback on a per scene basis or at quicklook resolution. Regions with lower performance, e.g. over sandy deserts, could be re-acquired with an optimized acquisition scenario. The majority of the data shows a reliable data basis as the mean coherence is above 0.6 for almost 90% of the data. The interferometric calibration of the SAR data pairs has further pushed the initial height accuracy of the majority of scenes (87%) to better than 10 m. In total, over 458,000 scenes have been analyzed in this work.

- The final DEM will be produced by calibrating and mosaicking all these individual DEM scenes into a homogeneous data set consisting of almost 20,000 tiles. Each tile has a size of about 110 km by 110 km. Up to February 2015, 8,856 final DEM tiles have been completed and analyzed for this paper. The final quality of these DEM products is well within the specified accuracy range. The relative height accuracy has already been predicted by using quicklook products of the input scenes. It corresponds very well with the first final DEM data. The absolute height accuracy far exceeds the specification as the first DEM data has been generated for mainly moderate terrain types. It is expected to complete the processing of the global DEM by the second half of 2016. Until then, the quality monitoring process will be further continued.

• April 2015: The WorldDEM DTM (Digital Terrain Model) is now commercially available for all users that need superior terrain information anywhere on the globe. The WorldDEM DTM is derived from the WorldDEM™ DSM (Digital Surface Model) product by removing vegetation and man-made objects to show the bare terrain of the Earth's surface. 68) 69) 70)

- This high-quality WorldDEM DTM provides an excellent foundation layer for applications such as civil engineering (e.g. road design, Earth work calculation), the management of natural resources as well as planning and implementation of military operations (e.g. vehicle trafficability analysis, 3D terrain visualization).

- The German radar satellites TerraSAR-X and TanDEM-X form a high-precision radar interferometer in space and acquire the data basis for the WorldDEM. This mission is performed jointly with DLR (German Aerospace Center). Airbus DS refines the Digital Surface Model (e.g. editing of acquisition, processing artefacts and water surfaces) or generates a Digital Terrain Model. Three product levels are offered: WorldDEMcore (output of the processing, no editing is applied), WorldDEM™ (guarantees a void-free terrain description and hydrological consistency) and WorldDEM DTM (represents bare Earth elevation). 71)

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Figure 33: Left: WorldDEMcore (unedited); Right: WorldDEM (edited); Site: Lake Ouachita, Hot Springs, Arkansas, USA (image credit: Airbus DS)

- Applications: Digital elevation models are input for a wide range of applications, from image orthorectification and base topographic mapping to the more specialized geospatial needs of defence, homeland security, intelligence and military engineering interests. As WorldDEM is globally available as a homogeneous, seamless and high accurate dataset, it provides a new dimension for applications on a global scale.

• April 1, 2015: Earth's cryosphere is particularly susceptible to climate change. Rising temperatures are certain to result in profound and widespread changes at high latitudes, where the ground remains frozen all year. Approximately one quarter of the Northern Hemisphere contains permafrost – an area so vast it can only be regularly and comprehensively monitored through satellite remote sensing. The Lena Delta in Siberia lies in such a permafrost zone; it consists of a number of islands and river channels that are covered by ice in winter. A part of this river delta was imaged by DLR (German Aerospace Center) TanDEM-X satellites in October 2014 (Figure 34). The colors correspond to the different signals that the satellites transmit and receive. For instance, the ice floes covering the river channels appear in blue, whereas the frozen ground takes on a greyish color. Yellow patches can be seen in the midst of this frozen ground, which correspond to shallow bodies of water. 72)

- Such bodies of water are abundant in many high-latitude regions, including the Lena Delta. The yellow color that they have in the radar image corresponds to a particular kind of interaction between the ice cover and the microwaves transmitted by the satellite. By analyzing such images, researchers can estimate the physical properties of the ice cover. Particularly, such microwave data provides: "Valuable information on whether these abundant shallow bodies of water in the Arctic completely freeze to the bottom or not, which affects permafrost conditions under these lakes, fish habitats, and hydrological connectivity between bodies of water in winter," says Guido Grosse from AWI (Alfred Wegener Institute). In the future, such data could be used operationally to monitor long-term changes in thermokarst lake ice conditions, resulting in a better understanding of climate warming impacts on lake systems, underlying permafrost and the potential release of the greenhouse gases methane and carbon dioxide in the Arctic.

- Permafrost: It is not just these bodies of water that are in constant flux; the surrounding land is also subject to changes, and some of these are expected to become more pronounced as the climate continues to warm. The soil contains ice – and with increasing temperatures – more and more of this ice is likely to melt. The thawing permafrost soils can become unstable, leading to slope failures and coastal erosion, as well as posing a risk to human infrastructure.

- These are among the many questions undertaken within the Helmholtz research alliance 'Remote Sensing and Earth System Dynamics', whose members include Sonya Antonova (AWI) and Simon Zwieback (ETH Zürich). Specifically, their research addresses the question of whether TanDEM-X data can be processed to show every summer's seasonal ground subsidence due to the melting of ice within the uppermost soil layer. "The magnitude of the subsidence can be used to derive the ice content within the active layer," explains Antonova. "This quantity is of the greatest interest for permafrost modelling." The subsidence can be assessed using the Differential SAR Interferometry (DInSAR) method, which requires acquiring at least two SAR images at different times over the same location. According to Zwieback: "This estimation is – in many regions, such as the Lena delta – hampered by the impact of additional surface processes, such as changes in the moss moisture content or vegetation growth, and it will be important to characterize the uncertainties that these processes cause. Subsidence measurements obtained on site help us to validate the interferometry results."

- Sonya Antonova and Julia Boike have installed several on site measurement stations for determining subsidence in the Lena Delta. These observations are one instance of the long-term measurements of the state of permafrost at Svalbard and in the Lena Delta at the Samoylov Research Station. This station is operated by the Russian Academy of Science in collaboration with AWI, and hosts many international research projects. "We need to monitor the surface indicators to identify the hot spots for changes in local hydrology, energy flux and moisture balances," says Boike. Detecting and observing the ongoing changes – such as thawing permafrost and the dynamics of bodies of water in a warming Arctic – have become critical to quantify the impacts of climate change. Recently developed space-based monitoring techniques provide key tools to estimate the global impacts of the changes in vast Arctic regions on all of us.

- The success of TanDEM-X forms the basis for the development of innovative radar technologies. Researchers at DLR and at the Helmholtz Alliance 'Remote Sensing and Earth System Dynamics' are already working on a new mission proposal with an innovative digital radar antenna – Tandem-L. A significantly higher imaging capability could be achieved by means of this new technology; it will exceed that of TanDEM-X by a factor of 100. While TanDEM-X only enables one global image of Earth to be acquired per year, Tandem-L will image Earth's entire landmass at a higher resolution twice a week. Hence, Tandem-L will be able to capture dynamic changes on Earth's surface with the necessary imaging frequency to provide information urgently needed for answering current scientific questions regarding the biosphere, geosphere, cryosphere and hydrosphere. Such a mission could be launched in 2020.

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Figure 34: TanDEM-X image of the month - changes in permafrost landscapes acquired with TanDEM-X in Oct. 2014 (image credit: DLR)

• In February 2015, the TanDEM (TanDEM-X and TerraSAR-X) missions are operating nominally, supporting the science mission. The new science phase, started in Sept./October 2014, applies to both missions of the formation, TerraSAR-X and TanDEM-X, and is planned to last until the end of 2015. 73)

- The first part of the science mission, from Sept. 20, 2014 to the end March 2015, is the so-called the "Pursuit Monostatic Phase", in which the TanDEM-X spacecraft is flying at a distance of 76 km behind its twin, TerraSAR-X. Each spacecraft acquires monostatic data of the same area which are then interferometrically ground-processed .

- At the end of March, the formation is changed again; the "Bistatic Phase" is started in which bistatic acquisitions with a large horizontal baseline (up to 3600 m) are taken up to September. After a drift phase of about a month, the bistatic phase is continued in October with a small baseline of ~250 m, until the end of the year. Note: The horizontal baseline is here the cross-track component to measure the surface topography.

- Independently from the science mission, the TerraSAR-X mission (generation of non-interferometric SAR products), is being served in parallel from both spacecraft.

Note: The TerraSAR-X has the primary objective to acquire 2D SAR observations in the operational modes Stripmap, ScanSAR and Spotlight. These objectives are being pursued in parallel to the TanDEM mission; these functions are actually supported by both spacecraft of the formation. — The TanDEM mission has the primary goal to generate a global DEM, while the secondary objective is to support also the goals of the current science mission.

• October 10, 2014: After four years of successful data acquisition for the new global topographical map of Earth, the Science Phase is beginning. The radar satellite TerraSAR-X has been orbiting the Earth since June 2007; in June 2010 its twin, TanDEM-X, followed it into space. For almost four years, the two satellites have been operated in a close flight formation by DLR (German Aerospace Center). During this time, the satellites have been acquiring data to generate a new global topographical map of the Earth. 74) 75)

- The goal of the TanDEM-X mission is to produce a highly precise, three-dimensional image of the Earth with uniform quality and unprecedented accuracy. For large parts of the Earth, there currently exist only approximate, inconsistent or incomplete elevation models derived from different data sources and collection methods. TanDEM-X is filling in these gaps and providing a homogeneous elevation model to be used as an indispensable basis for numerous commercial applications and scientific investigations.

- With the start of the TanDEM science phase, another significant milestone in the mission has been reached. "In the coming 15 month mission phase, the orbit and imaging mode will be configured and optimized so that new radar techniques and innovative applications can be tested and demonstrated. The expectations of the scientific user community for the science phase are very high, and more than 100 science proposals have already been submitted," explains Alberto Moreira, Director of the DLR Microwaves and Radar Institute and Principal Investigator for the TanDEM mission.

- The initial preparations for the science phase began with the transition to the new formation on 17 September 2014. TanDEM-X moved away from TerraSAR-X and has been flying at a distance of 76 km behind its twin since 20 September 2014. This has resulted in a time delay of 10 seconds. Since the Earth rotates at approximately 500 m/s at the equator, the orbit of TanDEM-X has also had to be displaced laterally by 5 km so that both satellites are imaging the same area (footprint) on the surface. TanDEM-X continues to follow a helical orbit. Unlike the imaging for the DEM (Digital Elevation Model) of the Earth, the helix will not be adhered to for weeks at a time; instead, significantly greater variations will be permitted. The distance between TanDEM-X and the nominal orbit of TerraSAR-X will vary between 0 and 1000 m over the next five months.

- The aim is to continue to operate both satellites using interferometry, to enable three-dimensional imaging of the surface of the Earth to continue. After changing the orbit of TanDEM-X in recent weeks, the two satellites are being operated independently of one another, in what is known as 'Pursuit Monostatic Mode'. The advantage of this new orbital configuration is that the distance between the satellites – the baseline – can be made substantially more flexible. In the new orbital configuration, data for elevation models can be generated with an elevation accuracy of a few tens of centimeters, for example. This opens up new applications in the areas of the geosphere, cryosphere and hydrosphere. This data is unique and will be used in the investigation of volcanic eruptions, the melting of ice as well as, for example, tomographic imaging of cities. The orbital configuration will be changed again in the spring of 2015 to enable other applications and demonstrations.

- One topical subject is the thawing of permafrost soils, which is being caused by global warming. It is causing massive damage to roads and houses and is leading to landslides. At present, it is known that huge areas are involved, although the precise extent is still unclear. "In the new phase of the mission, one of the things TanDEM-X will do is map these areas with a very high spatial resolution and contribute valuable insights into climate change," explains Irena Hajnsek, the Scientific Coordinator for the TanDEM-X mission.

Global elevation model of the Earth is being created: On 17 September 2014, the imaging for the DEM was completed, with the exception of a few images. A data set of over 2500 TB forms the basis for the new topographic map of the Earth. The quality of the elevation models generated to date exceeds all requirements. Final DEM tiles for more than a quarter of the land area – for example, for the flat areas of Australia, North America, Siberia, South and West Africa and South America – have already been processed. The new 3D map should be available in its entirety by the end of 2015. "Both satellites are functioning well, and the propellant supplies are certain to last until 2020," adds Manfred Zink, DLR scientist and Project Manager for the TanDEM-X ground segment. "We are already thinking beyond the science phase. The operation of two SAR satellites in close flight formation is a unique achievement and demonstrates Germany's leading position in radar technology. We can generate even more accurate elevation models or more precise coastline maps."

The success of TanDEM-X forms the basis for the development of innovative radar technologies. Researchers at DLR are already working on a new mission proposal with a digital radar antenna – Tandem-L. The aim is to achieve a significantly higher imaging capability, which will exceed that of TanDEM-X by a factor of 100. While TanDEM-X only enables one global image of the Earth to be acquired per year, Tandem-L will image the entire landmass of the Earth at a higher resolution twice a week. Hence, Tandem-L will be able to capture dynamic changes on the surface of the Earth with the required imaging repetition frequency and provide urgently needed information for solving topical scientific questions involving the areas of the biosphere, geosphere, cryosphere and hydrosphere. Such a mission could be launched in 2020.

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Figure 35: TanDEM-X timeline (image credit: DLR)

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Figure 36: TanDEM mission elevation model of the Palo Duro Canyon, Texas, USA (image credit: DLR)

• June 2014: After the TanDEM-X launch in June 2010 and the commissioning phase, close formation was achieved mid October 2010 and the operations at typical distances between 120 and 500 m is running remarkably smooth and stable since then. Global DEM acquisitions have started in December 2010 and the first and second global coverage (except Antarctica) was completed in March 2012 and March 2013, respectively. After an acquisition period for gap filling, Antarctica was mapped during local winter conditions (for reasons of better SNR) and since early August 2013 the Helix formation has been readjusted to allow imaging of difficult mountainous terrain from the opposite viewing geometry. After reversing the Helix formation in April 2014, a subsequent second coverage of Antarctica, and some further gap filling, it is anticipated to finalize data acquisitions for the global DEM in the second half of 2014. 76)

A comprehensive system has been established for continuous performance monitoring and verification, including feedback to the TanDEM-X acquisition planning for additional acquisitions. More than 350,000 Raw DEMs have been generated so far in a fully automated process employing multi-baseline interferometric techniques.

The final calibration and mosaicking chain is fully operational since 2013. Based on the first global acquisition, so-called intermediate DEMs have been produced for larger regions. Currently all efforts are concentrated on the generation of final DEMs for areas requiring only first and second acquisitions. Up to now, most of Australia and the rather flat areas of North America and Russia have been finished, areas in South Africa and South America are next in the production. The quality of the first final DEMs is well within specification for both the relative and the absolute height error. First examples for final DEMs over mountainous terrain will become available after the summer. It is expected to complete the global DEM by the end of 2015.

TanDEM-X has demonstrated the feasibility of an interferometric radar mission with close formation flight and delivers an important contribution for the conception and design of future SAR missions. One example is Tandem-L, a mission proposal for monitoring dynamic processes on the Earth surface with unprecedented accuracy.

- Interferometric performance: The interferometric performance of the TanDEM-X DEM is specified in terms of linear point-to-point relative height error. Each acquisition is processed into scenes of 50 km x 30 km in size, so-called RawDEMs. These RawDEMs are the basis for the evaluation of the relative height error. Detailed annotation statistics on the quality for each RawDEM as well as quicklook images for the coherence or the processing quality indicators are added by the processing system. 77)

• May 19, 2014: DLR is making the first elevation models of a new global topography for scientific use. Canyons in Australia's Flinders Ranges National Park, Canadian islands and the rugged volcanic landscape of Russia's Kamchatka Peninsula are revealed at a level of detail 30 times greater than anything seen to date. Over 800 scientists from 31 countries have already registered to work with these highly accurate elevation models. The complete and uniform terrain model is scheduled for completion by the end of 2015. Two sample DEMs are shown in Figures 37 and 38. 78)

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Figure 37: Elevation models of the volcanoes on the Russian Kamchatka Peninsula (image credit: DLR)

Legend to Figure 37: A detailed view around the Krasheninnikov Caldera and Kronotsky Volcano is presented as a shaded relief map. Maps of this kind using TanDEM-X data permit analyses of possible lava flow, used to determine endangered areas.

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Figure 38: TanDEM-X elevation model of the Kamchatka Peninsula (image credit: DLR)

Legend to Figure 38: A digital elevation model from the TanDEM-X mission showing an area on the Kamchatka Peninsula in the northeast of Russia. Wide expanses of the Pacific plate press against the Eurasian plate, causing unique volcanic activity with a very high density of volcanoes. Of the over 160 volcanoes, 29 are currently active and six of them erupt on average each year. Kamchatka is known as the 'Land of Ice and Fire' due to its combination of snow and ice and its high level of volcanic activity. Its highest peak is the 4835 m Kliuchevskoi. The simultaneous eruption of four volcanoes (Shiveluch, Bezymianny, Tolbachik and Kizimen) in January 2013 catapulted the region into the headlines. UNESCO declared the volcanic region of Kamchatka a Natural World Heritage Site in 1996.

• April 16, 2014: Airbus Defense and Space (former EADS Astrium) has commercially launched WorldDEM™, a Digital Elevation Model (DEM) that provides pole-to-pole coverage of unprecedented accuracy. The new model is based on data acquired by the high-resolution radar satellites TerraSAR-X and TanDEM-X, whose mission is to produce a global DEM at HRTE-3 (High Resolution Terrain Elevation, level-3), representing a significant jump forward in accuracy. In terms of resolution, it is setting new standards by providing 12 m grid spacing globally, compared to 90 m grid spacing on the existing global dataset from SRTM (Shuttle Radar Topography Mission). DLR (German Aerospace Center) is operating the mission and generating the global TanDEM-X DEM as a basis for WorldDEM™. 79)

The 12 m posting product meets a vertical accuracy of 2 m relative and better than 10 m absolute and guarantee full homogeneity and seamlessness. These specifications exceed any other global satellite-based elevation model available today. WorlDEM will improve the performance of worldwide operating systems and cross-border mission planning.

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Figure 39: Sample DEM of Quorn, Australia (image credit: Airbus Defence and Space) 80)

Legend to Figure 39: The WorldDEM™ area south of the city of Quorn in South Australia, Australia shows a diversified terrain with elevations ranging from 9 m to 968 m. The topography is characterized by a large valley with agriculture infrastructure (wheat production) in the eastern part and hilly relief in the western part of the sample.

• Feb. 2014: According to information provided by the TSX/TDX project of DLR, global data acquisition for DEM generation from TSX/TDX will be completed by mid-2014. The processing for the global TSX/TDX DEM is expected to last until early 2016. In early 2014, final DEMs for most of Australia, a large part of North America and Siberia are already available. The brandname or label selected by DLR for the final TSX/TDX DEM is actually called the global TanDEM-X DEM. 81)

• In early August (6-8, 2013), the two TanDEM-X mission satellites are reversing their formation. Until now, the TanDEM-X satellite has been circling around its twin, TerraSAR-X, in an anti-clockwise direction; after the reversal, it will circle clockwise. This complicated change to the formation in which they have been flying for almost three years is necessary to observe regions that are difficult to image, such as mountain ranges, from the opposite viewing angle.

The WorldDEMTM will be a global DEM (Digital Elevation Model) of unprecedented quality, accuracy, and coverage (Figure 40). This DEM is intended to be the replacement data set for SRTM (Shuttle Radar Topography Mission) and will be available from 2014 onwards for the Earth's entire land surface — pole-to-pole. WorldDEM is based on data acquired by the TerraSAR-X and TanDEM-X mission. The accuracy of the WorldDEMTM will surpass that of any satellite-based global elevation model available today and have the following unique features:

• Vertical accuracy of 2 m (relative) and 10 m (absolute)

• 12 m x 12 m raster

• Global homogeneity

• Highly consistent dataset as a result of an initial global data collection window of 2.5 years and the opportunity to continue to collect locally beyond the initial collect period.

• High geometric precision of the sensors make ground control information redundant.

Infoterra GmbH, the German unit of EADS Astrium GEO-Information Services, holds the exclusive commercial marketing rights and is responsible for the adaptation of the elevation model to the needs of the commercial users world-wide. Astrium will refine the DEM according to customer requirements, e.g. editing of water surfaces or processing to a Digital Terrain Model (representing the bare Earth's terrain).

• Very good progress has been made with the acquisition 84)

- Relative Height Error is well within specification

- Ongoing third and fourth acquisitions will improve mountains and desert areas

• Mosaicking and Calibration also continues on schedule with good results so far

• WorldDEM launches at GEOINT in April 2014.

Table 8: Introduction of WorldDEMTM in 2014 by Airbus DS (former EADS Astrium Geo-Information Services) 82) 83) 84) 85)

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Figure 40: Comparison between WorldDEMTM data of Death Valley with data from the SRTM missions (image credit: DLR)

Astrium provides intermediate Digital Elevation Models - for customers to perform a detailed assessment of the suitability of WorldDEMTM data for varied applications. 86)

The intermediate DEMs feature two different variants: the basic DSM (Digital Surface Model), which includes the heights of all natural and man-made objects, and the DSM hydro, where water body features derived from the radar data are extracted and edited. Five Quality Layers provide information on the data source and the production. These product variants and quality layers will constitute the core WorldDEMTM product offering. In the near future this offering will be complemented by a Digital Terrain Model and additional derived products.

• May 2013: Since December 2010, the two satellites TSX and TDX are flying as a large single-pass bistatic SAR interferometer in a close Helix formation at about 514 km of altitude (mean value across the equator). This orbit has an 11 day repeat cycle and is maintained for the entire mission within a 250 m toroidal tube around a predefined reference trajectory. TDX has a relative orbit to TSX and together they fly in a precise controlled formation. 87)

Originally designed for a nominal joint operation time of three years, the current status of the satellites resources (primarily fuel) will allow a mission extension of several years. In addition, the two satellites serve also for the TSX (TerraSAR-X) mission, where both satellites independently provide high-quality SAR products for the science community and for commercial customers.

In the first two years of operations, two global coverages of the Earth's land masses, excluding Antarctica, have been acquired. All the acquisitions have been carried out in the nominal right-looking observation mode.

The TanDEM-X mission will deliver a global and consistent DEM (Digital Elevation Model) with unprecedented accuracy. The acquisition of the first coverage has been completed on March 27, 2012; the acquisition of the second one has been finished in April 2013. The third phase of the acquisition, starting from May 2013, includes for the first time the acquisition over Antarctica and further acquisitions of difficult terrain such as deserts and mountainous regions. Performance has been studied in order to derive a consistent and appropriate acquisition strategy over these areas.

The left-looking imaging mode is required for the acquisitions over inner Antarctica, while in north-descending and in south-ascending acquisition mode, with a shift of 180º in the phase of libration is planned for the difficult terrain and on the outer rim of Antarctica.

The two satellites (TDX and TSX) are kept in a close helix-formation with a distance of less than 1 km. In order to reach the intended DEM accuracy, the baseline vector between the two spacecraft needs to be determined with an accuracy of 1 mm. To achieve this goal, both satellites are equipped with high grade dual-frequency GPS receivers.

The baseline vector between the two satellites is determined by GFZ and DLR/GSOC ( German Space Operations Center ) using independent software packages. The GSOC baseline solution is processed with the FRNS (Filter for Relative Navigation of Spacecraft) software. The underlying concept is to achieve a higher accuracy for the relative orbit between two spacecraft by making use of differenced GPS observations, than by simply differencing two independent POD (Precise Orbit Determination) results. The use of single-differenced code and carrier phase observations rigorously eliminates the GPS clock offset uncertainties and largely reduces the impact of GPS satellite orbit and phase pattern errors. Double differences are used for the integer ambiguity resolution of the carrier phase observations.

In studies prior to the TanDEM-X mission, comparisons between independent software packages showed biases of a few millimeters. In order to ensure the highest accuracy, a baseline calibration and combination process has been installed. The baseline products are validated by dedicated baseline calibration data takes over test sites, where the DEM is well known. Using those DEMs as a reference, height differences in the TanDEM-X scenes are estimated. Taking into account the incident angle and the height of ambiguity, these height differences can then be used to infer errors of the baseline products.. The resultant offset parameters are then applied in the baseline calibration process. The analyses show that the derived offset parameters are in the range of few millimeters. Finally the different solutions are merged to a combined product.

Prior to the mission, it was not sure, if the requirements on baseline accuracy of 1 mm, 1D-sigma, could be met. However,it was shown, that the GPS data quality remains constant on a high level. The routine baseline comparison has helped to assess the precision of the three individual baseline products and to identify systematic offsets between them. Since all baseline products employ identical GPS data sets, these offsets are mainly attributed to different processing concepts (such as ambiguity resolution and reduced dynamic versus dynamic trajectory models) in the employed software packages. Systematic biases of at most 2 mm can be observed in the cross-track direction, while biases in the radial direction are about ten times lower. Compared to the use of a single baseline product, the combination of multiple baseline solutions has, furthermore, helped to reduce the overall noise and to identify erroneous contributions.

On the other hand, the GPS-derived baseline can at best deliver accurate information on the relative position of the two spacecraft but is insensitive to potential errors in the adopted SAR antenna phase centers or uncalibrated differential delays between the two instruments. The SAR calibration data takes have therefore been used to determine the effective biases of the baseline products and of the instrumental effects in the SAR processing chain. While it is still not possible to exactly quantify the accuracy of the final calibrated baseline products, its use in DEM-processing has proven, that it is of fully acceptable quality to reach the demanding mission goal.

Table 9: Two years of TanDEM-X baseline determination 88)

• In 2013, the TanDEM-X mission, consisting of two spacecraft, namely TSX-TDX, is fully operational and continuous its close formation flight.

Beyond the primary mission objective of global DEM acquisition, the TSX-TDX satellite formation provides a configurable SAR interferometry test bed for demonstrating new SAR techniques and applications. In particular, it offers a unique chance to measure very slowly moving sea ice as well as ocean currents by means of ATI (Along-Track Interferometry). Due to the importance of the DEM acquisition the first three years of the mission are executed with a formation optimized for this purpose. After finalization of the global DEM formation flying will be dedicated to a variety of scientific interferometric campaigns including ATI. However, during the first years of operation already thousands of data sets have been acquired in the course of the TanDEM-X Science Program.

The secondary mission objectives of the TSX-TDX constellation, like SAR ATI support, are expected to start by the end of 2013, when the primary objective of global DEM acquisition has been completed. Because of the fact that spaceborne SAR ATI with along-track separations in the order of 50 m has not been demonstrated before, there is a strong interest in preliminary ATI experiments with TanDEM-X to validate the methods foreseen for both SAR acquisition and processing.

A first set of ATI experiments was already performed in February and March 2012 in the background of the on-going TerraSAR-X and TanDEM-X missions. At that time, the ground-controlled formation geometry comprised of minimum satellite distances of 150 m in the plane perpendicular to the flight direction and a favorable along-track separations over northern Europe. For the acquisition of the ocean current at the Pentland Firth on Feb. 26, 2012, the along-track separation was slightly adjusted to yield optimal observation conditions without affecting the routine DEM acquisition. The acquired data impressively illustrates the high potential of TanDEM-X for mapping water surface currents with high spatial resolution, which has applications in the field of optimal placement of renewable energy sites and to validate global circulation models which are used for climate research.

Table 10: ATI measurements of sea ice and ocean currents will follow the primary DEM mission by the end of 2013 (Ref. 32)

• July 2012: Global maps of Earth's topography: Within the last decade, three global mappings took place and especially those performed with radar brought dramatic progress: 89)

- The SRTM (Shuttle Radar Topography Mission) acquired the first global and consistent 30 m DEM between ± 60º latitude in the year 2000.

- After that the optical Terra/ASTER system (NASA) acquired another near global 30 m DEM between ±83º latitude since the year 2000.

- Since 2010, the German TanDEM-X mission is acquiring a global 12 m DEM with significantly improved resolution and accuracy (Ref. 9).

In the course of the quality control of the routine processing , TanDEM-X DEMs are continuously compared to SRTM DEMs. In this comparison not only the expected differences in resolution and accuracy have been found, but also significant changes that occurred to the Earth's surface over 10 years since the SRTM mission. Even the differences between TanDEM-X acquisitions separated by a couple of months reveal interesting details.

Accuracy : TanDEM-X data are currently (2012) being acquired and first accuracy assessments confirm that the specification of 2 m vertical point-to-point error (90%) will be fulfilled for the final DEM composed of two coverages, in many cases even in a single coverage (Ref. 89).

• In 2012, the TanDEM-X mission, consisting of two spacecraft, namely TSX (TerraSAR-X) and TDX (TanDEM-X), is fully operational and continuous its close formation flight. Both satellites are capable of monostatic and bistatic SAR operation, supporting several imaging modes and geometries like along- and cross-track interferometry, or new bistatic SAR techniques. 90) 91)

- The primary mission goal of TanDEM-X is the systematic acquisition of a global, homogeneous DEM (Digital Elevation Model). The nominal DEM acquisition is performed in the bistatic stripmap mode (single polarization). Figure 2 (in bistatic mode, left) shows one satellite transmitting and two satellites receiving the signal.

- On a secondary level, the system supports experimental applications in order to perform flexible multistatic configurations for scientific purposes. An alternating bistatic acquisition is depicted in Figure 3, where the transmitting signal is toggled between the satellites, while both satellites are always receiving. Further configurations can be commanded, but processing is on best effort for the following modes (bistatic and alternating bistatic): all spotlight variants with all polarizations, and stripmap dual-polarization for alternating bistatic. Quad-polarization (from dual-receive antenna mode) and pursuit monostatic acquisitions for all imaging modes are possible to be acquired too, but they are restricted to dedicated mission phases.

A key role for fast SAR processing of the bistatic data channel is the exact timing knowledge of the whole system. The knowledge is provided by the evaluation of synchronization pulses - exchanged via a special in-orbit sync link system- from one to the other satellite. These pulses are interspersed into the imaging radar pulses and exchanged via the sync-horn antennas. The synchronization pulses are embedded into a synchronization sequence which contains 4 pulses: the initialization of the synchronization, sync-pulse from one satellite to the other, sync-pulse from the other satellite back to the first one, and exit of the synchronization. Each of the 4 pulses is performed with a specific PRF in order to harmonize the receiving windows of the two independent SAR instruments (Figure 41). The duration of the 4 pulses is equal to an integer multiple of the imaging PRI (Pulse Repetition Interval) = 1/PRF.

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Figure 41: Insertion of synchronization sequence during bistatic image acquisition. The sync PRIs have different lengths than the nominal PRIs (image credit: DLR)

• In January 2012, after a year of formation flight of TanDEM-X with TerraSAR-X, the twin satellites have completely mapped the entire land surface of Earth for the first time. The data is being used to create the world's first single-source, high-precision, 3D digital elevation model of Earth. DLR controls both radar satellites, generates the elevation model, and is responsible for the scientific use of TanDEM-X data. 92) 93) 94)

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Figure 42: On Jan. 12, 2012, the TDX/TSX mission had completely mapped all land surfaces on Earth except Antarctica (image credit: DLR)

Legend to Figure 42: The color scale of the first global coverage stereo map shows the relative height error (10 m in red to 0 m in dark green) derived from the mean coherence of each individual Raw DEM. Gray-shaded strips have been recorded, but have yet to be processed (Ref. 94). 95) 96)

• Over the course of 2011, the distance between the satellites was progressively reduced down to the minimum permitted value of 150 m.

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Figure 43: The first TanDEM-X mosaic of Iceland (image credit: DLR)

• In the summer of 2011, the combined space and ground segment performs remarkably well and by now, more than half of the global land masses have been mapped. Currently the efforts concentrate on completing the processing and calibration chains. 97)

• In March 2011, the TanDEM-X and TerraSAR-X mission continue to acquire bistatic mode DEM data in their formation flight. So far, no problems in operational handling of bistatic flight configuration and related safety measures were encountered. 98) 99) 100) 101)

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Figure 44: Artist's view of TDX and TSX in formation flight (image credit: DLR)

• In January 2011, the TanDEM-X mission is operational, flying in close formation with TerraSAR-X (a single pass SAR interferometry configuration) and providing stereo SAR imagery. The collection of data for a global homogeneous DEM started - as planned - in early 2011.

Phase

Description

Epoch

Launch

 

June 21, 2010

Drift phase to acquire target orbit

The initial separation between TDX and TSX was 15700 km and after one month of drifting a formation in pursuit monostatic configuration with an along-track distance of 20 km was reached.

June 21 - July 22, 2010

Wide formation:

TDX monostatic radar instrument commissioning. The wide formation of 20 km was maintained for 3 months to calibrate the TDX radar instruments and to perform first bistatic and interferometric experiments employing large baselines.

July 22 - Oct. 12, 2010

Reconfiguration

On October 14, both satellites were maneuvered into a close formation to start the bistatic commissioning phase. During this phase, the radial and cross-track baselines were kept constant at 360 and 400 m, respectively, and the mean along-track distance was set to 0 m. The results from both the mono- and bistatic commissioning phase already demonstrated the unique interferometric performance of TanDEM-X.

Oct. 12 - 15, 2010

Close formation:

TSX/TDX bistatic instrument commissioning; Begin of routine monostatic radar operation on both satellites

Oct. 15 - Dec. 12, 2010

Begin of routine DEM acquisition with flexible baselines: operational phase

Operational DEM acquisition started on December 12, 2010, less than 6 months after satellite launch. Since then, the total landmass of the Earth has been mapped once with a height of ambiguity ranging from 40 to 60 m. Global DEM data acquisition with varying baselines will continue until 2013, mapping difficult terrain like mountains, valleys, tall vegetation, etc., with at least two heights of ambiguity as well as from multiple incidence/aspect angles. The latter will be achieved by swapping the Helix formation. This allows for a shift of the DEM acquisition quadrants from ascending to descending orbits in the northern hemisphere and vice versa in the southern hemisphere.

The fully mosaicked DEM shall become available in 2014 for 90 % of the global landmass. Figure 45 shows two examples of TanDEM-X DEMs that have been acquired during the commissioning phase.

Dec. 12, 2010

 

In order to collect sufficient measurements for a global DEM (Digital Elevation Model), three years of formation flying are foreseen with flexible baselines ranging from 200 m to a few kilometers.

 

 

Current fuel consumption and battery degradation on the TerraSAR-X satellite is well below specification and will probably allow for life time extensions of two to three years, i.e. close formation flying until 2015 seems feasible. The prolonged mission time will allow for additional DEM acquisitions with improved accuracy and resolution as well as the conduction of advanced bistatic and multistatic SAR experiments in unique configurations, modes and geometries. 102)

Fall 2012

Table 11: TanDEM-X (TDX) mission timeline leading to formation flight with TerraSAR-X (TSX) 102) 103)

The achieved relative orbit determination position accuracy is better than 50 cm in cross-track (2D, rms) and about 1 m in along-track direction (rms) and therefore well suited for the purpose of formation monitoring and control. After a short commissioning phase the GSOC formation control process has been fully automated. Daily in-plane maneuver pairs for execution on-board TDX are performed to compensate the natural eccentricity vector drift and to adjust the along-track separation while out-of-plane control is used to adjust the horizontal separation at less frequent control interval (typically 11 days). The achieved relative control accuracy is nominally 5 m (rms) in cross-track direction and can be up to 10 m (rms) during phases of large horizontal separation drifts as performed in the beginning of the DEM acquisition phase. The along-track control accuracy is better than 30 m (rms). The control requirements of 28 / 200 m (rms) in cross-track /along-track direction are clearly overachieved paving the way for the acquisition and processing of a global digital elevation model within the upcoming years of TSX-TDX formation flight (Ref. 103). 104)

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Figure 45: Examples of digital elevation models acquired by TanDEM-X acquired during the commissioning phase. Top: Italian volcano Mount Etna, located on the east coast of Sicily. Bottom: Chuquicamata, the biggest copper mine in the world, located in the north of Chile (image credit: DLR, Ref. 102)

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Figure 46: Overview of TanDEM-X commissioning phase and its sub-phases in 2010 (image credit: DLR, Ref. 98)

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Figure 47: Projected in-orbit lifetimes of TSX and TDX (image credit: DLR, Ref. 100)

• On December 14, 2010, the TanDEM-X mission reached another important milestone: the radar mission's test phase has been concluded in less than six months according to plan, paving the way for routine operations - the collection of elevation data. This meant in particular the start of systematic data acquisition for global DEM generation. 105) 106)

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Figure 48: This TanDEM-X DEM image shows Salar de Uyuni, the largest salt flats in the world covering ~10,000 km2, located next to the volcanic region of the Atacama Desert (image credit: DLR)

• In-orbit performance of TSX-1 and TDX-1: During the first three month the primary goal task was to check whether the TDX-1 satellite has the same performance as TSX-1 and to calibrate it to fulfill the requirement of the TerraSAR-X mission. Over 2000 acquisitions have been performed and analyzed w.r.t. various aspects like: 107)

- SAR instrument characteristics (noise, raw data balance, signal power and ADC saturation)

- Doppler centroid

- Noise equivalent sigma zero

- Signal to noise ratio

- Peak-to-side lobe ratio

- Integrated side lobe ratio

- Geometric resolution

- Radiometric resolution

- Repeat pass acquisition performance.

It could be validated that TDX-1 acquisitions fulfill the TerraSAR-X performance requirements and that the performance is very similar to TSX-1 acquisitions. TDX-1 has a slightly better noise performance, but on the other hand, transmits with slightly less power which in total results in the same imaging performance as TSX-1.

The commissioning period for the interferometric TanDEM-X aspects was very compacted; an important goal is to start the systematic DEM acquisition as soon as possible. Hence, the first priority was to ensure the quality of the DEM acquisitions which are acquired in bistatic stripmap single polarization mode. The bistatic commanding and the bistatic synchronization performance between the two satellites worked as expected, which allowed early acquisitions for further performance evaluations. In total more than 1500 bistatic acquisitions were acquired and analyzed during the two month lasting bistatic commissioning phase period. The acquired data was evaluated in two ways:

1) Statistically, which meant that the performance parameters like coherence or phase unwrapping errors for many acquisitions had to be averaged in bigger areas (e.g. 30 km x 50 km) and then evaluated on a large scale basis.

2) Individually, which meant that smaller areas were analyzed in detail to verify certain effects and to find new effects.

The main activities dealt with the topics of "height error analysis" and "polarization analysis" — resulting in the following conclusions: (Ref. 107)

The TanDEM-X (i.e., TSX+TDX) global DEM will be generated by merging at least two different acquisitions from the same area and thus increasing the performance. The height error requirement for 90% of the points is 2 m for slopes < 20º and 6 m for slopes > 20º.

All global DEM acquisitions will be performed in HH polarization since the performance prediction indicated a slightly better performance in comparison to VV. During the commissioning phase this prediction has been verified by real acquisitions. The slightly better performance for HH has been confirmed. - Please note that the absolute performance difference between HH and VV is very small.

 

• Calibration of the TanDEM-X SAR system- all items completed in early December 2010: The main part of all calibration activities, especially of measurements executed against precise reference targets, was concentrated on the monostatic CP (Commissioning Phase) for deriving all calibration parameters (see Figure 46). The activities executed during the interferometric CP were concentrated on the verification of all these parameters, i.e. whether they are still valid in bistatic constellation and enable consequently a precise synchronization of both systems. 108) 109)

The calibration effort and consequently the duration for commissioning the whole TanDEM-X (TDX) system could be optimized by the experience and the results which had been achieved for TSX since launch in 2007. This was the baseline for executing the TDX commissioning phase as fast as possible. Thus, a maximum overlap of the lifetime of both satellites required for the global DEM acquisition could be achieved.

- On top, most of the acquisitions against precise reference targets could be applied twice during one pass, i.e. once for TDX and once for TSX. These results approve once again that the stability and the accuracy of the TSX system is still of unprecedented quality, three years after launch.

- Internal calibration

- Geometric calibration

- Antenna pointing

- Antenna model verification

- Radiometric calibration

Calibration procedure

Goal

TDX

TSX

Internal calibration
Amplitude
Phase


0.25 dB
< 1.0º


< 0.1 dB
< 1.0º


< 0.1 dB
< 1.0º

Antenna pointing knowledge
Elevation
Azimuth


0.015º
0.002º


< 0.002º
< 0.002º


< 0.004º
< 0.001º

Pixel localization accuracy
Azimuth
Range


2 m
2 m


0.30 m
0.50 m


0.29 m
0.48 m

Antenna model verification
Elevation pattern
Azimuth pattern ††
Beam-to-beam gain offset


±0.2 dB
±0.1 dB
±0.2 dB


< ±0.2 dB
< ±0.1 dB
< ± 0.2 dB


< ±0.2 dB
< ±0.1 dB
< ± 0.2 dB

Radiometric calibration
Radiometric stability
Relative accuracy
Absolute accuracy


0.5 dB *
0.68 dB
1.1 dB ‡


0.15 dB **
0.17 dB ‡
0.48 dB ‡


0.15 dB **
0.16 dB ‡
0.39 dB ‡

Table 12: Results of the calibration procedure

Legend to Table 12: two-way, †† one-way, * requirement defined over a period of 6 months, ** measured by TSX after 2 years (2009), ‡ StripMap mode.

After the launch of the second satellite TDX, the whole TanDEM-X system could be successfully commissioned and consequently calibrated in 2010. Furthermore, the performance of the first satellite TSX is still of unprecedented quality, three years after launch.

Never before have two independent spaceborne SAR systems - at an altitude of > 500 km - been more accurately calibrated and consequently precisely matched to each other than TSX and TDX. The geometric offset between both SAR systems is smaller than half the wavelength. The radiometric offset is essentially < 0.1 dB. All requirements and/or goals have been achieved, even to a better extend than predicted. The results are listed in Table 12 (Ref. 108).

• Baseline calibration of TanDEM-X. Typical baseline lengths of the TDX/TSX constellation are in the order of 500-1500 m. The challenging baseline determination relative accuracy requirement is 1 mm, since the precise knowledge of this magnitude is essential for achieving the required height accuracy of the DEM (Digital Elevation Model). Previous baseline determination experiments did not exclude the existence of residual baseline biases in the order of 2 to 8 mm. Therefore, baseline calibration became one of the main TanDEM-X commissioning phase activities. 110)

The orbit positions are calculated based on the measurements of the on-board GPS navigation receivers, namely the TOR/IGOR system (Tracking, Occultation and Ranging / Integrated GPS Occultation Receiver). An absolute accuracy of 5 cm (1σ) has been achieved, as verified by the in-orbit tests of the TerraSAR-X satellite.

However, it is possible to track the relative changes between both satellites (baseline) with a much higher accuracy. This is performed through processing the navigation information derived from the DDGPS (Double Differential GPS) carrier phase measurements between both satellites and applying a Kalman filter method to the data. The use of the differential information even eliminates ionospheric errors and other characteristic GPS perturbations. The resulting relative baseline determination accuracy is expected to be in the order of 1 mm, based on the performance of the DDGPS method in similar missions like GRACE.

The calibration activities during the TanDEM-X satellite commissioning phase have proven their efficacy to characterize the baseline error in LOS over time and latitude location, as well as to identify its systematic. The first in-orbit results of these characterization activities show baseline accuracies of around 1.5 mm (1σ) for the calibrated baseline product (Ref. 110).

Legend to Figure 48: The blue to dark blue areas show the lowest lying parts of the salt flats. A trained eye can see the boundaries of rock deposits in the three-dimensional model. This information about landscape features helps the project to draw important conclusions about the origins and development of the area.

• From Oct. 14, 2010 onwards, the TanDEM-X (TDX) and TerraSAR-X (TSX) spacecraft are flying in a close flight formation starting the bistatic commissioning phase. During this phase, the radial and cross-track baselines were kept constant at 360 and 400 m, respectively, and the mean along-track distance was set to 0 m. - Bistatic DEMs are being acquired since then. With TDX delivering identical single SAR product quality as TSX, the TerraSAR-X mission is running operationally on both satellites since October 25, 2010.

• On October 11, 2010, the thrusters of TanDEM-X were fired to reduce the separation between TanDEM-X and TerraSAR-X from 20 km to 500 m. TerraSAR-X remained in its original, circular orbit, while TanDEM-X, moved on a slightly eccentric orbit in a plane that is rotated by a small angle with respect to that of its partner.

At the North Pole, TerraSAR-X overtakes TanDEM-X, as the latter, because of its eccentric orbit, is slightly higher and thus orbiting more slowly. At the South Pole, the situation is reversed; TanDEM-X orbits lower and faster, and overtakes TerraSAR-X. Of course, a collision must be prevented – 'touching' is not allowed! Viewed from side, the two orbits can be imagined as being like two links in a chain – one link round, the other elliptical – intertwined but not touching one another. 111)

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Figure 49: The circular orbit of TerraSAR-X (red) and the eccentric orbit of TanDEM-X (green) never cross, preventing a collision between the two satellites (image credit: DLR)

In this "narrow" formation flight configuration - flying over the same target area reduces the time difference into the region of ms (milliseconds), a condition most suitable for interferometric observations. Full commissioning of TanDEM-X and TerraSAR-X in formation flight is expected in December 2010.

• In early August 2010, the TanDEM-X and TerraSAR-X spacecraft of DLR are in their "wide" formation flight configuration - flying over the same target area in a time difference of ~ 3 seconds. In this formation flight configuration, interferometric imagery may be obtained automatically. 112) 113)

Already on July 20, 2010, TanDEM-X had achieved a "wide" formation with TerraSAR-X. The nominal distance to TerraSAR-X was 20 km in flight direction (300 m radial and 1305 m horizontal direction). The wide formation will be kept throughout the monostatic commissioning phase of TanDEM-X, planned to last to the end of September 2010. 114)

• During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes have been conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. - Two sets of innovative bistatic experiments have been carried out during the monostatic commissioning phase.

1) the first bistatic acquisition, complemented with a repeat-pass interferometric processing of consecutive bistatic surveys

2) the first single-pass bistatic interferometric acquisition.

In both cases, the geometrical configuration coincided with the one depicted in Figure 50: the beams used in pursuit monostatic operation are represented by solid lines, corresponding the dashed ones to a bistatic operation with symmetric azimuth steering. 115) 116) 117)

- Table 13 lists the main parameters of the bistatic acquisitions. The column 'Experiment 1' refers to the first bistatic imaging acquisition, as well as to the repeat-pass bistatic interferometric results; the column 'Experiment 2' refers to the first single-pass bistatic interferometric acquisition. All data-takes were acquired using the regular stripmap modes of the satellites. The data have been processed using the experimental TanDEM-X interferometric processor (TAXI), a flexible and versatile processing suite especially developed for the evaluation of TanDEM-X experimental data products.

Parameter

Experiment 1

Experiment 2

PRF (Pulse Repetition Frequency)

3182.52 Hz

2991.24 x 2 Hz

Incident angle

36.6º

35.8º

Squint angle TSX/TDX

±0.8º

±0.9º

Cross-track baseline

253 m

43 m

Table 13: Acquisition parameters

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Figure 50: Formation of the TSX and TDX satellites during the TDX monostatic commissioning phase (image credit: DLR)

The acquisition, carried out on Aug. 8, 2010, was planned over the city of Brasilia. For this first bistatic experiment, TSX operated monostatically with a squint of -0.8º, whereas TDX was set in receive-only mode with a squint of 0.8º. Due to the small bistatic angle, no relevant modifications of the timing schemes were required. Synchronization pulses were exchanged during the data-take, from which the clock phase error could be measured. A squinted monostatic image and a non-squinted bistatic one were obtained, but with no spectral overlap between them. Similar acquisitions were conducted in consecutive passes of the system over the same area to produce bistatic repeat-pass interferograms, i.e., after 11 days.

The first bistatic image acquired by TanDEM-X is shown in Figure 51 (green channel), overlaid with the simultaneous TSX monostatic image (magenta channel). Two different aspects of the previous images can be outlined. In the city center, the dominant scattering mechanism seems to be monostatic, but there exist distinct building areas near the lake, where the bistatic scattering dominates. The conclusion is that, even for the small bistatic angle of the experiment, about 1.6º, significant changes in target reflectivity, especially in man-made structures, can be expected between monostatic and bistatic observations. This feature might be very helpful to enhance the performance of existing identification or classification algorithms. The second one is the distribution of the azimuth ambiguities. Considering the azimuth ambiguities of the point target of opportunity in the center of Figure 51, which are mapped on the lake and zoomed within the ochre rectangle on the bottom right corner of the figure, a range difference in the position of the monostatic and bistatic ambiguities appear. This happens because the monostatic image is squinted, whereas the bistatic is not, and therefore the 2D monostatic impulse response, unlike the monostatic, is skewed. This feature might be exploited to develop ambiguity identification/suppression strategies.

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Figure 51: TDX first bistatic image (green channel) over Brasilia. TSX monostatic image (magenta channel) overlaid. Radar coordinates (horizontal range, vertical azimuth time), image credit: DLR

Figure 52 shows the repeat-pass interferogram using the two bistatic images after eleven days. Note that the images are rotated 90º with respect to Figure 51. The figure on the left shows the non-synchronized bistatic interferogram. The azimuthal fringes of the interferogram are due to the differential clock frequency change between the two passes, of about 1 Hz. The right figure shows the synchronized interferogram; note that the SRTM DEM has been used in Figure 52 to remove topography. The residual fringes correspond mainly to a priori DEM errors and (possibly) marginally to unaccounted atmospheric effects. The mean value of the coherence is 0.35; in urban areas, this value increases to about 0.5. There are no significant differences between the values obtained from the monostatic repeat-pass and the bistatic interferograms.
Concerning the interferometric performance, the baselines are practically the same, as is the SNR of both acquisitions. Note that the monostatic image has a squint, but since no significant changes in target reflectivity other than for certain man-made structures have been observed, the results are definitely consistent. Besides its novelty, the relevant conclusion of this experiment was that we could obtain with the new system bistatic images and interferograms of similar quality to the monostatic (more mature) TerraSAR-X counterparts, a quite relevant information at the time.

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Figure 52: Results of bistatic 11-day repeat-pass interferometry with TanDEM-X (image credit: DLR, Ref. 115)

- Bistatic single-pass interferometry: Following the success of the bistatic imaging acquisitions, a natural next experiment consisted of performing a single-pass bistatic interferometric experiment with a large along-track baseline before the end of the pursuit monostatic commissioning phase. However, a way to overcome the spectral decorrelation of the previous bistatic configuration was needed. Because of the small bistatic angle, simultaneous monostatic and bistatic images with similar equivalent squint angles have Doppler spectral overlap, i.e., the images are coherent. This equivalency is depicted in Figure 50.

To achieve this, an imaginative commanding of the satellites was designed, with a switch of the azimuth antenna patterns of TSX and TDX on a pulse-to-pulse basis. Both satellites transmitted one pulse using the non-squinted beams (solid lines) in Figure 50; in the next pulse TSX transmitted with a squint of -0.9º and TDX only received with a squint of +0.9º (depicted with dashed lines in Figure 50). All things considered, one pursuit monostatic interferogram with full baseline, plus two symmetric bistatic interferograms with half baseline could be computed.

However, the acquisition had a couple of drawbacks: firstly, the PRF needed to be doubled, i.e., the swath was halved; secondly, due to the specifics of the commanding, no calibration nor synchronization pulses were available. The acquisition was carried out over the Parque nacional del volcán Turrialba, in Costa Rica, a gracefully mountainous area. Note that this experiment was conducted early October 2010, about a week before the first official bistatic TanDEM-X interferograms in close formation were obtained, and are therefore the first bistatic single-pass spaceborne SAR interferometric acquisitions.

TDX uses a direct X-band link to measure the differential phase between the two radar master clocks. For the cases where this synchronization information is missing, TAXI has an automatic synchronization module which is capable of estimating the synchronization error using the bistatic data, which definitely substantiates the scientific character of the experiment. The estimated clock carrier frequency difference of about 124 Hz is also consistent with available contemporary SyncLink samples.

Figure 53 shows the DEM generated using one of the bistatic interferograms. The validity of the approach can be shown by cross-checking the results obtained using the single-pass bistatic interferogram of Figure 53 and the one resulting from the conventional pursuit monostatic one.

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Figure 53: Geocoded (North points rightwards) DEM of the are surrounding Turrialba volcano, the first single-pass bistatic interferometric TanDEM-X acquisition (image credit: DLR)

Figure 54 shows the height difference between the two DEMs. A mask has been used to avoid including values with low coherence. No trends in range or azimuth can be identified, which qualitatively validates the automatic synchronization approach. The standard deviation in the height error of the DEMs computed using single-look interferograms corresponds to 23.3 m, which results in an effective averaging factor of about 20 for the DEM error of the figure.

TanDEMX_AutoD

Figure 54: Difference between pursuit monostatic and single-pass bistatic DEMs. Effective averaging factor is roughly 20, (image credit: DLR, Ref. 115)

• On July 22, 2010, DLR published the first 3D images from the TanDEM-X satellite mission. A digital elevation model test image was created of an ice cap located on one of a group of Russian islands in the Arctic Ocean, referred to as "October Revolution Island." The image of Figure 55 represents a large ice cap in the center of the island, it is mapped accurately to a few cm in elevation.

Close formation flying of TanDEM-X and TerraSAR-X is yet to be achieved. Nevertheless, DLR researchers were able to generate the first 3D images by waiting for the optimum time when the two satellites – in their near-polar orbits – were fairly close together. Such a close approach incident of the two spacecraft occurred on July 16, 2010 when the interferometric imagery was observed.

The DLR project started the orbital correction maneuvers on July 12, 2010 - a deceleration maneuver, to prevent TanDEM-X from overtaking TerraSAR-X. On July 16, the distance between the satellites had shrunk to 370 km. This distance corresponded to a time gap of 48 seconds, during which the Earth continues to revolve, so that the trailing satellite did not cover the identical target area. Although the orbits are nearly identical for both satellites, Earth's rotation caused the ground tracks at the equator to be separated from one another by more than 20 km. This distance is too great for interferometric observations. At the poles, the ground tracks approach one another and eventually intersect. This provided a unique opportunity to observe the same area with two satellites, one after another, from similar positions and with a reduced distance between them. However, because of the 48 second time delay and Earth's rotation, adjustments had to be made in the analysis of the imagery for each different imaging location. 118)

• The initial separation between TDX and TSX was 15700 km and after one month of drifting a formation with an along-track distance of 20 km was reached (20 km correspond to ~ 3 seconds). This formation was maintained for 3 months to calibrate the TanDEM-X radar instruments and to perform first bistatic and interferometric experiments employing large baselines.

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Figure 55: Digital elevation model of an ice cap in the middle of October Revolution Island (image credit: DLR)

TanDEMX_AutoB

Figure 56: Ice floes on the coast of October Revolution Island (image credit: DLR)

• On June 24, 2010, only 3 days into the mission, TanDEM-X sent its first imagery back to Earth. The data transmission was received and automatically processed by the DLR ground station in Neustrelitz, Germany. This ended the LEOP (Launch and Early Orbit Phase) and started the commissioning phase which is expected to last for about 3 months. 119) 120)

TanDEMX_AutoA

Figure 57: First downlinked image of the TanDEM-X mission showing northern Madagascar (image credit: DLR)

Legend of Figure 57: The colored pale yellow on the right side of the image are the waves of the Indian Ocean. The change in the waves at the entrance to the Diego Suarez Bay is clearly visible. The water in the bay itself, on the shore of which lies the provincial capital city, Antsiranana, can be recognized. The water in the bay is very flat – in contrast to the undulating ocean – and reflects the radar signals from TanDEM-X more uniformly. The area of valleys to the south drains the volcanic cone of Ambre-Bobaomby into the Indian Ocean.

• A first signal of the TanDEM-X spacecraft was received via the Troll ground station in Antarctica. TrollSat is Norway's 7.5 m X-band ground station of KSAT (Kongsberg Satellite Services AS) in Antarctica (72º S, 2º E, since 2007) operated from Tromsø, Norway.

 


 

Sensor complement and support modes:

The TDX-SAR instrument is identical to the TSX-SAR (TerraSAR-X SAR instrument) in layout, operational performance and support modes. Hence, the reader is referred to the TSX-SAR instrument description of the TerraSAR-X mission.

Each spacecraft in the formation is equipped with 6 small horn antennas that are used to exchange X-band synchronization pulses with a frequency of about 10 Hz. This allows for a phase synchronization with an accuracy in the order of 1º.

TanDEMX_Auto9

Figure 58: Accommodation of the synchronization horn antennas with the beams shown in red (image credit: DLR)

The intersatellite X-band synchronization is established by a mutual exchange of radar pulses between the two satellites. For this information exchange, the nominal bistatic SAR data acquisition is shortly interrupted, and a radar pulse is redirected from the main SAR antenna to one of six dedicated synchronization horn antennas mounted on each spacecraft. The pulse is then recorded by the other satellite which in turn transmits a short synchronization pulse. This trigger establishes the bidirectional link between the two radar instruments which allows for mutual phase referencing without the exact knowledge of the actual distance between the satellites.

The TanDEM-specific SAR instrument features provide a scheme for transmission and reception of USO (Ultra Stable Oscillator) phase information between the SAR instruments with adequate SNR (at X-band for low atmospheric sensitivity). The synchronization horn antennas are connected to the unused polarization inputs/outputs of the leaf amplifier assembly TRMs. The major driver was to find a suitable accommodation (see Figure 58) and RF-design of the horns to give full solid-angle coverage with low phase disturbance. The data take start time accuracy has to be improved (relative to onboard GPS time) to allow for synchronization of data takes of both SARs. These modifications have already been implemented on the TerraSAR-X spacecraft.

Note: The TSX-SAR is already built for repeat pass interferometry and thus provides several essential features for the TanDEM-X mission, like precise orbit control, a single-frequency GPS for orbit determination, and excellent RF phase stability.

Parameter

Value

Parameter

Value

Satellite orbit altitude

514 km

Antenna length

4.8 m

Nominal swath width

30 km

Antenna width

0.7 m

Swath overlap

6 km

Antenna elements

32 x 12

Carrier frequency

9.65 GHz

Antenna tapering

Linear phase

Chirp bandwidth

100 MHz

Antenna mounting

33.8º

Sampling frequency

110 MHz

Data quantization

3 bit/sample

Peak Tx power

2.26 kW

Image pair misregistration

< 0.1 pixel

Duty cycle

18%

Proc. azimuth bandwidth

2266 Hz

Noise figure TRM

4.3 dB

Misregistration

0.1 pixel

Losses (proc., atm., taper, degradation)

3.1 dB

Sigma nought (σο) model (Ulaby, 90%, X-band)

Soil and rock, VV

Independent post spacing

12 m x 12 m

Along-track baseline

< 1 km

Table 14: X-SAR instrument parameters of TanDEM-X (TDX)

RF center frequency

9.65 GHz (X-band, 3.1 cm wavelength)

Bandwidth

up to 300 MHz

Incidence angle range

20º to 55º

Polarization

single, dual, quad

SAR modes

Stripmap, ScanSAR, Spotlight,
Dual-phase (split-antenna) center mode

Table 15: Main characteristics of the X-SAR instrument on TSX and TDX satellites

Regarding the X-SAR instrument on TDX, several optimizations were performed to further improve the producibility of the TRM (T/R Module) and to guarantee the excellent quality reached with the TSX TRMs. 121)

 


 

Secondary payloads of TanDEM-X (TOR, LRR, LCT)

TOR (Tracking, Occultation and Ranging)

TOR, a dual-frequency GPS flight receiver with two zenith antennas for POD (Precise Orbit Determination) applications support, is of TerraSAR-X heritage provided by GFZ Potsdam and CSR (Center of Space Research) of the University of Texas at Austin. - The excellent data provided by the TOR device flown on the TerraSAR-X mission led to the decision to duplicate this instrument for TanDEM-X. The additional TanDEM-X requirement of intersatellite baseline determination between both satellites with mm accuracy is essential for the success of the anticipated generation of global DEMs (Digital Elevation Models). 122)

The TOR payload consists of the dual-frequency GPS receiver IGOR (Integrated GPS Occultation Receiver), developed and built by Broad Reach Engineering Company of Tempe, AZ, and LRR (Laser Retro Reflector) for evaluation of GPS-based orbit data as an independent tracking technique. - The general description of TOR and LRR is provided under the TerraSAR-X mission.

LCT (Laser Communication Terminal)

LCT is again of Tesat-Spacecom GmbH manufacture and a contribution in kind of DLR. The objective of LCT on TanDEM-X is to demonstrate and verify the performance of a 2 Gbit/s optical LEO-to-GEO link as part of an experimental broadband data relay transmitting a 300 Mbit/s user data stream from TanDEM-X to ground. 123) 124)

TanDEMX_Auto8

Figure 59: Future application scenario of the LCT payload on various spacecraft (image credit: ESA, Tesat)

Compared to the TerraSAR-X mission, the enhanced TanDEM-X LCT terminal allows optical laser links to a relay satellite in the geostationary orbit. The technology demonstration of optical high data rate intersatellite links and the subsequent Ka-band downlink is important to cope with the increased demand of data downlink capacity in future EO missions (LCTs are being planned to support the European GMES program missions). 125)

Homodyne binary phase shift keying (BPSK) is based on coherent detection, i.e. the signal to be detected is superposed to a local oscillator laser running on the same frequency as the signal's carrier. - The general description of LCT is provided under the TerraSAR-X mission.

TDX/TSX applications:

Beyond the global HRTI-3 DEM as the primary mission objective, TanDEM-X will demonstrate several enabling technologies like: VLBI (Very Large Baseline Interferometry), along-track interferometry, polarimetric SAR interferometry, four phase center moving target indication, bistatic SAR imaging, and digital beamforming.

• The VLBI concept takes advantage of the large bandwidth of the TSX-SAR instrument to significantly improve the height accuracy for local areas by combining multiple interferograms with different baseline lengths. This can e.g. be used to acquire DEMs with HRTI-4 like quality on a local or even regional scale. A temporal comparison of multiple large baseline TanDEM-X interferograms (either phase or coherence) provides furthermore a very sensitive measure for vertical scene and structure changes. 126)

• Along-track SAR interferometry can either be performed by the so-called dual receive antenna mode with a baseline of 2.4 m from each of the satellites or by adjusting the along-track distance of the two satellites to the desired size (Figure 60). The HELIX orbit concept allows this distance (called along-track baseline) to be adjusted from zero to several kilometers. This technical feature is essential as this application requires velocity measurements of different fast and slow objects. Mainly four scientific application areas are identified to explore the innovative along-track mode: oceanography, traffic monitoring, glaciology and hydrology.

TanDEMX_Auto7

Figure 60: Along-track interferometry modes in TDX/TSX configuration (image credit: DLR)

• Polarimetric SAR Interferometry combines interferometric with polarimetric measurements. This allows e.g. for the extraction of vegetation density and vegetation height. Fully polarimetric operation uses the split antenna and is susceptible to ambiguities which limit the swath width. This could be avoided by a restriction to dual polarized measurements and/or an acquisition of multiple polarizations in successive passes.

• Bistatic Imaging provides additional observables for the extraction of new scene and target parameters. A combination of bistatic and monostatic images can e.g. be used to improve segmentation, classification and detection. Data takes with large bistatic angles are planned at the beginning and end of the TanDEM-X mission.

• The TerraSAR-X/TanDEM-X mission will be the first operational mission requiring a post-facto baseline reconstruction with an accuracy of 1 mm. The feasibility of achieving this goal using GPS dual-frequency measurements of the IGOR GPS receiver has earlier been demonstrated in the GRACE mission.

• Digital beamforming (DBF) and super resolution techniques can be used to suppress ambiguities and to enhance the ground resolution. The combination of the four independent phase centers in TSX and TDX enables also a first demonstration of high resolution wide swath (HRWS) SAR imaging.

With TanDEM-X, innovative SAR techniques will be demonstrated and exploited, which open up new perspectives for future SAR systems. The focus will be on the following application areas and the associated application topics:

Application area

Application topics

Cross-track SAR interferometry

Land environment

Navigation, Crisis and security management, Urban areas,

Glaciology/hydrology

Ice and snow, Sea-ice, Morphology/hydrology

Geology

Geological mapping/morphology, Earthquakes/volcanoes, Landslides, Subsidence (land and urban areas)

Land cover and vegetation

Land cover/surface parameters, Forestry,

Oceanography

Wind and waves/ocean dynamics, Ship detection, Coastal zones

Along-track SAR interferometry

Oceanography

Ocean currents, Coastal zones, Ship detection, Wind and waves

Traffic

Traffic flow monitoring, Development of new SAR techniques

Glaciology

Ice flow monitoring

Hydrology

River flow monitoring, Development of new SAR techniques

New SAR techniques

Multistatic processing

Bistatic/multistatic processing, Land cover, Development of new SAR techniques

Pol-InSAR (polarimetric SAR interferometry)

Forest, Agriculture, Snow and ice, Ship detection, Wind and waves, Geological mapping, Urban areas, Landslides/subsidence, Earthquakes/volcanoes, Development of new SAR techniques

Digital beam forming (DBF)

Wide swath imaging and ambiguity suppression, Development of new SAR techniques

Super resolution

Development of new SAR techniques

InSAR processing

Development of new SAR techniques, Ultra high resolution DEM with multiple baselines

Formation flying

Development of new SAR techniques, Precise baseline determination and orbit control

Table 16: Overview of application areas and topics for the various science teams

TanDEMX_Auto6

Figure 61: General outline of the data acquisition plan (image credit: DLR)

TanDEMX_Auto5

Figure 62: The TanDEM-X timeline (image credit: DLR, outdated image)

 


 

TanDEM-X ground segment:

The TanDEM-X and TerraSAR-X missions are closely linked and share resources of the space and ground segments. Consequently, for the overall TanDEM-X system, besides the additional TanDEM-X satellite, the TerraSAR-X ground segment has to be extended. The ground segment consists of three major parts all covered by DLR: 127) 128)

• MOS (Mission Operations Segment) to simultaneously operate two satellites in close formation and to optimally merge the acquisition plans for both missions. The MOS service is provided by the German Space Operation Center (GSOC).

• PGS (Payload Ground Segment) to handle the increased data volume, to include a network of receiving stations, to adapt the processing chain for new data products and to generate the global HRTI-3 DEM. The PGS service is provided by the German Remote Sensing Data Center (DFD) and the Remote Sensing Technology Institute (IMF).

• IOCS (Instrument Operations and Calibration Segment) to operate the two SAR sensors in bistatic mode including synchronization, to calibrate and validate interferometric products. The IOCS service is being provided by the Microwaves and Radar Institute (IHR).

The GFZ (Geoforschungszentrum) Potsdam is responsible for the IGOR data analysis for precise baseline determination.

The scientific exploitation of the TanDEM-X data is coordinated by the DLR Science Service Segment. Commercial customers are served by Infoterra GmbH.

TanDEMX_Auto4

Figure 63: Overview of the TanDEM-X overall system architecture (image credit: DLR)

The ground stations for data reception are: 129)

- Neustrelitz, Germany (DLR station)

- Weilheim, Germany (DLR station)

- GARS (German Antarctic Receiving Station) at O'Higgins in Antarctica with 9 m L/S/X-band antenna

- Inuvik, NWT (Northwest Territories), Canada: DLR station with 13 m L/S/X-band antenna. Inuvik is a CCRS-controlled facility located just outside the town of Inuvik (68º 19' N; 133º 32' W). On August 10, 2010, DLR inaugurated the new Inuvik station for TanDEM-X use. 130) 131) 132) 133) 134)

The Inuvik station is fully automated, from antenna control, through the complete reception chain, to the recording of the encrypted raw data onto tape. These tapes are then sent to DLR for processing. Monitoring of the station operations is performed at the Earth Observation Center in Oberpfaffenhofen, where all the data from the various TanDEM-X ground stations comes together and is assessed for completeness and quality.

- ERIS Chetumal, Mexico (Yucatan Peninsula): DLR transportable ground station with L/S/X-band 9 m antenna (ERIS = Estacion para la Reception de Imagenes Satelitales).

- Kiruna, Sweden, a partner ground station of SSC (Swedish Space Corporation).

TanDEMX_Auto3

Figure 64: Structure of the twin SAR missions on an organizational level (image credit: DLR)

TanDEMX_Auto2

Figure 65: Overview of the German spaceborne SAR development line (image credit: DLR)

SAR products

DEM products

Experimental products from operational modes (co-registered complex images –"CoSSCs")

HRTI-3 specified DEM

Experimental mode raw data (processing with help from DLR contact scientist)

Intermediate DEM: close to HRTI-3 specified DEM

TS-X mission basic products* from selected TanDEM-X raw data sets
* : TS-X basic product performance parameter specification does not apply

FDEMs: DEMs processed to finer pixel spacing and higher random height error

"Byproduct" of operational DEM processing chain: archive of CoSSCs from all acquisitions for DEM generation (multi-temporal global coverage)

HDEMs: HRTI-4 like DEMs (high resolution DEM, were additional acquisitions are needed)

Table 17: TanDEM-X data products

Dual Satellite and Dual Mission Operations Concept: 135)

The basic operations concept is to handle TSX and TDX operations independently as far as possible. TSX and TDX spacecraft are nearly identical from hard- and soft-ware point of view. There is no master/slave configuration between both spacecraft but an equal level operations approach. Figure 66 shows a simplified work flow for TSX and TDX operations in the control center. Each satellite, TSX and TDX, has a dedicated TT&C link from ground to space. There is no telecommand routing from one spacecraft to the other and also no complete telemetry routing.

TanDEMX_Auto1

Figure 66: Schematic, simplified workflow for dual satellite/dual mission operations (image credit: DLR)

Legend to Figure 66: The green box summarizes elements being part of the Control Center. TDX and TSX space craft are each controlled by a satellite specific monitoring & control system. The elements Mission Planning and Flight Dynamics are combined systems for both satellites, generating output to both monitoring & control systems. The Mission Planning System receives external orders from the TerraSAR-X mission and the TanDEM-X mission. Relative orbit control performed by Flight Dynamics is based on the required geometry of the TanDEM-X mission phase.

 

GSCDA (GMES Space Component Data Access)

The GMES Space Component (GSC) includes the Sentinel satellites and the coordinated access to ESA and European EO missions.

In 2007, ESA and the EC (European Commission) signed an agreement to allow ESA to ensure the coordinated and timely supply of satellite-based Earth Observation data for the preoperational phase of GMES from 2008 to 2010.

ESA is managing the GSCDA project in the frame of the FP7 space program as part of the European Space Policy focusing on coordinating the access to space-based observation data to support GMES services.

ESA targets the introduction of the following capabilities to achieve a coordinated access to data from current and future missions:

HMA (Heterogeneous Mission Access). GMES data access implies also a coherent data access to ~40 different EO missions (inside and outside of ESA). Aside from the current and future ESA missions (Envisat, GOCE, SMOS, CryoSat-2, MSG-3, Swarm, ADM/Aeolus, GMES Sentinels, etc.), the European space agencies are also cooperating with their EO missions to make HMA become possible for a global EO community. 136)

QA4EO (Quality Assurance Framework for Earth Observation data). 137)

LTDP (Long Term Data Preservation). 138)

DLR is a cooperative member of the GSCDA initiative. The ground segment of the missions TerraSAR-X (launch June 15, 2007), TanDEM-X (launch planned for Oct. 2009), and EnMAP (launch 2013) are part of GSCDA (implementation of HMA customization). The GSCDA/HMA feature will be made compatible through the DLR DIMS-based (Data and Information Management System) catalog/archive. 139)

TanDEMX_Auto0

Figure 67: German GMES EO data interfaces (image credit: DLR)

Minimize References
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2) G. Krieger, H. Fiedler, I. Hajnsek, M. Eineder, M. Werner, A. Moreira, "TanDEM-X: Mission Concept and Performance Analysis," Proceedings of IGARSS 2005, Seoul, Korea, July 25-29, 2005

3) G. Krieger, A. Moreira, H. Fiedler, I. Hajnsek, M. Eineder, M. Zink, M. Werner, "TanDEM-X: A Satellite Formation for High Resolution SAR Interferometry," FRINGE 2005 Workshop, ESA/ESRIN, Frascati, Italy, Nov. 28-Dec. 2, 2005, URL: http://earth.esa.int/fringe2005/proceedings/papers/382_krieger.pdf

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13) H. Fiedler, G. Krieger,M. Zink, M. Geyer, J. Jäger, "The TanDEM-X Acquisition Timeline and Mission Plan," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany

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15) I. Hajnsek, A. Moreira, "A TanDEM-X : Mission and Science Exploration during the Phase A Study," Proceedings of EUSAR 2006, Dresden, Germany, May 16-18, 2006

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35) "TerraSAR-X's 'twin' satellite, TanDEM-X, certified ready for space," DLR, April 29, 2010, URL: http://www.dlr.de/en/desktopdefault.aspx/tabid-6216/10226_read-24034/10226_page-3/

36) http://www.astrium.eads.net/media/document/astrium_tandem-x_en.pdf

37) J.-S. Ardaens, S. D'Amico, D. Ulrich, D. Fischer, "TanDEM-X Autonomous Formation Flying System," 3rd International Symposium on Formation Flying, Missions (FFMT) and Technologies, ESA/ESTEC, Noordwijk, The Netherlands, April 23-25, 2008, URL: http://www.weblab.dlr.de/rbrt/pdf/FFMT_08b.pdf

38) "TanDEM-X Autonomous Formation Flying Experiment (TAFF),"URL: http://www.weblab.dlr.de/rbrt/GpsNav/TAFF/TAFF.html

39) J. Herman, D. Fischer, D. Schulze, S. Löw, M. Licht, "AOCS for TanDEM-X, Formation flight at 200m separation in low-Earth orbit," Proceedings of the SpaceOps 2010 Conference, Huntsville, ALA, USA, April 25-30, 2010, paper: AIAA 2010-2375

40) J.-S. Ardaens, R. Kahle, D. Schulze, "In-Flight Performance Validation of the TanDEM-X Autonomous Formation Flying System," Proceedings of the 5th International Conference on Spacecraft Formation Flying Missions and Technologies (SFFMT), Munich, Germany, May 29-31, 2013, URL of paper: http://www.sffmt2013.org/PPAbstract/4115p.pdf , URL of presentation: http://www.sffmt2013.org/PPAbstract/4115pr.pdf

41) Daniel Schulze, Jaap Herman, Sebastian Löw, "Formation Flight in Low-Earth-Orbit at 150 m Distance - AOCS In-Orbit Experience," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL: http://elib.dlr.de/76296/1/id1274713-Paper-007.pdf

42) A. Schwab, Ch. Giese, D. Ulrich, "TDX-TSX - On-board autonomy and FDIR of whispering brothers," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL: http://www.spaceops2012.org/proceedings/documents/id1290887-Paper-001.pdf

43) R. Kahle, "TSX/TDX Formation Collision & Illumination Aspects," TD-MOS-TN-4060; GSOC, 2008

44) J. Herman, D. Fischer, D. Schulze, S. Loew, M. Licht, "AOCS for TanDEM-X – Formation Flight at 200 m Separation in low-Earth Orbit"; SpaceOps conference, Huntsville, Al, USA, April 25-30, 2010, URL: http://elib.dlr.de/67867/1/Man204529.pdf

45) Manfred Zink, Alberto Moreira, Markus Bachmann, Paola Rizzoli, Thomas Fritz, Irena Hajnsek, Gerhard Krieger, Birgit Wessel, "The Global TanDEM-X DEM – A Unique Data Set," Proceedings of IGARSS 2017 (IEEE International Geoscience and Remote Sensing Symposium), Fort Worth, Texas, USA, July 23–28, 2017

46) Christopher Wecklich, Carolina Gonzalez, Paola Rizzoli, "TanDEM-X Height Performance and Data Coverage" Proceedings of IGARSS 2017 (IEEE International Geoscience and Remote Sensing Symposium), Fort Worth, Texas, USA, July 23–28, 2017

47) Christopher Wecklich, Michele Martone, Paola Rizzoli, José-Luis Bueso-Bello, Carolina Gonzalez, Gerhard Krieger, "Production of a Global Forest/Non-Forest Map Utilizing TanDEM-X Interferometric SAR Data,",Proceedings of IGARSS 2017 (IEEE International Geoscience and Remote Sensing Symposium), Fort Worth, Texas, USA, July 23–28, 2017

48) "Airbus releases WorldDEM4Ortho, the most accurate elevation model for global orthorectification," Airbus DS, 06 July 2017, URL: http://www.airbus.com/newsroom/press-releases/en/2017/07/airbus-releases-worlddem4ortho--the-most-accurate-elevation-mode.html

49) "Gabon's Towering Mangroves," NASA Earth Observatory, May 19, 2017, URL: https://earthobservatory.nasa.gov/IOTD/view.php?id=90251

50) Bernadette Jung, "How researchers use the latest Earth observation data – Part two," DLR blog, Oct. 19, 2016, URL: http://www.dlr.de/blogs/en/home/tandem-x/How-researchers-use-the-latest-Earth-observation-data-part-two.aspx

51) David Lagomasino, Temilola Fatoyinbo, Seung-Kuk Lee, Emanuelle Feliciano, Carl Trettin, Marc Simard, "A Comparison of Mangrove Canopy Height Using Multiple Independent Measurements from Land, Air, and Space," Remote Sensing, Vol. 8, No, 4, April 2016, doi:10.3390/rs8040327, URL: http://www.mdpi.com/2072-4292/8/4/327/pdf

52) Seung-Kuk Lee, Temilola Fatoyinbo, David Lagomasino, Batuhan Osmanoglu, Marc Simard, Carl Trettin, Imran Ahmed, Mizanur Rahman, "Large-­‐scale Mangrove Canopy Height Map Generation from TanDEM-X by Means of Pol-InSAR Techniques," IGARSS 2015, URL: https://mangrovesciencedotorg.files.wordpress.com/2015/10/igarss2015-mangrovesklfinal4web.pdf

53) Information provided by Stefan Buckreuss of DLR/HR (Microwave and Radar Technology Institute).

54) "The future of radar – scientific benefits and potential of TerraSAR-X and TanDEM-X," DLR, Oct. 17, 2016, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-19779/year-all/151_page-3/#/gallery/24722

55) "New 3D world map – TanDEM-X global elevation model completed," DLR News, October 4, 2016, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-19509/year-all/#/gallery/24516

56) Manfred Zink, Alberto Moreira, Markus Bachmann, Benjamin Bräutigam, Thomas Fritz, Irena Hajnsek, Gerhard Krieger, Birgit Wessel, "TanDEM-X Mission Status: The complete new topography of the Earth," Proceedings of the IEEE IGARSS (International Geoscience and Remote Sensing Symposium) Conference, Beijing, China, July 10-15, 2016

57) Daniela Borla Tridon, Markus Bachmann, Michele Martone, Daniel Schulze, Manfred Zink, "The Future of TanDEM-X: Final DEM and Beyond," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016

58) Birgit Wessel, Markus Breunig, Markus Bachmann, Martin Huber, Michele Martone, Marie Lachaise, Thomas Fritz, Manfred Zink, "Concept and First Example of TanDEM-X High-resolution DEM," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016

59) Christopher Wecklich, Carolina Gonzalez, Benjamin Bräutigam, Paola Rizzoli, "Height Accuracy and Data Coverage Status of the Global TanDEM-X DEM," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016

60) Carolina González, Benjamin Bräutigam, Paola Rizzoli, "Relative Height Accuracy Analysis of TanDEM-X DEM Products," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016

61) Edith Maurer, Ralph Kahle, Falk Mrowka, Andreas Ohndorf, Steffen Zimmermann, "Operational aspects of the TanDEM-X Science Phase," Proceedings of the 14th International Conference on Space Operations (SpaceOps 2016), Daejeon, Korea, May 16-20, 2016, paper: AIAA 2016-2459, URL: http://arc.aiaa.org/doi/pdf/10.2514/6.2016-2459

62) Fotios Stathopoulos, Guillaume Guillermin, Carlos Garcia Acero, Karin Reich, Falk Mrowka, "Evolving the Operations of the TerraSAR-X / TanDEM-X Mission Planning System during the TanDEM-X Science Phase," Proceedings of the 14th International Conference on Space Operations (SpaceOps 2016), Daejeon, Korea, May 16-20, 2016, paper: AIAA 2016-2571, URL: http://arc.aiaa.org/doi/pdf/10.2514/6.2016-2571

63) "Bundeswehr Acquires TanDEM-X Global Elevation Model," Airbus DS Press Release, Nov. 24, 205, URL: http://www.netherlandscorporatenews.com/archive/en/2015/11/24/T013.htm

64) C. Grigorov, D. Schulze, M. Bachmann, B. Bräutigam, M. Zink and TerraSAR-X / TanDEM-X Team, "TerraSAR-X and TanDEM-X Mission Status," CSA(Canadian Space Agency) 10th ASAR (Advanced SAR) Workshop, Oct. 20-22, 2015, Saint-Hubert, Quebec, Canada, URL: http://elib.dlr.de/99324/1/20151020_TSM_TDM_Mission_Status_ASAR_CSA.pdf

65) "WorldDEM™ for Africa," Airbus DS, Sept. 2015, URL: http://www.geo-airbusds.com/en/6751-worlddem-for-africa

66) Manfred Gottwald, "Cosmic scarring in radar view," DLR, June 12, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13876/year-all/#/gallery/19675

67) B. Bräutigam, M. Martone, P. Rizzoli, C. Gonzalez, C. Wecklich, D. Borla Tridon, M. Bachmann, D. Schulze, M. Zink, "Quality assessment for the first part of the TanDEM-X global Digital Elevation Model," Proceedings of ISRSE (36th International Symposium on Remote Sensing of Environment), Berlin, Germany, May 11-15, URL: http://www.int-arch-photogramm-remote-sens-spatial-inf-sci.net/XL-7-W3/1137/2015/isprsarchives-XL-7-W3-1137-2015.pdf

68) WorldDEM™DTM Now Available — Precise Terrain Information Globally for Effective Analysis," Airbus DS ,GEO-Intelligence, April 28, 2015, URL: http://www.geo-airbusds.com/en/6418-worlddem-dtm-now-available

69) "WorldDEMTM: The New Standard of Global Elevation Models," Airbus DS, 2014, URL: http://www2.geo-airbusds.com/files/pmedia/public/r5434_9_geo_022_worlddem_en_low.pdf

70) "WorldDEM™ Supports Identification of Hazard Zones Affected by Potential Future Sea-Level Rise," Airbus DS, 2015, URL: http://www.geo-airbusds.com/en/6377-worlddem-supports-identification-of-hazard-zones-affected-by-potential-future-sea-level-rise

71) G. Riegler, S. D. Hennig, M. Weber, "WORLDDEM – A Novel Global Foundation Layer," The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XL-3/W2, 2015 PIA15+HRIGI15 –Joint ISPRS (International Society for Photogrammetry and Remote Sensing) Conference (Volume II-3/W4), Munich, Germany, 25–27 March 2015, URL: http://www.int-arch-photogramm-remote-sens-spatial-inf-sci.net/XL-3-W2/183/2015/isprsarchives-XL-3-W2-183-2015.pdf

72) Irena Hajnsek, Alberto Moreira, Manuela Braun, "TanDEM-X image of the month - Changes in permafrost landscapes," DLR, April 1, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13210/#/gallery/19021

73) Information provided by Stefan Buckreuss of DLR/MRI (Microwave Radar Institute), Oberpfaffenhofen.

74) "TanDEM-X – Start of the Science Phase of the mission," DLR Press Release, Oct. 10, 2014, URL: http://www.dlr.de/dlr/presse/en/desktopdefault.aspx/tabid-10312/475_read-11787/#/gallery/16691

75) I. Hajnsek, T. E. Busche, G. Krieger, M. Zink, A. Moreira, "TanDEM-X Ground Segment — Announcement of Opportunity: TanDEM-X Science Phase," DLR/MRI, May 19, 2014, URL: https://tandemx-science.dlr.de/pdfs/TD-PD-PL_0032TanDEM-X_Science_Phase.pdf

76) Manfred Zink, "TanDEM-X: Key Features and Mission Status," Proceedings of EUSAR 2014 (10th European Conference on Synthetic Aperture Radar), Berlin, Germany, June 3-5, 2014

77) Daniel Schulze, Markus Bachmann, Benjamin Bräutigam, Daniela Borla-Tridon, Paola Rizzoli, Michele Martone, Manfred Zink, Gerhard Krieger and further members of the TanDEM-X Team, "Status of TanDEM-X DEM Acquisition, Calibration and Performance," Proceedings of EUSAR 2014 (10th European Conference on Synthetic Aperture Radar), Berlin, Germany, June 3-5, 2014

78) "A new map of Earth – in 3D and extremely precise," DLR, May 19, 2014, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-10323/year-all/#gallery/14760

79) "Airbus Defense and Space Sets New Accuracy and Quality Standards for Global Elevation Models with WorldDEM™ Launch," Airbus Defence and Space, April 16, 2014, URL: http://www.astrium-geo.com/en/5734-airbus-defense-and-space-sets-new-accuracy-and-quality-standards-for-global-elevation-models-with-worlddem-launch

80) http://www.astrium-geo.com/en/5402-experience-the-quality-and-accuracy-of-the-worlddem

81) Information provided by Manfred Zink of DLR

82) Thomas Schrage, Juergen Janoth, Alexander Kaptein, Noemie Bernede, Steffen Gantert, Ralf Duering, "TerraSAR-X Next Generation – Mission Overview," Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B1.2.8

83) "WorldDEMTM Reaching New Heights," Infoterra GmbH, 2012, URL: http://www.astrium-geo.com/worlddem/

84) Brian Cutler, Jon Collins, "WorldDEM™ Acquisition and Processing Status," Proceedings of JACIE 2014 (Joint Agency Commercial Imagery Evaluation) Workshop, Louisville, Kentucky, USA, March 26-28, 2014, URL: https://calval.cr.usgs.gov/wordpress/wp-content/uploads/14.024_Cutler_WorldDEMforJACIE2014.pdf

85) "Airbus Defense and Space sets new accuracy and quality standards for global elevation models with WorldDEM launch," Airbus Group, April 16, 2014, URL: http://www.airbus-group.com/airbusgroup/int/en/news/press.20140416_airbus_defence_and_space-worlddem_launch.html

86) "Experience the Quality and Accuracy of the WorldDEMTM," Astrium, URL: http://www.astrium-geo.com/en/5402-experience-the-quality-and-accuracy-of-the-worlddem

87) Daniela Borla Tridon, M. Bachmann, D. Schulze, C. J. Ortega Miguez, M. D. Polimeni, M. Martone, "TanDEM-X: DEM acquisition in the third year era," Proceedings of the 5th International Conference on Spacecraft Formation Flying Missions and Technologies (SFFMT), Munich, Germany, May 29-31, 2013, URL of paper : http://www.sffmt2013.org/PPAbstract/4038p.pdf , URL of presentation: http://www.sffmt2013.org/PPAbstract/4038pr.pdf

88) Martin Wermuth, Rolf König, Yongjin Moon, John Mohan Walter Antony, Oliver Montenbruck, "Two years of TanDEM-X baseline determination," Proceedings of the 5th International Conference on Spacecraft Formation Flying Missions and Technologies (SFFMT), Munich, Germany, May 29-31, 2013, URL of paper: http://www.sffmt2013.org/PPAbstract/4098p.pdf , URL of presentation: http://www.sffmt2013.org/PPAbstract/4098pr.pdf

89) Michael Eineder, Thomas Fritz, Wael Abdel Jaber, Cristian Rossi, Helko Breit, "Decadal Earth Topography Dynamics Measured with TanDEM-X and SRTM," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012

90) José Luis Bueso Bello, Christo Grigorov, Ulrich Steinbrecher, Thomas Kraus, Carolina González, Daniel Schulze, Benjamin Bräutigam, "System Commanding and Performance of TanDEM-X Scientific Modes," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

91) Manfred Zink, "TanDEM-X Mission Status," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012

92) Elisabeth Mittelbach, Manfred Zink, "A step closer to mapping the Earth in 3D," DLR, Jan. 13, 2012, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-2451/year-2012/

93) "A Step Closer to Mapping the Earth in 3D - First TanDEM-X Coverage of the World Completed," Astrium, Feb. 2012, URL: http://www.astrium-geo.com/na/2953-first-tandem-x-coverage-completed

94) M. Zink, Michael Bartusch, Dieter Ulrich, "TanDEM-X Mission Status," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

95) Benjamin Bräutigam, Michele Martone, Paola Rizzoli, Markus Bachmann, Gerhard Krieger, "Interferometric Performance of TanDEM-X Global DEM Acquisitions," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

96) Helko Breit, Marie Lachaise, Ulrich Balss, Cristian Rossi, Thomas Fritz, Andreas Niedermeier, "Bistatic and Interferometric Processing of TanDEM-X Data," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

97) M. Zink, Michael Bartusch, David Miller, "TanDEM-X Mission Status," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

98) Manfred Zink, "TanDEM-X Mission Status & Commissioning Phase Overview," 3rd TanDEM-X Science Team Meeting, Feb. 17, 2011, URL: http://www.dlr.de/Portaldata/32/Resources/dokumente/tdmx/sciencemeeting3/02-Mission_Status_CP_Overview.pdf

99) Agenda of the TanDEM-X Science Meeting along with all presentation, Feb. 17, 2011, Oberpfaffenhofen, Germany, URL: http://www.dlr.de/Portaldata/32/Resources/dokumente/tdmx/sciencemeeting3/Agenda-3rd-TanDEM-X-ScienceTeamMeeting_2011-02-22.pdf

100) Christoph Giese, "The TanDEM-X Space Segment," 3rd TanDEM-X Science Team Meeting, Feb. 17, 2011, URL: http://www.dlr.de/Portaldata/32/Resources/dokumente/tdmx/sciencemeeting3/03-Space_Segment_Astrium.pdf

101) Rolf Scheiber, "Die Dynamik der Eisbewegung," March 1, 2011, URL: http://www.dlr.de/blogs/de/desktopdefault.aspx/tabid-5919/9754_read-357/

102) Gerhard Krieger, Manfred Zink, Alberto Moreira, "TanDEM-X: A Radar Interferometer with two Formation Flying Satellites," Invited paper, Proceedings of the 63rd IAC (International Astronautical Congress), Naples, Italy, Oct. 1-5, 2012, paper: IAC-12-B4.7B.3

103) Ralph Kahle, Benjamin Schlepp, Michael Kirschner, "TerraSAR-X / TanDEM-X Formation Control - First Results from Commissioning and Routine Operations," Proceedings of the 22nd International Symposium on Space Flight Dynamics (ISSFD), Feb. 28 - March 4, 2011, Sao Jose dos Campos, SP, Brazil, URL: http://www.issfd22.inpe.br/S4-Flight.Dynamics.Operations.1-FDOP1/S4_P1_ISSFD22_PF_037.pdf

104) Martin Wermuth, Oliver Montenbruck, Anna Wendleder, "Relative navigation for the TanDEM-X mission and evaluation with DEM calibration results," Proceedings of the 22nd International Symposium on Space Flight Dynamics (ISSFD), Feb. 28 - March 4, 2011, Sao Jose dos Campos, SP, Brazil, URL: http://www.issfd22.inpe.br/S5-Orbit.Dynamics.2-ODY2/S5_P1_ISSFD22_PF_030.pdf

105) "TanDEM-X: ready for routine operations in 2011," DLR, Dec. 15, 2010, URL: http://www.dlr.de/en/desktopdefault.aspx/tabid-6840/86_read-28307/

106) "TanDEM-X: Mapping the world in 3D," Astrium, Dec. 16, 2010, URL: http://www.astrium.eads.net/en/news2/tandem-x-mapping-the-world-in-3d.html

107) Daniel Schulze, Paola Rizzoli, Benjamin Bräutigam, Gerhard Krieger, "In-orbit Performance of TSX-1 and TDX-1," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

108) Marco Schwerdt, Jaime Hueso Gonzalez, Markus Bachmann, Dirk Schrank, Björn Döring, Nuria Tous Ramon, John Mohan Walter Antony, "In-orbit Calibration of the TanDEM-X System," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

109) Marco Schwerdt, Dirk Schrank, Markus Bachmann, Jaime Hueso Gonzalez, Björn J. Döring, Nuria Tous-Ramon, John Walter Antony, "Calibration of the TerraSAR-X and the TanDEM-X Satellite for the TerraSAR-X Mission," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

110) Jaime Hueso González, John Walter Antony, Gerhard Krieger, Marco Schwerdt, "Baseline Calibration in TanDEM-X," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

111) Stefan Buckreuss, "Everybody waltz," October 12, 2010, URL: http://www.dlr.de/blogs/en/desktopdefault.aspx/tabid-5919/9754_read-264/

112) Birgit Schättler, "TanDEM-X ground segment kicks off," DLR, Aug. 2, 2010, URL: http://www.dlr.de/blogs/en/desktopdefault.aspx/tabid-5919/9754_read-218/blogmonth-8/blogyear-2010/

113) http://www.dlr.de/blogs/desktopdefault.aspx/tabid-5919/

114) Ralph Kahle, "Die 20-Kilometer Formation ist eingestellt," DLR, July 20, 2010, URL: http://www.dlr.de/blogs/de/desktopdefault.aspx/tabid-5919/9754_read-210/blogmonth-7/blogyear-2010/

115) Marc Rodriguez-Cassola, Pau Prats, Daniel Schulze, Nuria Tous-Ramon, Ulrich Steinbrecher, Luca Marotti, Matteo Nannini, Marwan Younis, Paco Lopez-Dekker, Manfred Zink, Andreas Reigber, Gerhard Krieger, Alberto Moreira, "First Bistatic Spaceborne SAR Experiments with TanDEM-X," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

116) Marc Rodriguez-Cassola, Pau Prats, Daniel Schulze, Ulrich Steinbrecher, Nuria Tous-Ramon, Marwan Younis, Paco López-Dekker,Manfred Zink, Andreas Reigber, Alberto Moreira, Gerhard Krieger, "Non-nominal Experimental Bistatic SAR Acquisitions with TanDEM-X," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

117) Pau Prats, Rolf Scheiber, Josef Mittermayer, Steffen Wollstadt, Stefan V. Baumgartner, Paco López-Dekker, Daniel Schulze, Ulrich Steinbrecher, Marc Rodríguez-Cassolà, Andreas Reigber, Gerhard Krieger, Manfred Zink, Alberto Moreira, "TanDEM-X Experiments in Pursuit Monostatic Configuration," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

118) Gerhard Krieger, "The first 3D experiment," July 22, 2010, URL: http://www.dlr.de/blogs/en/desktopdefault.aspx/tabid-5919/9754_read-212/blogmonth-7/blogyear-2010/

119) "TanDEM-X sends its first images in record time," DLR, June 25, 2010, URL: http://www.dlr.de/en/desktopdefault.aspx/tabid-6221/10233_read-25278/

120) Space Daily, June 28, 2010, URL: http://www.spacedaily.com/reports/TanDEM_X_Sends_Its_First_Images-In_Record_Time_999.html

121) K. Biller, M. Adolph, H. Dreher, A. Fleckenstein, U. Hackenberg, G. Hoefer, R. Rieger, M. Wahl, R. Zahn, "Design and Performance of the TanDEM-X T/R Modules," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany

122) L. Grunwaldt, "The TOR Payload on TanDEM-X," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany

123) http://telecom.esa.int/telecom/www/object/index.cfm?fobjectid=26368

124) Robert Lange, Frank Heine, Hartmut Kämpfer, Rolf Meyer, "High Data Rate Optical Inter-Satellite Links," 35th ECOC (European Conference on Optical Communication) Sept. 20-24, 2009, Vienna, Austria

125) Berry Smutny, Hartmut Kaempfner, Gerd Muehlnikel, Uwe Sterr, Bernhard Wandernoth, Frank Heine, Ulrich Hildebrand, Daniel Dallmann, Martin Reinhardt, Axel Freier, Robert Lange, Knut Boehmer, Thomas Feldhaus, Juergen Mueller, Andreas Weichert, Peter Greulich, Stefan Seel, Rolf Meyer, Reinhard Czichy, "5.6 Gbps optical intersatellite communication link," Free-Space Laser Communication Technologies XXI,. Edited by Hemmati, Hamid, Proceedings of the SPIE, Volume 7199., pp. 719906-719906-8, 2009

126) Irena Hajnsek, Thomas Busche, Alberto Moreira & TanDEM-X Team, "Mission Status and Data Availability: TanDEM-X," Proceedings of the 4th International POLinSAR 2009 Workshop, Jan. 26-30, 2009, ESA/ESRIN, Frascati, Italy, URL: http://earth.esa.int/workshops/polinsar2009/participants/444/pres_5_hajnsek_444.pdf

127) Information provided by Irina Hajnsek of DLR, Oberpfaffenhofen, Germany

128) B. Schättler, R. Kahle, R. Metzig, U. Steinbrecher, M. Zink, "The Joint TerraSAR-X / TanDEM-X Ground Segment," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

129) Erhard Diedrich, Norbert Bauer, Robert Metzig, Max Schwinger, "Ground Station Network for Payload Data Reception of German TanDEM-X Mission," Proceedings of the SpaceOps 2010 Conference, Huntsville, ALA, USA, April 25-30, 2010, paper: AIAA 2010-1976

130) Mikael Stern, Erhard Diedrich, Jean-Marc Soula, "The Inuvik Station in Canada: An Example on how Space Agencies and Industry Share Risks and Benefits," Proceedings of the SpaceOps 2010 Conference, Huntsville, ALA, USA, April 25-30, 2010, paper: AIAA 2010-1901

131) "Inauguration Of First DLR Ground Station In Canada," Space Daily, Aug. 12, 2010, URL: http://www.spacedaily.com/reports/Inauguration_Of_First_DLR_Ground-Station_In_Canada_999.html

132) "Set-up of DLR-receiving antennas for the TanDEM-X mission in INUVIK (North Canada) has been successfully completed," Aug. 12, 2010, URL: http://www.dlr.de/caf/en/desktopdefault.aspx/tabid-5568/9076_read-19419/

133) Jan Wörner, Inauguration of the DLR ground station in Inuvik" Aug. 12, 2010, URL: http://www.dlr.de/blogs/de/desktopdefault.aspx/tabid-5896/9578_read-229/

134) R. Metzig, E. Diedrich, R. Reissig, M. Schwinger, F. Riffel, H. Henniger, B. Schättler, "The TanDEM-X Ground Station Network," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

135) E. Maurer, S. Zimmermann , F. Mrowka, H. Hofmann, "Dual Satellite Operations in Close Formation Flight," Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012

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139) Gunter Schreier, Jürgen Janoth, "TerraSAR-X, TanDEM-X, EnMAP," GSCB (Ground Segment Coordination Body) Workshop, ESA/ESRIN, Frascati, Italy, June 18-19, 2009, URL: http://www.congrex.nl/08c33/papers/3.4_Schreier.pdf
 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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