PAZ SAR satellite mission of Spain
PAZ ("peace" in Spanish), formerly known as SEOSAR/PAZ (Satélite Español de Observación SAR - SAR Observation Spanish Satellite), is an X-band SAR (Synthetic Aperture Radar) mission of Spain, in fact a flagship mission of the Spanish Space Strategic Plan 2007-2011. The Spanish Earth Observation Program, referred to as PNOTS (Programa Nacional de Observación de la Tierra por Satélite), is based on two complementary satellites, namely:
• PAZ is an X-band SAR (Synthetic Aperture Radar) spacecraft based on the TerraSAR-X platform, to serve the security and the defense needs. The PAZ mission is a dual-use mission (civil/defense) funded and owned by the Ministry of Defense and managed by Hisdesat (Hisdesat Servicios Estratégicos, S.A., a Spanish private communications company providing also its services to the Ministry of Defense, since 2001.
• Ingenio , formerly also referred to as SEOSat/Ingenio, with the optical payload to serve the civilian users (space segment lead by CDTI/ESA). A launch is planned for 2015.
PNOTS is funded and owned by the government of Spain. INTA (Instituto National de Técnica Aeroespacial), the Space Agency of Spain, is managing the common ground segment of the two missions. Hisdesat together with INTA, will be responsible for the in-orbit operation and the commercial operation of both satellites. Airbus Defence and Space Spain (former EADS CASA Espacio) is the prime contractor leading the industrial consortia of both missions. 1) 2) 3)
A major goal of PNOTS is to maximize the common development, services, and to share the infrastructure between both missions (whenever possible). Both missions will also contribute to the GMES (Global Monitoring for Environment and Security) program of Europe. Hence, as third party missions of ESA, they must comply with the EO multi-mission environment scheme - and therefore to support HMA (Heterogeneous Mission Access) services.
The PAZ satellite owner is Hisdesat Servicios Estratégicos, S.A., Madrid, Spain, the PAZ ground segment owner is INTA. The dual use nature (civil/defense) of the PAZ mission imply security constraints within its ground segment. 4)
- CDTI (Centro para el Desarrollo Tecnológico Industrial - Spain's Center for Development of Industrial Technology) is the PNOTS funding agent and is responsible for the programmatic aspects of the program.
- Astrium España (or EADS CASA España) is the prime contractor, responsible for developing and building the satellite within a period of 48 months.
- INTA (Instituto National de Técnica Aeroespacial - or Spanish National Institute for Aerospace Technology) is responsible for the ground segment, which includes two control stations: one in Torrejon near Madrid and the other in Maspalomas, located on the island of Gran Canaria.
- Hisdesat will be the satellite's operator, responsible for all commercial exploitation. The Spanish Ministry of Defense will be one of Hisdesat's main customers, and one of the main beneficiaries of the satellite's capabilities.
Figure 1: Overview of SAR imaging from space: mission timeline (image credit: EADS Astrium) 7)
With a total mass of around 1350 kg, the satellite is 5 m in length and 2.4 m in diameter. The SM (Service Module), platform and instrument back-end, is based on that of TerraSAR-X and TanDEM-X. The satellite bus is manufactured by Airbus Defence and Space Geo-Intelligence/Infoterra GmbH of Friedrichshafen, Germany (formerly EADS Astrium GmbH) and delivered to Airbus DS, Spain (former EADS CASA Espacio) for further integration with the front-end of the PAZ-SAR instrument, including the SAR antenna. The AstroBus platform is designed as a hexagonal shaped body, with side panels at an angle of 60º. A deployable boom is needed to position the X-band downlink antenna, for data transmission. 8)
The S/C bus design features a central hexagonal CFRP structure as the main load carrying element. Three sides of the hexagon are populated with electronics equipment, while the sun-facing side is additionally carrying the solar array. The SAR antenna is mounted on one of the hexagon sides, which in flight attitude points 33.8º off nadir. The other nadir looking side is reserved for the accommodation of an S-band TT&C antenna, a SAR data downlink antenna - carried by a deployable boom of 3.3 m length in order to avoid RF interferences during simultaneous radar imaging and data transmission to ground - and a LRR (Laser Retro Reflector) to support precise orbit determination.
Figure 2: Artist's view of the PAZ spacecraft (image credit: Hisdesat)
Onboard data handling: The newly developed ICDE (Integrated Control and Data System Electronics) system is being used as the central component for all avionics services. The ICDE core consists of two redundant 32 bit processor modules, implementing the ATMEL ERC32SC (Embedded Real‐time computing Core ‐ 32 bit Single Chip) processor, giving it a processing performance of more than 18 MIPS and enough memory capacity to handle full AOCS and data handling software tasks, leaving sufficient margins in performance and memory capacity for future extensions and redundancy concepts. A dedicated, hot-redundant reconfiguration module provides all necessary surveillance, reconfiguration, command and telemetry functions. The ICDE modules are cross-coupled, providing a fully redundant unit.
The ICDE provides the spacecraft and payload interfaces with the following standard link protocols: MIL-1553 bus, HDLC and SpaceWire. An optional GPS receiver module with optional star sensor processing fits seamlessly into the architecture; it is capable of acquiring and independently tracking of up to eight GPS satellites and provides position, velocity and time. The ICDE uses full duplex UART (Universal Asynchronous Receiver/Transmitter) interfaces to all "intelligent" onboard equipment, except for the LCT experiment, where a MIL-STD-1553B bus is being used. The ICDE has a mass budget of 12-18 kg and a power demand of 15-30 W, depending on the configuration selected.
Figure 3: Functional architecture of the PAZ spacecraft platform (image EADS Astrium)
Table 1: Platform main characteristics
Figure 4: Photo of the PAZ spacecraft (image credit: EADS Astrium) 9)
RF communications: A standard S-band TT&C system with 360º coverage in uplink and downlink is used for satellite command reception and telemetry transmission. The uplink path is encrypted. Generated payload (SAR) data are stored onboard in a SSMM (Solid State Mass Memory) unit of 256 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.
The X-band antenna is mounted on a deployable boom 3.3 m in length (the only deployable item on the S/C) to prevent interference with the X-band SAR instrument. This arrangement enables for simultaneous SAR observations and X-band downlink.
Status of PAZ development:
• March 2017: Airbus Defence and Space, as prime contractor, integrated the PAZ satellite at its facilities in Madrid, and was also responsible of the development of the radar instrument, leading a group of more than ten European companies (Ref. 15).
• In October 2014, the launch of the PAZ spacecraft is scheduled for Q1 2015 and the TerraSAR-X/PAZ constellation is planned to be operational in Q4 2015. This constellation program will be the blue print of a first of its kind coordinated constellation concept amongst X-band SAR satellites and is further scalable to add future satellite programs like TerraSAR-X-NG (Next Generation). 10)
• In the summer of 2014, the PAZ Ground Segment validation and verification phase is being conducted (Ref. 31).
• In September 2013, Astrium and Hisdesat are reporting a successful integration of the PAZ satellite. 11)
• In April 2013, the PAZ Ground Segment CDR (Critical Design Review) was completed.
Figure 5: Photo of the Paz satellite (image credit: Hisdesat)
Launch: The launch of the PAZ spacecraft is now targeted for Q4 of 2017 on a SpaceX Falcon-9 Full Thrust vehicle from VAFB, CA. Hisdesat has signed an agreement with SpaceX — after waiting for a launch approval from the Russian government to launch PAZ on a Dnepr vehicle of Kosmotras for more than 2 years. 12) 13)
The Spanish satellite operator Hisdesat is trying to retrieve money it paid Kosmotras for a long-overdue Dnepr launch of a radar satellite that Hisdesat has rebooked on a SpaceX Falcon 9 for a flight late this year. The satellite was originally supposed to launch on a Dnepr rocket in 2014 under an agreement Hisdesat signed with Kosmotras, a Ukrainian-Russian joint venture that has not launched a Dnepr since March 2015 because of difficulties winning Russian government approval for additional launches of the Ukrainian-built rocket. 14)
Hisdesat has since turned to the International Court of Arbitration in Paris to recover money it paid Kosmotras for the repeatedly postponed Dnepr launch. According to the Russian news service TASS, Hisdesat is seeking 15 million euros.
Orbit: Sun-synchronous dawn-dust orbit, altitude = 514 km, inclination = 97.44º, LTAN (Local Time of Ascending Node) at 18:00 hours. The nominal revisit period is 11 days (167 orbits within revisit period, 15 2/11 orbits per day).
Following the satellite's launch during the last quarter of 2017, PAZ will be positioned in the same orbit as the German owned TerraSAR-X and TanDEM-X satellites and form a high-resolution SAR satellite constellation with them. The addition of this third satellite will reduce revisit time and increase acquisition capacity, leading to subsequent benefits to various applications. All three satellites will feature exactly identical ground swaths and acquisition modes. The new setup will be jointly exploited by Hisdesat and Airbus Defence and Space. 15)
Sensor complement: (PAZ-SAR, AIS, ROHPP, LRR)
PAZ-SAR (X-band SAR) instrument:
The PAZ-SAR instrument is designed and developed at Airbus Defence and Space, Spain [former EADS CASA Espacio (ECE)], Madrid (Barajas site), Spain. The instrument's radar antenna will incorporate printed-radiator technology which has already been space qualified (for L-band, C-band, and X-band) implementations at ECE for photo-printed patch antennas (Ref. 2).
The objective of the PAZ-SAR instrument is to provide high quality SAR imagery in a variety of sizes and resolution ranging from medium over wide regions up to very high resolution (e.g. meter and sub-meter). Operational flexibility with multi-mode, multi-polarization and left and right looking attitude is one of the major PAZ System requirements leading to a quite large number of different instrument configurations and antenna beams. 16) 17)
The PAZ-SAR instrument comprises an X-band active phased array antenna with an operation instantaneous bandwidth up to 300 MHz. The SAR antenna (with a size of 4.8 m x 0.7 m) consists of 12 panels in azimuth direction – assembled in three mechanical leaves – each with 32 dual-polarized subarrays. Each individual subarray is driven by a TRM (Transmit-Receive Module) adjustable in amplitude and phase by applying complex excitation coefficients. This enables beam steering and adaptive beam forming in both azimuth and elevation directions. For the variety of standard acquisition modes possible, such as Stripmap, ScanSAR or Spotlight, and some experimental modes more than ten thousand beams can be programmed and commanded. This multitude of beams is one highlight but also a great challenge for the whole mission and it represents the main driver for the need of an accurate antenna model. 18) 19) 20)
Compared to conventional slotted waveguide radiators, the instrument offers attractive features such as low profile, low-mass, flexibility of BFN (Beam Forming Network) topologies (possibility of using series or parallel feeding), easiness of manufacturing (capability of maintain a series production) while providing good radiation efficiency and low losses. The beam forming network is full corporate and it is implemented in low-losses stripline technology. Tests on first subarray breadboards are presented, showing ohmic losses around 0.5 dB, good polarization purity (cross-polar better than -25 dB) and good bandwidth performances regarding radiation patterns and sidelobes.
Each panel consists of 32 dual-polarized stripline technology subarrays, arranged in elevation, each with a dedicated TRM (Transmit/Receive Module). Each panel has a PDN (Panel Distribution Network) for splitting/combining radar (Tx/Rx) signals, a PCN (Panel Calibration Network) for splitting/combining calibration signals, two PSUs (Panel Supply Units) to power the TRMs, and one PCU (Panel Control Unit) to control the TRMs. 21)
A subarray is a 16-element array of size 400 mm x 22 m. The antenna thickness is 9 mm (excluding connectors) and the estimated mass is 80 gram.
Figure 6: Top view of an X-SAR panel (left ) and panel rear view (right), image credit: ESA CASA Espacio
Array radiator: The array radiator is a circular patch excited by electromagnetic coupling through two orthogonal stripline probes, coupled to a ring aperture and providing both horizontal and vertical polarizations. Both stripline probes and radiators are photo-printed in a low-loss Teflon-based substrate. The aperture coupled patch radiators offer good electrical performances in terms of input matching (better than -18 dB in the full 300 MHz bandwidth) isolation between ports (better than -22 dB), low efficiency losses and high directivity (since the substrate has a very low dielectric constant), and good polarization purity (cross-polar better than -20 dB that is further improved at the array level).
BFN (Beam Forming Network): The BFN provides uniform amplitude and phase to the elements in the array. It is a fully corporate network to cover the entire bandwidth (3% at 9.65 GHz) and so it prevents the decrease of directivity, null filling and increase of the sidelobes level that occur in a series feeding at the band edges. Parallel feeding is more insensitive to any kind of variation such as thermal effects, tolerances and manufacturing errors.
Due to available space, BFN is divided into three levels as shown in Figure 7.
• Probes layer: It includes horizontal and vertical probes for each radiator and the first two-way divider stage for the array. In the case of the vertical polarization, it is a two-way equal-phase reactive divider while for the horizontal polarization to two-way divider includes a 180º transmission line to implement the sequential rotation.
Figure 7: Schematic layout of the subarray layers ( image credit: ESA CASA Espacio)
• BFN 1 to 8 is being used for horizontal polarization. It is a reactive divider based on λ/4 transformers feeding the eight pairs of radiators.
• BFN 1 to 8 is also being used for vertical polarization. It features the same design as in the horizontal polarization.
The probes layer is joined to the BFN 1 to 8 through vertical transitions based in small coaxial sections and properly tuned to the working frequency. The subarray output uses a male SMA (SubMiniature type A) connector.
First subarray breadboards show adequacy of the X-band design featuring low losses and low efficiency losses, while adhering to the attractive performances of this technology in terms of low profile and low-mass design.
Figure 8: Photo of a subarray breadboard in the anechoic chamber at EADS (image credit: ESA CASA Espacio)
Subarray test results (embedded performances): The subarrays have also been tested in an embedded configuration in order to characterize the radiation pattern performances in their actual panel configuration. The measured subarray is surrounded by other 8 subarrays in order to simulate the most relevant couplings in the panel (composed by 32 subarrays).
Figure 9: Subarray qualification model at EADS CASA Espacio anechoic chamber. Embedded radiation pattern tests (EADS CASA Espacio)
Figure 10: Illustration of the X-SAR antenna front end (top and rear view), image credit: ESA CASA Espacio (Ref. 18)
The X-SAR system design offers the following support modes of operation:
- Spotlight: Imagery of size 10 km x 5 km @ 1m resolution, or 10 km x 10 km at 2 m resolution
- ScanSAR: Imagery on a 100 km swath @ 15 m resolution
- Stripmap: Imagery on a 30 km swath @ 3 m resolution (single polarization), or 15 km swath @ 6 m (dual polarization).
The mission goal is to provide more than 200 SAR images per day.
Figure 11: Basic operational modes of PAZ-SAR (image credit: INTA)
Figure 12: Functional block diagram of the PAZ-SAR instrument (image credit: ECE)
Table 2: PAZ-SAR instrument parameters
Table 3: Summary of PAZ-SAR instrument performance of all sensing modes
PAZ-SAR instrument calibration:
The requirements call for a minimum end-to-end calibration time to impact on the operation time of the instrument (to cope with system degradation, SAR performance, or with need for long re-calibration periods). After the lessons learnt during the TerraSAR-X (TSX) commissioning, an effective calibration philosophy emerges which basically shifts the calibration effort from space to the ground in order to assure a commissioning phase of less than six months (Ref. 16).
The PAZ-SAR instrument architecture is similar to the TSX-SAR instrument of TerraSAR-X; hence, a similar instrument calibration strategy has been established, including internal and external calibration methods, as well as phase-coded individual TRM measurements. However, even though this is a consolidated strategy, the new front-end design and the high degree of flexibility in the instrument calibration and operation requires a careful evaluation of the error sources, correction methods and the impacts in the overall instrument radiometric budget (Ref. 8). 22) 23)
Figure 13: Main error contributions to the PAZ-SAR radiometric budget (image credit: ECE)
The PAZ-SAR operation and calibration approach is built around an antenna model called AMOR (Antenna MOdelleR). This tool is used to accurately determine the antenna beam patterns combining mathematical models with highly precise on-ground characterization data, post-launch external verification measurements and periodic in-flight TRM calibration (Ref. 16).
AMOR is developed by the UPC (Universitat Politècnica de Catalunya), Barcelona, under EADS Casa Espacio contract. AMOR permits the reconstruction of the antenna radiation patterns by superposition of measured embedded subarray patterns weighted by beam excitation coefficients, considering the antenna geometry and elements referencing. Another important function to be implemented into the SW tool is the beam excitation coefficients optimization, in order to maximize the performance w.r.t the NESZ (Noise Equivalent Sigma Zero) and ambiguity ratio parameters; an eventually re-computation may be required in response to TRMs failure or degradation. In addition, the tool includes the analysis of the effects due to eventual antenna subarray misalignment and planetary errors, favoring a better fit to the desired patterns. The frequency dependence as well as thermo-elastic deformations can be assessed.
The key element of this new calibration concept is a mathematical antenna model (AMOR) based upon accurate on-ground measurements of the instrument, a set of post-launch external measurements to be performed during the initial commissioning period, periodic in-flight internal characterization, and the internal calibration data to be performed during and together with the sensing data. This tool plays an important role in the performance prediction of the entire instrument and allows for dynamic re-calibration during operational lifetime, so its required accuracy has to be carefully managed and verified.
Figure 14: AMOR validation/verification flow chart (image credit: ECE, UPC)
AIS (Automatic Identification System)
The AIS payload on board PAZ is part of the exactEarth constellation (exactEarth Ltd. is based in Cambridge, Ontario, Canada). The overall objective is to provide a SAR data fusion capability for maritime surveillance from space with AIS data services. Hisdesat is a 27% shareholder of exactEarth in Canada in joint venture with COM DEV. 24)
• Commanding is done through the PAZ main ground station at Torrejón (Madrid). Nominal downloading of AIS information is in Svalbard (located near Longyearbyen on the island of Spitsbergen, Norway). Other possibilities are in Guildford and Cork (UK).
• AIS antenna footprint has narrower size due to lower satellite orbit altitude.
• Dual polarization receiver. The entire signal spectrum is downloaded for de-collision of the AIS messages.
There are complementaries between AIS and SAR data:
• Although it was born as an anti-collision system, S-AIS (Satellite AIS, also written as "SAIS") complements other systems such as VMS (Vessel Monitoring System) and LRIT (Long Range Identification and Tracking) as a generic source of vessel positioning data.
• Still, in order to achieve the most accurate maritime situational picture, an independent observation is required in order to verify the vessels that are present in the area. The SAR can provide such source of data due to its all-weather, day/night capability.
• The use of SAR images for maritime reconnaissance requires of the balancing between image coverage and resolution in order to set the optimum degree between detection rate and level of information to be extracted from the image.
• Intelligence applications normally require very high resolution data while environmental applications (e.g. fisheries, pollution, etc) concentrate in larger areas.
• However, unlike optical images, SAR detected vessels are more difficult to identify.
There are some challenges in fusing AIS and SAR data.
• In order to use VDS (Vessel Detection System ) given at an instant and S-AIS information, interpolation and matching of the data has to be performed.
• The problem is that AIS/S-AIS data is not always available at the moment of the acquisition therefore interpolation is required.
• The interpolation of data has several problems:
- Navigational routes do not always follow straight lines (e.g. Central Atlantic) in all areas of the globe (e.g. Red Sea).
- Navigational information is not always accurate (e.g. speed over ground, etc) therefore the most accurate speed parameters have to be estimated from the vessel positions rather than from the AIS messages.
- Time interval between image acquisition time and AIS targets becomes critical and information easily de-correlates beyond ΔT = 5-10 minutes (depending on the area) and type of ship (e.g. a cargo ship normally moves in straight line therefore is more predictive while a fishing vessel is more erratic).
• Vessel speed can not be measured from the image unless the wake is visible. Therefore AIS speed and heading has to be taken into account to correct positions prior to SAR-SAIS target matching. Recently, other methodologies to estimate the speed from the image itself based on sub-looks images analysis have been developed but results are not yet fully verified. The problem is that with current systems, SAIS data is interpolated therefore it is not sure that this effect can be accurately corrected.
• Size properties are difficult to measure as it has been seen from defocusing. However an estimation has to be measured in order to use the size as a input value for the cost function in the VDS-SAIS correlator.
• Dense traffic areas are a problem for SAIS due to IS message collisions coming from different SODTMA (Self Organizing Time Division Multiple Access) cells. Therefore the quality of the SAIS is a key to avoid mismatching of erroneous SAIS and VDS contacts; de-collision methodology improves the detection of messages and seems more convenient in order to avoid errors and improve id of VDS targets.
• For the first time a satellite will integrate a SAR and an AIS receiver onboard.
• PAZ SAIS data will be integrated into exactEarth and the processing chain is independent from PAZ processing. SAIS delivery is done via web services data access.
• The synergistic use of both payloads shall overcome the current limitations of SAR-SAIS correlation. In particular, the time synchronization between both data sources which is critical for the successful matching of VDS-SAIS contacts.
• The information contained in SAIS messages (speed and heading) will need to be accounted (correct position of the targets) for in order to achieve a correct matching between SAIS and VDS targets therefore quality SAIS data is required. To this aim, processing seems a ‘must' specially in areas of high density traffic. ExactEarth is the only company that possesses such technology.
• The availability of a polar station or a DAS (Direct Access Station) can enhance the systems to a Near Real Time (NRT-30 minutes) delivery capability on areas of interest (e.g. Europe).
• For some users (DAS) in addition to baseline SAR processing capability, VDS and SAIS correlator processor can be incorporated to the facility to reduce latency periods (time between image acquisition and VDS-SAIS fused product delivery to end users).
• The analysis of SAIS anomalies seems promising to direct the datatakes of SAR sensors, in particular for PAZ where tracking of vessels shall be more straight forward due to the SAIS payload onboard.
ROHPP (Radio Occultations and Heavy Precipitation with PAZ)
The global atmospheric community (weather and climate) is seeking EO missions which can support RO (Radio Occultation) observations to continue the global service which has been provided over the past years. However, the missions that are currently flying RO instruments, are close to the end of their mission life or already beyond their mission life. In order to avoid an observation gap in the near future, the PAZ project was approached by the international community to include RO observations in their mission.
In 2009, the Spanish Ministry for Science and Innovation (MICINN) approved the RO proposals, aimed to include a polarimetric Global Navigation Satellite System (GNSS) Radio-Occultation (RO) payload on board of the Spanish Earth Observation satellite PAZ. The PAZ mission was initially designed to carry a Synthetic Radar Aperture (SAR) as primary and sole payload, and included an IGOR+ advanced GPS (Global Positioning System) receiver and corresponding antennas for precise orbit determination. The design and software of this GPS receiver allows the tracking of occulting signals, that is: signals transmitted by GPS satellites setting below the horizon of the Earth (or rising above it). 25)
RO profiles are assimilated operationally into several global NWPMs (Numerical Weather Prediction Models). The latest results at NCEP (National Centers for Environmental Prediction) show that ROs improve anomaly correlation scores by ~8 hr starting at day 4 and increases with extended forecast range. It also helps reducing model biases. ECMWF (European Centre for Medium-Range Weather Forecasts) compared the impact of 24 operational observation systems, GPS-RO impact resulting among the top-five. The use of GPS RO has been shown to significantly improve models forecast skill, and is a key component of the operational observing system.
• FormoSat-3 / COSMIC (Taiwan/USA constellation of 6 spacecraft) with a launch on April 15, 2006. Instrument: IGOR (Integrated GPS Occultation Receiver).
• MetOp mission of EUMETSAT with a launch on Oct. 19, 2006. Instrument: GRAS (GNSS Receiver for Atmospheric Sounding).
• GRACE (US-German mission) with a launch of March 17, 2002. Instrument: BlackJack (GPS Flight Receiver).
The above missions provide about 2500 daily RO profiles evenly distributed all over the Globe. They are most useful for climate studies (long time series of RO data required) and for weather predictions (short time series of RO data required). In particular, a post FormoSat-3 / COSMIC observation gap may occur until the follow-up constellation (12 spacecraft) of FormoSat-7 / COSMIC-2 is operational (2 launches are planned for 2015 and 2017).
The ROHPP instrument makes use of the original IGOR+ receiver on PAZ. Minor modifications on IGOR+, together with deploying the dedicated antennas, are the basic steps required to achieve the desired capabilities. Moreover, polarimetric RO will be added, becoming a proof-of-concept experiment for the detection of heavy precipitation. This will be the first time that polarimetric GPS signals are captured from space, and the first attempt to monitor precipitation by means of RO techniques.
The PI of the ROHPP instrument is Estel Cardellach of ICE (Institut de Ciencies de L'Espai), Barcelona, Spain. The funding institution is MICINN. The ROHPP collaborative institutions are: NOAA and UCAR in the ground segment.
ROHPP experiment (Ref. 26):
• 1 single RO antenna (setting observations) with two linear polarizations
• The two linear polarization can be recombined in post-processing to get both Right-Hand and Left- Hand Circular Polarization components (RHCP and LHCP, respectively)
• RHCP: standard RO measurements
• LHCP: This represents the polarimetric experiment aiming to capture [heavy] precipitation.
• Heavy precipitation increases drops size, resulting in a flattened shape
• Signal crossing drops along the flattening axis suffers depolarization
• RO cross lowermost atmosphere tangentially (aligned with the most common flattening axis)
Figure 15: Schematic view of the ROHPP measurement concept( image credit: ICE, IEEC)
Legend to Figure 15: In the low troposphere, the RHCP GPS signal propagates tangentially, aligned with the plane of flatness of the rain drops, situation in which the de-polarization effect is maximal. The receiver on board the PAZ mission will collect both RHCP (Right-Hand Circular Polarization) and LHCP (Left-Hand Circular Polarization) components.
• The IGOR+ receiver is modified (as in other RO missions). The modifications are made by BRE (Broad Reach Engineering); IGOR is a product of Broad Reach Engineering.
- Azimuth range: ± 55-65º
- Elevation range: ± 3-7º
- Dual Polarization: V/H (Vertical/Horizontal)
- Dual Frequency: GPS L1/L2
- Gain 10-12 dB each port (V/H) ⇒ 13-15 dB circular.
Figure 16: IGOR receiver history (image credit: BRE) 28)
LRR (Laser Retro Reflector)
The PAZ spacecraft features LRR for an independent orbit determination of LEO (Low Earth Orbiting) satellites. LRR is developed at GFZ Potsdam and is of CHAMP, GRACE, TerraSAR-X,TanDEM-X , KOMPSAT-5, and Swarm constellation heritage.LRR has four corner cube prisms mounted in a compact frame. 29)
Table 4: Main parameters of the GFZ LRR for LEO satellites
Figure 17: Illustration of the LRR device (image credit: GFZ Potsdam)
PGS (PAZ Ground Segment):
A ground segment (within the framework of PNOTS) for both missions, PAZ and Ingenio) is allocated to INTA (Instituto National de Técnica Aeroespacial) in Torrejón de Ardoz near Madrid (primary or nominal system) and in Maspalomas, located on the Canary Islands (backup system). INTA and Hisdesat will be the operators of the system and are involved in the ground segment development (Ref. 1). 30) 31)
PAZ is a dual use (civil/defense) mission - providing also its data to the GMES (Global Monitoring for Environment and Security) program of the EU. The defense implementation requires security constraints.
Figure 18: PNOTS - PAZ and Ingenio organization scheme (image credit: INTA)
The PGS (PAZ Ground Segment) is a multi-mission ground segment featuring a HMA (Heterogeneous Mission Access) implementation as used in the GMES initiative.
The PGS is providing the following functions: 32)
• FOS (Flight Operations Segment): The main functions of FOS are the operation and control of the satellite and the provision of telemetry. It is divided in a FOCC (Flight Operations Control Center) and the following subsystems:
- G/S (Ground Station) operations, which includes the S- and X-band functions for signal modulation/demodulation.
- ARF (Acquisition & Routing Facility), in charge of the second level of synchronization, generation of ISP files and routing to the Nominal PDGS or the Defence one.
- SMC (Secure Management Center. CFI) in charge of TC authentication/ciphering and key management.
- FMC (FOS Monitoring and Control), in charge of segment monitoring and G/S interfacing.
The main functionality of the FOS is included in the FOCC , which performs, between others, the tasks of planning and satellite commanding. The main subsystems involved here are:
- MPF (Mission Planning Facility), in charge of planning the satellite activities for the following days, including platform and instrument ones.
- IO (Instrument Operations), in charge of generating the commands needed to perform a data take acquisition. This subsystems works on a day-to-day basis leaving the task of planning the other instrument activities needed to allow the correct data acquisition to the MPF.
- FD (Flight Dynamics), in charge of orbit control and the generation of the required orbit and attitude products.
- MCS (Mission Control System), in charge of convert the schedule generated by MPF to TC and the uplink to the satellite. Also it receives the S-band TM and generates the HK reports.
- SSIM (Spacecraft SIMulator, CFI), to simulate the satellite. This simulator doesn't support instrument TCs.
• PDGS (Payload Data Ground Segment): The PDGS is in charge of managing the user requests, data archiving, processing and calibration/validation. To perform these tasks, PDGS consists of the following systems:
- MUS (Master User Services). This subsystem is the only that interacts with the user, receiving the user requests and providing to it the obtained products. It also generates the internal orders required to start the PGS flow.
- PDS (Payload Data System). This subsystem is the core of the PDGS and it is in charge of deciphering and processing the raw data in order to obtain the desired products and also circulating all data flow inside the segment.
- CALVAL Center (Calibration and Validation). The tasks performed in this subsystem are the maintenance of the SAR system configuration (satellite and PGS), the calibration, characterization and validation of SAR products and the management of the scientific exploitation.
- MAC (Mission Archive & Catalog), in charge of the storage and catalog of processed data and all the necessary information required in the PDGS.
- PM (Performance Monitoring), in charge of the monitoring of all system performance.
• PGS Operational Concept: PGS operations are performed in a data driven model, conducted by user request reception. User requests are introduced in PGS via the MUS (Master User Services) subsystem. Three different types of request are supported by PGS:
- Dissemination Request; with this operation the users are able to get L1B products currently present in PAZ archive.
- Processing Request; in this case the users consult the PAZ catalog to know past acquisitions that match their interests (specific area of interest, acquisition time period, imaging mode, etc), performing a request of the product in a Level1b different to the stored in the PAZ archive.
- Acquisition Request; intended to obtain a future raw data acquisition. Users can select acquisition and processing parameters such as: area of interest, time acquisition window, imaging mode, beam, polarization and L1B processing level.
All the requests have to pass an approval stage to be ingested in the PGS chain. After this operation the working flow for the requests depends on their type. Dissemination requests are directly sent from MUS to the MAC. MAC extracts from PAZ archive the desired product that is transferred to MUS to be delivered to the user.
Processing requests are managed by PDS, that extracts from MAC the L0 product to be processed up to desired L1B level and it solves the dependencies related to the auxiliary data needed to perform the processing. Once all the dependencies are solved, PSP performs the processing and the resulting product is temporally archived into the MAC and delivered to the user via MUS.
Acquisition requests are managed in a first stage by MUS. Once the user has inserted in the subsystem his request, MUS performs a feasibility analysis giving to the operator a list of potential acquisitions matching his search criteria. The user will select from the list the acquisition or set of acquisitions to be performed, generating the final User Request. Feasibility analysis takes into account satellite maneuvers that prevent acquisitions as well as possible conflict problems due to another user request. Nevertheless, although the possible conflicts between requests are showed to the user, the acquisition can be requested anyway.
Taking into account all the final acquisition requests introduced in the system, MUS generates in a daily basis the DTOL (Data Take Order List), which contains all the possible data takes that can be acquired in the next programming horizon of the mission (at present set to 72 hours).
Civilian and Defence DTOL are sent to the MPF, which is in charge of generating the final plan. This plan is performed taking into account all the acquisitions requests as well as the satellite maneuvers, ground station unavailability and visibilities and satellite constrains. MPF has to solve the conflicts that can arise between users requests coming from nominal and defence centers, performing an algorithm which uses the different user roles and priorities.
• PAZ Basic Products Portfolio: The PAZ system can operate in four imaging modes:
1) Stripmap, with single or dual polarization. Azimuth resolution 3.3 m (single pol.), 6.6 m (dual pol.); L1B ground range x product length 30 x 50 km (single pol.); 15 x 50 km (dual pol.).
2) ScanSAR, with single polarization. Azimuth resolution 18.5 m ; L1B ground range x product length 100 x 150 km.
3) Spotlight, with single or dual polarization. Azimuth resolution 1.7 m (single pol.) | 3.4 m. (dual pol.); L1B ground range x product length 10 x 10 km.
4) High Resolution Spotlight, with single or dual polarization. Azimuth resolution 1.1 m (single pol.); 2.2 m. (dual pol.); L1B ground range x product length 10 x 5 km.
The operational image products will have a level L1B of processing, with four variants:
- SSC (Single Look Slant Range Complex); available for Stripmap, ScanSAR and High Resolution Spotlight.
- MGD (Multi Look Ground Range Detected); available for all imaging modes.
- GEC (Geocoded Ellipsoid Corrected); available for all imaging modes.
- EEC (Enhanced Ellipsoid Corrected); available for all imaging modes.
The PAZ SAR Processor is a DLR development integrated in the Ground Segment. The format of L1B products keeps the structure of TSX/TDX ones, allowing compatibility between applications made for products of these missions.
A backup capability is provided in Maspalomas (Canary Islands) with an S/X-band station, FOCC, and a temporal archive.
The PGS advances in coordination with the IGS (Ingenio Ground Segment), also supporting the HMA (Heterogeneous Mission Access) interface. HMA is a technique which is being implemented for the ground segment of the GMES program in Europe to accomplish coherent access to archives to support scientific exploitation like the Climate Change Initiative. HMA is being implemented by ESA, DLR, CNES, EUMETSAT, MDA (RADARSAT), INTA, etc.
Figure 19: Overview of the PAZ ground system and its functions (image credit: INTA) 33)
Figure 20: Ground segment data flow of the PAZ mission (image credit: INTA)
Figure 21: Overview of the PGS elements (image credit: INTA)
Figure 22: Overview of PNOTS users (image credit: INTA)
TerraSAR-X and PAZ Constellation:
In April 2012, Astrium GEO-Information's German unit, Infoterra GmbH, and Hisdesat, the Spanish government satellite service operator, have signed a framework agreement for a joint technology development project that aims to establish a constellation approach with the TerraSAR-X/TanDEM-X and PAZ radar satellites.
The agreement between Airbus DS and Hisdesat defines the joint development and coordination of their space, ground and service segments. The two satellite operators will establish interfaces enabling them to have a system overview of both satellites' tasking plans and establish simplified data ordering and delivery procedures. The owner companies of the respective systems will retain complete control of their satellites, while being able to better coordinate acquisition planning and satellite tasking. It is the first international collaboration to bring together two national X-band radar remote sensing satellites in an effort to improve revisit and acquisition capabilities, enhance service levels and application opportunities for both public and commercial customers. 37) 38) 39) 40) 41) 42) 43)
The cooperation will include the development of an aligned tasking approach for the two missions, harmonized ground segment operation, an integrated processor for both satellites as well as complementary order and delivery interfaces. The agreement also envisions joint marketing of products and services generated by the constellation approach.
Operating the two virtually identical satellites will afford Astrium and Hisdesat more efficient and flexible management of system capacity. Both companies will be able to offer their customers and partners enhanced performance and services levels. The key advantages of the TerraSAR-X / TanDEM-X / PAZ constellation will be:
• Shorter revisit times
• Increased data acquisition capacities
• Improved service reliability.
A wide range of time-critical and data-intensive applications will benefit from the constellation, including defense and security, surface movement monitoring, maritime surveillance, disaster management. Shorter revisit times and increased capacity will provide improved levels of responsiveness and acquisition opportunities worldwide. The constellation will offer improved InSAR capabilities for precise monitoring and detection of faster surface-movement activities. The constellation approach will also provide improved system redundancy and back-up for both satellites during maintenance phases. 44)
Figure 23 highlights the various options for positioning the PAZ satellite in the TerraSAR-X orbital plane. The satellites will fly in the same orbit tube (i.e. with a diameter of 500 m) and shall keep the same ground track, in order to limit the interferometric baseline and ensures a regular update of data stacks (Ref. 10).
Figure 23: PAZ positioning options in TerraSAR-X orbital plane (image credit: Airbus DS, Hisdesat)
As shown in Figure 23, the TerraSAR-X / TanDEM-X satellites are assumed to be located at position 0 in the orbital plane. According to the 11 day TerraSAR-X repeat cycle and with the objectives of achieving an interferometric revisit with the PAZ satellite there is an option of selecting 10 slots that allow having the same ground tracks with the same orbital plane. Five out of the ten orbital slots, separated by 32.73º, are symmetric (offering same interferometric revisit) in nature, so for practical purposes the possible candidate for the PAZ satellite are shortlisted from position 1 to position 5.
In the available 5 positions, the PAZ satellite has been decided to be placed in position 3 due to the following reasons:
• Optimized interferometric revisit 4 and 7 days thus reducing the current TerraSAR-X interferometric revisit into half.
• Position 3 means an approximate 30 minute orbit separation between TerraSAR-X and PAZ; this arrangement makes it possible for direct access customers (even having single receiving antennas) to access both the satellites in one orbit pass.
• Daily global revisit is achieved by the constellation (left and right looking mode) for mapping and monitoring applications (Figure 24).
• Also, the positioning of PAZ in front of the TerraSAR-X / TanDEM-X formation flight gives Airbus DS and DLR the flexibility to experiment different separations for the pursued formation flight.
Figure 24: Average global data access capability. (green line: TSX + PAZ in right and left looking mode; red line: TSX in right and left looking mode; purple line: TSX and PAZ in right looking mode and blue line: TSX in right looking mode), image credit: Airbus DS, Hisdesat
Figure 25: High level architecture of the TerraSAR-X / PAZ constellation mission (image credit: Airbus DS, Hisdesat, Ref. 10)
Constellation CONOPS (Concept of Operations):
For commercial constellation operations, a detailed CONOPS has been agreed by Airbus DS and Hisdesat. In this section we highlight the key features of the planned CONOPS:
• The TerraSAR-X and PAZ satellites, although commanded from their respective ground segments in Torrejón and Neustrelitz, would share the same commercial order submission deadline (2 times a day).
• A harmonized ordering and priority access scheme for both the missions has been devised and agreed to for the constellation.
• Both the missions will have the same customer service procedures, policies and working times. The customer service teams in both companies will work very closely together, with a joint acquisition plan encompassing both the missions to meet the customer demands in the fastest possible manner.
• Both TerraSAR-X and PAZ imagery products will have harmonized product structure and delivery formats and quality standards.
• Both companies have also agreed to market the products at the same commercial price list and present one face to the customer through both the commercial ordering systems.
• Depending upon special customer and application needs, the customers would have the option to choose one mission over the other.
• Customers requiring near real time access will have the option of owning a joint direct access capability or can also choose to use one of the polar stations like Svalbard for quick data access from both missions.
• A copy of the TerraSAR-X and PAZ mission data accessed by customers via the multimission direct access capability would be repatriated to the respective ground segments (due to mission data ownership regulations).
• Harmonized ordering and data secrecy provisions for sensitive customers accessing the missions.
As the market for spaceborne products and services grows, efficient business practices are spreading through the industry. Commercial companies look for further opportunities to improve efficiency, and reach beyond the frame of the so far mostly national PPPs (Public Private Partnerships). International collaborations emerge and bring several advantages (Ref. 37):
- Government to Government (G2G) agreements, complex and difficult to put in place, are no longer necessary to foster new programs. However, public national players do keep a large influence in the overall construct thanks to their respective PPP.
- Programmatic objectives of public and commercial partners, often compatible, can be simultaneously met, allowing efficient risk management, and capacity sharing.
- Improved system performances (e.g. revisit, information freshness, lower latency) provide significant value add to end-users.
- As the commercial market for data and services is limited, only a small number of satellites can capture sufficient revenues to be profitable. Hence, partners cooperating in a constellation present more robust business cases and increased return on investment.
As products and services derived from space programs increasingly gain in popularity and become crucial elements of the world economy, competition increases between the commercial players. The requirements of the end-customers must be respected and anticipated, and have a direct impact on the design of new missions. Continuous innovation is thus a key differentiating factor.
In the context of SAR Earth observation programs, four main topics drive customer needs: data freshness, update frequency, high resolution and high reliability. The first two criteria are directly related to the number of satellites available in orbit. Operating -or having access to- a constellation is thus a necessity for service providers to stay competitive and meet customers' demands.
Hisdesat and Airbus DS Geoinformation Services have in this context convinced their partners to leverage the synergies of the PAZ and TerraSAR-X (TSX) programs to create an innovative X-band high resolution SAR constellation. The companies have focused their investment on establishing the key features to bring to end-users all the benefits of a true constellation.
- Identical orbits and phasing optimized for regular revisit
- Identical imaging modes, and performance
- One multi-mission interface to check available capacity of both satellites, in a transparent fashion
- Optimized capacity booking through coordination of customer service teams
- DAS (Direct Access Station) service with reception and processing of both satellites.
Roles and responsibilities in the constellation: The creation of a constellation between PAZ and TerraSAR cannot impact the sovereignty of the national missions, who continue to operate independently. The commercial partners Hisdesat and Airbus DS have thus concentrated their efforts on formalizing the key elements necessary to enable constellation operations and commercialization of the related products and services.
Constellation operations: The two main requirements structuring the technical concept are:
- the necessity for both GS (Ground Segments) to maintain sovereignty on the operations of their respective satellite, and
- the necessity for the commercial partners to provide "one face to the customer."
Efficient coordination is thus required between the GS, and between the commercial entities, to transfer relevant ordering, tasking, downloading or delivery information.
Ordering and delivery: A scheme describing the main blocks of the systems, and their interactions is shown in Figure 26. Airbus DS and Hisdesat, the commercial entities, manage the direct interactions with the customers. Ordering and delivery are done via either of the partner's CS (Customer Service) team or directly by the DA (Direct Access) customer via the DAS (Direct Access Station). The MMT (Multi-Mission Planning Tool) suggests an acquisition planning taking into account both PAZ and TerraSAR, based on anticipated satellites orbit positions. Both Service Segments then check with their respective GS the feasibility of the part of the acquisition planning attributed to their satellite. Once feasibility is confirmed, the orders are passed to each GS, which in turn task their satellite accordingly. Communications between the Service Segments is ensured by electronic interfaces,such as HMA (Heterogeneous Mission Access). In the standard customer case, data reception and processing is done at the respective GS.
Delivery of data is performed electronically to the customer using the delivery mechanisms established by the respective mission. A joint pricing strategy will be established, and invoicing for data & services from both missions is done by the commercial partner that was approached by the user. - In the Direct Access case, the DAS receives and processes data from both satellites on site.
DAS (Direct Access Station): The DAS enables the direct reception and processing of SAR payload data acquired by both PAZ and TerraSAR-X. Near-real time production workflows give DA customers the shortest time possible between image acquisition and product delivery. Re-processing of image data with further processing variants or with high accuracy attitude and orbit data is also possible.
Once an order is confirmed, the appropriate TSX or PAZ Ground Segment performs the acquisition planning, timeline generation, spacecraft commanding, and provides DLI (Downlink Information), TLE (Two Line Elements), Key (Decryption Key), attitude and orbit products, and further auxiliary data to the DAS.
At the DAS, the received SAR payload data are decrypted and processed into basic products at L1B level. Consistent product quality is ensured by the joint PAZ/TerraSAR processor, developed and maintained by the DLR. The L1B product is delivered directly to the DA customer, for immediate image analysis, to perform external value-added processing or for further product distribution to the own customer group.
Figure 26: Scheme of the constellation ordering and delivery procedure (image credit: Airbus DS, Hisdesat)
Modes harmonization: Acquisition modes onboard PAZ and TerraSAR will be harmonized to enable consistent constellation products and services. Both PAZ and TerraSAR images will meet the same quality criteria. In particular, interferometry analyses with joint TSX-PAZ image stacks shall be possible. The harmonization process ensures the synchronization of several parameters between both missions:
- orbit phasing between the satellites compatible with interferometry analysis (overlapping ground tracks)
- the adaptation of the space segment, in particular beams configuration
- the upgrade of the ground segment capabilities: image commanding and relevant antenna parameters.
Applications such as visual interpretation, surface deformation monitoring and ship detection will significantly benefit from these upgrades. Besides the StripMap, ScanSAR and SpotLight modes, the Wide ScanSAR and Staring SpotLight will also be harmonized. The latter two modes were recently implemented on TerraSAR-X and TanDEM-X to extend the sensors' capabilities with respect to coverage and resolution. Wide ScanSAR, particularly adapted to maritime monitoring services, will profit from a very large image swath (up to 270 km, 40 m resolution). The Staring SpotLight mode will feature an unprecedented spatial resolution (down to 0.25 m), ideal for object detection and identification. Both modes will be available on TerraSAR for commercial operations from mid-October 2013 onwards.
Application examples: The benefits of a constellation of SAR satellites are manifold, as a constellation provides a larger coverage, higher system capacity, increased revisits, and higher system reliability, than a single spacecraft. Two conditions are necessary to perform InSAR (Interferometric SAR) measurements): harmonized modes and the same orbit plane with specific phasing of the satellites. Amongst the phasings compatible with InSAR, 32.73º is currently envisaged to optimize the distribution of revisit opportunities across a cycle.
• Site Monitoring – revisit analysis: An operational scenario was assumed, to monitor 56 different sites (cities, critical infrastructure) across China and South-East Asia, between the equator and 50° N. Two use cases were studied:
- repeat pass: same incidence angle for InSAR
- maximum revisit: as often as possible.
Figure 27: Example test sites for regional monitoring scheme (image credit: Astrium, Hisdesat)
• Interferometric repeat pass: To efficiently monitor fast evolving sites, a revisit rate lower than a week is sometimes necessary. This can be achieved by the PAZ-TerraSAR constellation, as acquisition opportunities are doubled over a dedicated period. Assuming the above mentioned phasing, acquisition opportunities are well distributed throughout the 11 days cycle: t0 with TSX, t0+5 days with PAZ and t0+11 days again with TSX.
• Maximum revisit: For the use case of maximum acquisition possibilities (regardless of orbit direction and incidence angle), Figure 27 shows the results of TSX alone and TSX/PAZ constellation for the 56 test sites. Depending on the orbit footprint intersection and the latitude, the revisit times for TSX nominal right looking mode are between 44 and 88 hours, whereas the average is around 67 hours. The constellation reduces the revisit times of the selected locations significantly. The minimum revisit time in right looking mode is 26 hours, the maximum 44 hours with an average of 33 hours. Revisit times below 1 day in average can even be reached, using right and left looking modes for both satellites.
• Mapping – coverage analysis: In terms of mapping, the scenario assumptions are to cover an area of interest (say, Malaysia) in the most efficient manner. As described on Figure 28, the selected acquisition strategy includes one orbit direction, and strips of similar geometry (i.e. with moderate incidence angle differences between adjacent strips). A strategy that would combine both ascending and descending acquisitions would not reduce the overall coverage time, but increase the complexity of the mosaic production.
For this area of interest 18 orbits in total are available for TSX and 36 for the TSX/PAZ constellation, with a potential acquisition capacity per 11 day cycle of 3600 s or 7200 s, respectively (in right looking mode). Based on the additional acquisition capacity of PAZ, the time for covering the area is reduced from 3.6 (10 cycles) to 1.8 month (5 cycles), as described in Table 5. Beside the reduction of coverage time, also the potential conflicts with other acquisitions (e.g. site monitoring use case) are reduced thanks to the doubled potential capacity.
Table 5: Example coverage analysis over Malaysia
Figure 28: Coverage of Malaysia with TSX and TSX/PAZ constellation. For the eastern part the ascending orbit and for the western part the descending orbit are chosen (right looking mode), image credit: Astrium, Hisdesat
• Surface deformation monitoring: One application that will significantly benefit from the constellation increased revisit is interferometric surface deformation monitoring. As mentioned above, harmonization of the modes will allow the construction of joint image stacks. The precision of interferometric techniques such as InSAR is directly related to the temporal density of the data stacks, and can reach the cm to mm level. In summary, the constellation benefits for interferometry analyses will be:
- Lower temporal decorrelation
- Improved sensitivity for fast movements (e.g. triggered landslides). The problem of unwrapping bias is reduced.
- More reliable atmospheric correction
- Less impact of potential data gaps in temporal stack, e.g. due to acquisition conflicts.
Outlook and summary:
The PAZ-TerraSAR constellation is planned to be operational shortly after the PAZ spacecraft launch, in late 2017. This will be the blue print of a first of its kind CCC (Coordinated Constellation Concept) amongst X-band SAR satellites. This scalable concept will enable further satellite programs to join PAZ and TerraSAR, for the mutual benefit of users, public and commercial partners alike. It shall particularly raise interest of nations intending to set-up a SAR Earth observation program. It allows the sharing of risks and benefits between partners, each of whom is responsible for a part of the constellation.
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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 (email@example.com).