Minimize Tandem-L

Tandem-L Interferometric Radar Mission

Mission Concept    Spacecraft    Launch    Sensor Complement   References

Tandem-L is a DLR proposal for a highly innovative radar satellite mission to monitor dynamic processes on the Earth's surface with hitherto unknown quality and resolution. Important mission goals are the global measurement of forest biomass and its temporal variation for a better understanding of the carbon cycle, the systematic monitoring of deformations of the Earth's surface on a millimeter scale for the investigation of earthquakes and risk analysis, the quantification of glacier motion and melting processes in the polar regions, the fine scale measurement of variations in the near-surface soil moisture as well as observations of the dynamics of ocean surfaces and ice drift.

The Tandem-L mission concept builds upon the success of TanDEM-X and utilizes two formation-flying radar satellites operating in L-band (Figure 1). The use of the SAR (Synthitic Aperture Radar) technique enables the systematic acquisition of high-resolution radar images independent of weather and daylight and constitutes therefore an ideal basis for the continuous monitoring of dynamic processes on Earth's surface. Furthermore, the wavelength of Tandem-L (23.6 cm) optimally fulfils the requirements for a tomographic imaging of the three-dimensional structure of vegetation and ice bodies, as well as for a systematic measurement of wide-area deformations with millimeter precision. To ensure regular observations with short repeat intervals, Tandem-L will employ cutting-edge radar technology based on the latest digital beamforming techniques which allow for the mapping of ultra-wide image swaths with high azimuth resolution. The goal of Tandem-L is to interferometrically map large parts of the Earth's landmass up to two times per week. Beyond the primary mission objectives, the data set recorded with Tandem-L represents a tremendous opportunity for the development of novel scientific applications and commercial services. 1) 2) 3)

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Figure 1: Artist's rendition of the Tandem-L satellites (image credit: DLR)

The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the DLR (German Aerospace Center) and the German space industry.

User and mission requirements: The Tandem-L user requirements have been defined and elaborated in close cooperation with a large international science community. Important mission objectives are:

- Global measurement and monitoring of 3-D forest structure and biomass for a better understanding of ecosystem dynamics and the carbon cycle.

- Systematic recording of small and large scale deformations of the Earth's surface with millimeter accuracy for earthquake, volcano and landslides research as well as risk analysis and mitigation.

- Quantification of glacier movements, 3-D ice structure and melting processes in the polar regions for improved predictions of future sea level rise.

- Fine scale measurements of soil moisture and its variations close to the surface for a better understanding of the water cycle and its dynamics.

- Systematic observation of coastal zones and sea ice for environmental monitoring and ship routing.

- Monitoring of agricultural fields for crop yield forecasts, as well as the generation of highly accurate global digital terrain and surface models which form the basis for a wide range of further remote sensing applications.

These objectives address subjects of great societal importance and encompass a broad science and application spectrum that ranges from basic Earth system research to environmental monitoring and disaster mitigation. Tandem-L will moreover contribute to the measurement of 7 essential climate variables (Figure 2). The unique Tandem-L observations will therefore provide also important and currently missing information about the extent and influence of climate change, based on which improved scientific forecasts and socio-political decisions can be made.

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Figure 2: Examples of dynamic processes within the bio-, geo-, cryo- and hydrosphere and the observation intervals required for their systematic monitoring. The processes denoted by a star represent essential climate variables (image credit: DLR)

Based on the user requirements, a set of 26 preliminary geophysical products have been defined during Phase A and summarized within the Mission Requirements Document. Table 1 provides an excerpt of the most important products and their main parameters. Most of the products are unique in terms of their quality, quantity, resolution and coverage and rely on special data acquisition modes such as single-pass polarimetric SAR interferometry (PolInSAR) and multi-baseline coherence tomography. Implicit to most products is moreover the demand for high-resolution SAR acquisitions with short repeat intervals. Due to the limitations of current spaceborne SAR systems, such radar data can only be provided by a new generation of multi-channel SAR instruments.

 

Products

Coverage

Resolution

Accuracy

Repetition Rate

Biosphere

Forest height



All forest areas

50 m (global), 30 m (regional)

~10%



seasonal (bi-weekly
acquisitions)

Vertical forest structure

50 m (global), 30 m (regional)

~20 % for 10 m layers

Forest structure change

50 m (global), 30 m (regional)

~ 15 % (goal) for each layer

Above ground biomass

100 m (global), 50 m (regional)

~ 20 % (or 20 t/ha)

Biomass change

100 m (global), 50 m (regional)

~ 10 % (goal) (or 10 t/ha)

Geo-Lithosphere

LOS deformation (tectonics)

High strain areas

50 m

1 mm/year (E/V), 10 mm/year
(N) (after 10 years)

weekly acquisitions
from multiple angles

3-D deformation (tectonics)

Subsidence & landslides (PSI)

Urban & risk areas

7 m

1 mm/year (after 10 years)

LOS displacement (volcanoes)

 

50 m

10 mm

Cryosphere

Glacier velocity maps

Most glaciers

50-500 m

1 – 50 m / year

4 / year

Sea ice type and thickness

Arctic & Antarctic

5 km – 50 km

5% – 20% /0.5 m – 1 m

Bi-weekly to monthly

3-D ice structure

Greenland & selected areas

100 m

10 m vertical resolution

Yearly

Ice sheet elevation change

Ice sheets worldwide

50 m

0.5 m – 1 m

Seasonal

Permafrost

Selected Arctic regions

10 m (quad)

1 cm LOS displacement./season

Bi-weekly

Hydrosphere

Soil moisture

Selected areas

50 m

0.05 – 0.1 m3/m3

Weekly

Agriculture mapping

Selected areas

20 m (16 looks, quad pol)

1 dB rad. res., NESZ ≤-28dB

Weekly

Wind speed & wave height

Coastal regions

4 km

Speed: 2 m/s, height: 0.1 m

Weekly

Ocean currents

Selected areas

4 km

0.05 m/s

Weekly

DEM

Digital Terrain & Surface Model

Global

~ 12 m (bare), ~ 25 m (forest)

2 m (bare), 4 m (vegetated)

Annual

 

Global basemap and landcover

All land surfaces

10-20 m

single (2/year), quad (2/year)

4 / year

 

Emergency

Local

1 m

single/dual/quad

On demand

Table 1: Main geophysical products of Tandem-L

 


 

Mission Concept

To satisfy the challenging user and mission requirements, a dedicated data acquisition concept has been developed which consists of two basic measurement modes:

1) The 3-D structure mode employs fully-polarimetric single-pass SAR interferometry to acquire structural parameters of semitransparent volume scatterers. By combining multiple interferometric acquisitions with varying cross-track baselines ( Figure 3), it becomes moreover possible to derive tomographic images with fine vertical and horizontal resolutions as required for the accurate measurement of 3-D forest and ice structure as well as for the generation of digital terrain and surface models.

2) The deformation mode employs repeat-pass interferometry to measure small displacements on the Earth surface with accuracies down to centimeters or even millimeters. To minimize errors from atmospheric disturbances and temporal decorrelation, special attention has been paid to maximize the number of image acquisitions. For this, a special SAR imaging mode has been developed which allows for the systematic mapping of 350 km wide swaths with an azimuth resolution of 7 m.

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Figure 3: Left: The formation flight of the two Tandem-L satellites exploits the naturally occurring differential secular variations of the right ascension of the ascending nodes in response to slightly different inclinations. Right: Evolution of the equatorial baseline over the course of one year (image credit: DLR)

The Tandem-L satellites will fly on a sun-synchronous dawn-dusk orbit with a repeat cycle of 16 days (Table 2). During each repeat cycle, up to four global data acquisitions can be performed from different viewing directions in single- and dual-pol mode. Deformation measurements are further supported by flying the master satellite in a closely controlled orbital tube with a radius of 250 m (3σ). To obtain the required cross-track baselines for single-pass interferometry and tomography, the inclination of the slave satellite will be periodically adjusted. This results in a natural drift of the ascending node and allows for large periodic baseline variations with a minimum amount of fuel. 4)

A challenge in Tandem-L is the large amount of data that has to be transferred to the ground. For this, a high- performance Ka-band downlink with a net data rate of up to 2.6 Gbit/s will be employed. Together with an appropriate ground station network, 8 Terabyte can be downlinked every day. To show the mission feasibility, a first data acquisition plan has been developed and Figure 4 shows how the available data volume is distributed among the different applications. 5)

The systematic observation of dynamic processes will be further supported by the long mission lifetime of 10 years, which may even be extended as all consumables are planned for 12 years. At the end of the mission, the satellites will be deorbited via dedicated thrusters.

Parameter

Value

Comments

Orbital height

745 km

231 cycles / 16 days

Orbital tube diameter

500 m (3σ)

Refers to master satellite

Horizontal baselines

1 km to 18 km

Variable horizontal baselines for tomography

Radial baselines

0 m to 600 m

Radial baselines for passive safety (Helix concept)

Local time

6 h / 18 h

Dawn/dusk orbit

Inclination

98.4º

Sun-synchronous orbit

Revisit time

16 days

Enables up to 4 global data acquisitions from different directions every 16 days

Downlink capacity

~ 8 TB/day

Ka-band downlink and ground station network

Mission time

> 10 years

Consumables for 12 years

Table 2: Key parameters and performance figures of the Tandem-L satellites

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Figure 4: Daily data volume acquired for the different application areas over one year. Note that most data acquisitions serve not only one but multiple applications (image credit: DLR)

The phase A study of Tandem-L has confirmed both the feasibility and the unique opportunities of this highly innovative SAR mission. In the summer of 2016, Tandem-L is proceeding to Phase B1 which will last until the mid of 2017.

 

Greater insight into climate research with the Tandem-L satellite mission 6)

The Earth system is multifaceted, complex and in constant motion. Our home planet is ever changing – the ground rises and falls, glaciers calve into the ocean, and fires destroy forest areas. "But alterations to the environment are not only natural – today, human intrusion is playing a major role too – from deforestation and construction activities through to the impact we have on the climate," says Alberto Moreira, Director of the Microwaves and Radar Institute of the German Aerospace Center (DLR) and Principal Investigator of the Helmholtz Alliance ‘Remote Sensing and Earth System Dynamics'. Understanding these changes is an essential part of climate research and is important for enabling sustainable, positive development in the long term. Since the 1990s, Moreira has been working on radar technologies that enable Earth's dynamics to be recorded and depicted globally. The Tandem-L mission will bring science one step closer to achieving this goal.

Earth observation using radar satellites offers unique insights into our planet's dynamic processes. Radar satellites deliver reliable data regardless of weather and time of day, and enable the use of highly precise interferometric and even tomographic measurement techniques. A recent example is the interferometric imaging of Earth provided by the twin satellites TerraSAR-X and TanDEM-X, which since 2010 have been measuring the Earth's surface in close formation flight at a few hundred meters apart. Numerous fields within climate and environmental research are benefiting from the highly precise, three-dimensional images of our planet. Although more than 1000 scientists across the globe are working with the elevation model of Earth, it quickly became clear that TanDEM-X is merely scratching the surface – quite literally – of what such a satellite mission is capable of.

Numerous processes important to environmental research are occurring within the three-dimensional structure of forest ecosystems. In addition, many of the changes take place within a relatively short period of time, which is why it is necessary to continuously monitor their status. Consequently, a solution was needed that could not only penetrate deeper into the vegetation layer but also enable regular imaging at short time intervals. "We carried out the first experiments in X-band and L-band using our airborne radar system in the early 1990s, and realized how accurately we could determine the Earth's topography with radar. And so, the idea of TerraSAR-X and TanDEM-X was conceived. When TanDEM-X was officially approved in 2006, the question of how it could be developed further immediately arose at the Institute. My answer to this – even back then – was very clear: the next project will be Tandem-L," recalls Moreira.

From vegetation to plate tectonics – innovative technology with great potential: Similarly to TanDEM-X, Tandem-L will consist of two identical satellites orbiting Earth in helical formation flight and scanning the surface by radar, swath after swath. Radar systems are usually operated at various frequencies between 3 cm (X- band) and 25 cm (L- band). The X-band waves, used for TanDEM-X, are short and are already reflected by the tree canopies. The longer L-band waves, which will be used for Tandem-L, can penetrate much deeper into vegetation, ice or soil. Hence, it will be possible to measure the entire biomass – from the tree canopy to the forest floor. Two different operating modes are used for this. Firstly, there are 3D measurements based on polarimetric SAR interferometry (Pol-InSAR). The combination of interferometry (the superimposition of the radar waves) and polarization (the vertical and horizontal alignment of the waves' oscillation) enables clear conclusions to be drawn with respect to the type of vegetation, as well as its density and structure. Secondly, in deformation mode it is possible to measure topographical changes – tectonic shifts, volcanic activity or landslides. " The two modes can be operated alternately, as required. For the operation of the satellite, there will be an observation plan based on the requests of the participating scientists in the Helmholtz Alliance," Moreira explains.

A particular innovation in the satellite design is the circular foldable 15 m diameter reflector antenna. Thanks to its large surface area, up to 350 km-wide swaths can be imaged. To maintain high resolution even at this large swath width, Tandem-L uses the DLR-developed concept of digital beamforming. This technology takes advantage of the fact that the echoes reflected back from the surface of the Earth are received by the satellite in temporal sequences. In the so-called digital feed array, individual antenna elements are combined in such a way that a tightly bundled antenna diagram precisely follows these temporally displaced signals. As a result, despite its 350 km-wide coverage, Tandem-L can guarantee a resolution down to 5 m. While it takes TanDEM-X approximately one year to record the Earth's entire surface, Tandem-L will be able to do this up to twice a week. Changes on Earth that occur within a very short time can therefore be recorded.

Better information on forests: Earth's forests store huge quantities of carbon. As such, they play a critical role in the global carbon cycle and are important for the global climate. But in the last two centuries, our planet's forest area has been halved as a result of human land use. Due to the heterogeneous and multifaceted structure of the forests, it is only possible to determine their condition to a limited extent at the moment. The Intergovernmental Panel on Climate Change (IPCC) stresses the need for programs to measure forest biomass – and changes to it – on a global scale. Yet national forest inventories are based on estimates produced from local inventory plots. In the tropics, often disturbed forest areas are not necessarily considered. The next generation of high-resolution radar satellites, such as Tandem-L, can fill these knowledge gaps. An important objective of the Tandem-L mission is to determine forest biomass globally and its changes over time. Using data on forest height and structure, biomass can be estimated with much greater accuracy. Important structural characteristics here are tree density and vertical heterogeneity.

In this respect, the L- band stands out in comparison with the X- and C- bands due to its penetration depth into vegetation. This new technology enables scanning forests all the way to the forest floor – even in dense vegetation. In addition to information on the forest structure, knowledge of soil moisture and the level of disturbance in the forest can also be derived. By linking information from radar remote sensing with forest simulation models, researchers can derive important forest properties at a large scale, also thanks to the high spatial resolution.

Such approaches – meaning the combination of forest inventories with radar measurements and forest modelling – are currently being tested at a newly established super test site in Froscham (Traunstein, Bavaria, Figure 6). In the 25-hectare forest area, 16,000 trees have been analyzed and flights involving both radar and lidar have been carried out. The acquired data will be freely available and the forest plot will become part of the international ForestGeo network. Findings from this and other campaigns, for example AfriSAR in Gabon, provide a better understanding of the connection between forest biomass, forest structure and carbon flows. Contribution: Andreas Huth, Helmholtz Center for Environmental Research, Leipzig.

• Weather forecasting and water management: Exchange processes involving water and energy take place on Earth's surface. These processes drive mass movements both in the lower atmosphere and in the soil. Tandem-L will be able to observe such processes directly and indirectly via changes in the soil moisture, and it will do so with previously unattained spatial resolution and precision. Tandem-L has been conceived as a monitoring mission. The high temporal repetition rate, combined with improved resolution and precision, will open up new areas of application for satellite remote sensing.

A whole range of important exchange processes on the surface are barely taken into consideration in current weather forecasting models – and sometimes not at all. Developing these models at a regional or even local level is still exposing a data gap that can be filled with Tandem-L. Integrating Tandem-L information on soil moisture into numerical weather prediction models will improve weather forecasting significantly. The regional management of water resources can also benefit from Tandem-L. Large-scale monitoring of soil moisture is important for the management of reservoirs or for optimal irrigation in agriculture.

Yet Tandem-L will provide important information not only for the hydrology of land surfaces, but also about the ocean surface. Using SAR along-track interferometry, it is even possible to determine the velocity of ocean currents near the surface. All in all, the possibilities offered by Tandem-L are quite varied and offer great potential for science. Contribution: Carsten Montzka, Jülich Research Center.

Capturing Earth's movements: Tandem-L not only underlines the power of innovation and the leading role of German space technology, but also offers unique opportunities to carry out precise and reliable geodetic measurements from space. Researchers will have the opportunity to, for the first time, measure geological or man-made deformations on a global scale, systematically and over large areas, all with high resolution (down to one meter) and greatly improved temporal coherence. This is possible thanks to the long wavelengths of the L- band, which make Tandem-L impervious to measurement errors caused by structural changes and/or massive deformations, and superior to other existing SAR (Synthetic Aperture Radar) missions.

We will receive updated information on the topography down to the millimeter range on a weekly basis. With this, we can track diverse geological processes accurately – whether they are tectonic movements and fault-zone deformations, magmatic processes in volcanoes, mass movement or erosion processes. Other processes caused by humans can also be documented, such as deformations triggered by underground liquid injection or extraction. We can also draw conclusions on the stability of dams, among other things.

Unlike standard radar systems, Tandem-L will not simply measure one component of the three dimensional deformation field – by using a combination of different observation geometries, it will enable 3D movement vectors to be reconstructed. But more than this: for emergency purposes, such as an earthquake or a volcanic eruption, the affected area can be placed under observation within a day or two. This makes Tandem-L a completely new player in the rapid provision of data from space. The information on destroyed or endangered areas will advance research and also be useful to the public. Contribution: Mahdi Motagh, German Research Center for Geosciences (GFZ).

Increase in the knowledge of ice sheets: Tandem-L will, for the first time, give us the opportunity to use SAR tomography to analyze the structure of the topmost 100 meters of the ice sheets in Greenland and Antarctica in high spatial resolution and with complete coverage – a gigantic step for glaciologists! Layers in the firn that occur as a result of the seasonal compaction of snow will finally be observed on a large scale. The Tandem-L mission therefore latches directly onto the signal of climate change, and I expect it to provide an enormous increase in knowledge of spatial changes in the ice sheets. Contribution: Angelika Humbert, Alfred Wegener Institute, Bremerhaven.

Detailed global information for science and policy: "The focus of the Tandem-L mission is different from that of previous missions: TanDEM-X targets the commercial and security technology sector, whereas Tandem-L is a high-caliber science mission," says Richard Bamler, director of DLR's Remote Sensing Technology Institute, which is developing methods for processing the vast quantities of data. "From the outset, Tandem-L has been tailored to the requirements of the scientists, and not just restricted to commercial interests. Naturally, the data are also of relevance to companies."

When planning for the L-band mission began after the successful launch of TanDEM-X, DLR brought additional partners of the Helmholtz Association of German Research Centers (Helmholtz-Gemeinschaft Deutscher Forschungszentren; HGF) to the table. The Helmholtz Alliance ‘Remote Sensing and Earth System Dynamics' was founded under the aegis of DLR in 2012 – one year after initial ideas were laid out. Some 140 scientists are now working in the Alliance, meeting regularly to exchange information. Specific research focal points were defined in the areas of the geo-, hydro-, cryo- and biosphere, and these alone already cover seven of the essential climate variables, which include sea-ice coverage, soil moisture and forest biomass. The climate variables defined by the GCOS ( Global Climate Observing System) provide information about the Earth system and changes in the climate. "Scientists who, for example, provide data for the IPCC (Intergovernmental Panel on Climate Change) report will be able to use Tandem-L data for their work," explains Bamler. "Another example is the measurement of biomass. This is very important for the climate agreement in connection with global carbon dioxide emissions. Countries want to know, of course, the amount of carbon bound by their ecosystems. To determine this, one must first establish how much biomass – in forests, for example – is available to act as a carbon sink. And this requires regular monitoring."

A decisive year met with great anticipation: 2017 has been a very exciting year for Moreira and his colleagues so far. In January 2016, the Tandem-L team submitted a proposal to the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung; BMBF) for a large-scale research infrastructure. "The project went through a review process by the German Council of Science and Humanities. In addition to scientific excellence, commercial viability was also examined," the radar expert explains. "In July 2017 Tandem-L, together with 10 other proposals, was selected for the final step – the research- and social-policy evaluation. We expect a final decision regarding the implementation of Tandem-L by March 2018."

The conception and design of the new radar mission benefits from DLR's experience with the TanDEM-X mission, which has been in orbit for seven years. The Tandem-L mission will be operated from the DLR site in Oberpfaffenhofen, where some 100 employees from four DLR institutes – the Microwaves and Radar Institute, the Remote Sensing Technology Institute, the German Remote Sensing Data Center (Deutsche Fernerkundungsdatenzentrum; DFD) and Space Operations – are working closely together and, in doing so, are controlling the entire system chain for the mission. Both Moreira and Bamler emphasize that this is the great strength of the Tandem-L project – no one else can offer this.

Launch in late 2022 following green light: If Tandem-L gets the go-ahead and the funds are approved, the rocket carrying the two satellites, which are designed for a mission duration of 10 years, could launch in 2022. The geoscientific information that the mission will yield may well spark many scientific findings for several years after the mission ends. "Tandem-L is designed to serve a wide range of applications," Bamler says. "It will not always provide the solution by itself – sometimes additional information will be needed. Yet Tandem-L comes fantastically close to the ideal, providing standalone, informative data on each technical issue."

In a time of fast-paced change and in which science – as well as policy – depends on reliable information, a mission aimed specifically at climate research is of great significance. "What drives us is the vision of knowing what is changing on our planet – at any time," says Moreira. "Tandem-L is just the answer to a radar mission designed specifically for climate research and environmental monitoring."

Table 3: ,Listening to Earth's Heartbeat — Greater insight into climate research with the Tandem-L satellite mission (Ref. 6)

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Figure 5: Overview of DLR SAR mission scenario (image credit: DLR)

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Figure 6: City forest in Traunstein in Upper Bavaria: The forest is thoroughly mapped and has been used as a stable reference area for the development of radar remote sensing technology for more than 10 years. At the time of the flight in late June 2016, a large part of the fields was overgrown. The various crops and stages of growth result in differently colored polarimetric signatures (image credit: DLR)

 


 

Spacecraft:

A spacecraft description will be provided when available.

 

Launch: According to current planning, and subject to timely financial approval, the Tandem-L satellites could be launched at the end of 2022.

Orbit: Sun-synchronous dawn/dusk orbit, altitude = 745 km, inclination = 98.4º, revisit time = 16 days.

 


 

Sensor complement (L-SAR)

L-SAR instrument

A particular challenge of the Tandem-L mission is the development of two extremely capable but at the same time also cost-efficient SAR instruments that shall map a 350 km wide swath in single/dual pol mode and a 175 km wide swath in quad-pol mode, both with an azimuth resolution of 7 m and a range bandwidth of up to 84 MHz (Table 4). Moreover, the noise equivalent sigma zero (NESZ) shall be better than -25 dB and the ASR (Ambiguity-to-Signal-Ratio) shall be better than -25 dB in single/dual pol mode (-22 dB in quad-pol mode). These requirements exceed by far the capabilities of current spaceborne SAR systems. Therefore, a new instrument concept has been developed that combines a large unfurlable mesh reflector with a digital feed that is composed of 32 patch elements in elevation and 6 patch elements in azimuth. The 6 azimuth patches are connected to a single T/R module via fixed power dividers to obtain, for each elevation direction, an optimized azimuth antenna pattern. The outputs of the T/R modules are then individually digitized and combined in real-time to form multiple elevation beams that follow the simultaneously arriving radar echoes from subsequent transmit pulses. By this, it becomes possible to map a very wide swath with high azimuth resolution.

The emergence of blind ranges is moreover avoided by a systematic variation of the pulse repetition interval . 7) Figure 7 demonstrates that such a staggered SAR mode provides an excellent performance that can meet the demanding science requirements for both the fully polarimetric 3-D structure mode and the ultra-wide swath deformation mode. 8)

Parameter

Value

Parameter

Value

Frequency

L-band

Reflector diameter

15 m

Bandwidth

≤ 84 MHz

Focal length

13.5 m

NESZ(Noise Equivalent Sigma Zero)

<-25 dB

Feed offset

9 m

Azimuth resolution

7 m (1 m spot)

Feed size

5.2 m x 0.86 m

Swath width

350 km

Patches

32 x 6

Incident angle range

26.3º - 47.0º

T/R (Transmit/Receive) modules

2 x 32

Inc. (quad)

28.4º - 39.5º

TRM (Transmit Receive Module) power

56.6 W

ASR(Ambiguity-to-Signal-Ratio)

< -25 dB

Total losses

3.6 dB

ASR (quad)

< -22 dB

Noise figure

3.5 dB

Look direction

right & left

Duty cycle

4% (8% quad)

Table 4: Key parameters of the L-SAR instrument

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Figure 7: Top row left:AASR (Azimuth Ambiguity-to-Signal Ratio) for single-pol; right: quad-pol mode. Middle row left: RASR (Range Ambiguity-to-Signal Ratio) for single-pol; right: quad-pol mode. Bottom row left: NESZ (Noise Equivalent Sigma Zero) for single-pol; right: quad-pol mode (image credit: DLR)

 

Some background on SAR technology:

As the staggered SAR mode is associated with a notable oversampling of the SAR signal, a new onboard data reduction technique will be employed to keep the data rate even below that of a conventional SAR system. This will maximize the science output for a given downlink budget. 9)

Conventional SAR systems are limited, in that a wide swath can only be achieved at the expense of a degraded azimuth resolution. This limitation can be overcome by using systems with multiple receive apertures, displaced in along-track, but a very long antenna is required to map a wide swath. If a relatively short antenna with a single aperture in along-track is available, it is still possible to map a wide area: Multiple swaths can be, in fact, simultaneously imaged using digital beamforming in elevation, but "blind ranges" are present between adjacent swaths, as the radar cannot receive while it is transmitting. Staggered SAR overcomes the problem of blind ranges by continuously varying the PRI (Pulse Repetition Interval). If the sequence of PRIs is properly chosen, the samples, missing because the radar is transmitting, are distributed across the swath and along azimuth, such that they can be then recovered by interpolation of neighboring azimuth samples. This concept therefore allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple apertures. In order to provide satisfactory suppression of azimuth ambiguities, some azimuth oversampling is required. This may cause (1) increased range ambiguities, which can be suppressed by jointly processing the data acquired by the available multiple elevation beams, and (2) an increased data volume, which can be reduced by on-board Doppler filtering and decimation.

In conventional stripmap SAR, the swath width constrains the PRI (Pulse Repetition Interval): To control range ambiguities, the PRI must be larger than the time it takes to collect returns from the entire illuminated swath. On the other hand, to avoid significant azimuth ambiguity levels, a large PRI, or equivalently a low PRF (Pulse Repetition Frequency), implies the adoption of a small Doppler bandwidth and limits the achievable azimuth resolution. A wide swath can be also mapped using ScanSAR or TOPS (Terrain Observation with Progressive Scan), but the azimuth resolution is still impaired.

To overcome these limitations, new radar techniques have been developed, which allow for the acquisition of spaceborne high-resolution SAR images without the classical swath limitation imposed by range and azimuth ambiguities .10) These techniques are mainly based on DBF (Digital Beamforming) and multiple aperture signal recording. DBF on receive is used to steer in real-time a narrow beam towards the direction of arrival of the radar echo from the ground, exploiting the one-to-one relationship between the radar pulse travel time and its direction of arrival, this is also referred to as SCORE (Scan-On-Receive) or Sweep-SAR. A large receiving antenna can hence be used to improve the sensitivity without narrowing the swath width. As the unambiguous swath width is limited by the antenna length, a long antenna is deployed to map a wide swath. Moreover, to improve the azimuth resolution, the receive antenna is divided into multiple sub-apertures, mutually displaced in the along-track direction and connected to individual receive channels. By this, multiple samples of the synthetic aperture can be acquired for each transmitted pulse. The coherent combination of all signals in a dedicated multichannel processor enables the generation of a high-resolution wide-swath SAR image. The need for a very long antenna represents the main limitation of the mentioned system: A 40 m antenna is, in fact, required to map a 350 km swath width on ground in stripmap imaging mode.

In order to keep the antenna length down, several new instrument architectures and modes have been proposed. 11) One example is the combination of displaced phase centers in azimuth with ScanSAR or TOPS mode (Figure 8 (a)). As in classical ScanSAR, azimuth bursts are used to map several swaths. The associated resolution loss from sharing the synthetic aperture among different swaths is compensated by illuminating a wider Doppler spectrum and reducing the PRF by collecting radar echoes with multiple displaced azimuth apertures. A possible drawback of multichannel ScanSAR or TOPS approaches is the rather high Doppler centroid for some of the imaged targets, in case high resolution is desired. Moreover, high squint angles may also challenge co-registration in interferometric applications. Besides multichannel ScanSAR, of great interest are concepts based on simultaneous recording of echoes of different pulses, transmitted by a wide beam illuminator and coming from different elevation directions. This enables an increase of the coverage area without the necessity to either lengthen the antenna or to employ burst modes.

Figure 8 (b) provides an illustration, where three narrow receive beams follow the echoes from three simultaneously mapped image swaths that are illuminated by a broad transmit beam. A sufficiently high antenna is needed to separate the echoes from the different swaths by digital beamforming on receive, while a wide beam can either be accomplished by a separate small transmit antenna or a combined transmit-receive antenna together with tapering, spectral diversity on transmission or sequences of subpulses. An interesting alternative to a planar antenna is a reflector, fed by a multichannel array, as illustrated in Figure 8 (c). A parabolic reflector focuses an arriving plane wave on one or a small subset of feed elements. As the swath echoes arrive as plane waves from increasing look angles, one needs hence to only read out one feed element after the other to steer a high-gain beam in concert with the arriving echoes.

A drawback of the multi-beam mode is the presence of blind ranges across the swath, as the radar cannot receive while it is transmitting. The Staggered SAR concept (Figure 8 (d)) overcomes this drawback by continuously varying the PRI in a cyclic manner, so allowing the imaging of a wide continuous swath without the need for a long antenna with multiple apertures (Ref. 7). 12)

TandemL_Auto0

Figure 8: Advanced concepts for high-resolution wide-swath (HRWS) imaging. (a) ScanSAR with multiple azimuth channels. (b) Single-channel SAR with multiple elevation beams. (c) Digital beamforming with reflector antenna. (d) Staggered-SAR (image credit: DLR)

 


1) Alberto Moreira, Gerhard Krieger, Irena Hajnsek*, Konstantinos Papa thanassiou, Marwan Younis, Paco Lopez-Dekker, Sigurd Huber, Michelangelo Villano, Matteo Pardini, Michael Eineder, Francesco De Zan, Alessandro Parizzi, "Tandem-L: A Highly Innovative Bistatic SAR Mission for Global Observation of Dynamic Processes on the Earth's Surface," IEEE Geoscience. Remote Sensing Magazine, Volume 3, Issue 2, pp. 8-23, July 31, 2015, URL: DOI: 10.1109/MGRS.2015.2437353, URL: http://www.dlr.de/Portaldata/32/Resources/dokumente/tdml/Tandem-L-IEEE-GRSM-2015.pdf

2) G. Krieger, A. Moreira, M. Zink, I. Hajnsek, S. Huber, M. Villano, K. Papathanassiou, M. Younis, P. Lopez Dekker, M. Pardini, D. Schulze, M. Bachmann, D. Borla Tridon, J. Reimann, B. Bräutigam, U. Steinbrecher, C. Tienda, M. Sanjuan Ferrer, M. Zonno, M. Eineder, F. De Zan, A. Parizzi, T. Fritz, E. Diedrich, E. Maurer, R. Münzenmayer, B. Grafmüller, R. Wolters, F. te Hennepe, R. Ernst, C. Bewick, "Tandem-L: Main Results of the Phase A Feasibility Study," Proceedings of the IEEE IGARSS (International Geoscience and Remote Sensing Symposium) Conference, Beijing, China, July 10-15, 2016

3) Michael Eineder ,Irena Hajnsek,Gerhard Krieger, Alberto Moreira, Kostas Papathanassiou, "Tandem-L Brochure, Satellite Mission Proposal for Monitoring Dynamic Processes on the Earth's Surface," German Aerospace Center, April 2016, URL: http://www.dlr.de/dlr/en/Portaldata/1/Resources/documents/2016-Tandem-L-Brochure-2016-06.pdf

4) G. Krieger, I. Hajnsek, K. Papathanassiou, M. Eineder, M. Younis, F. De Zan, P. Prats, S. Huber, M. Werner, H. Fiedler, A. Freeman, P. Rosen, S. Hensley, W. Johnson, L. Veilleux, B. Grafmueller, R. Werninghaus, R. Bamler, A. Moreira, "The Tandem-L Mission Proposal: Monitoring Earth's Dynamics with High Resolution SAR Interferometry," Proceedings of the IEEE Radar Conference (RadarCon), Pasadena, CA, USA, May 4-8, 2009, URL: http://elib.dlr.de/58480/1/PID841883.pdf

5) Markus Bachmann, Daniela Borla Tridon, Francesco De Zan, Gerhard Krieger and Manfred Zink, "Tandem-L Observation Concept - An Acquisition Scenario for the Global Scientific Mapping Machine," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016

6) Elisabeth Schreier, "Listening to Earth's Heartbeat," Earth Observation, DLR magazine No 154 ·155, pp: 9-14, Sept. 2017, URL: http://www.dlr.de/dlr/en/Portaldata/1/Resources/documents/dlr_magazin-155/DLRmagazine_154-155_EN.pdf

7) Michelangelo Villano, Marc Jäger, Ulrich Steinbrecher, Gerhard Krieger, Alberto Moreira, "Staggered SAR: High-Resolution Wide-Swath Imaging by Continuous PRI Variation," IEEE Transactions on Geoscience and Remote Sensing , Volume: 52, Issue: 7, pp: 4462–4479,July 1 2014 , DOI: 10.1109/TGRS.2013.2282192 , URL: http://elib.dlr.de/90943/1/YSA-F-Wave-Propagation-and-Remote-Sensing-Michelangelo-Villano.pdf

8) S. Huber, M. Villano, M. Younis, G. Krieger, A. Moreira, B. Grafmüller, R. Wolters, "Tandem-L: design concepts for a next generation spaceborne SAR system," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016

9) Michelangelo Villano, Gerhard Krieger, Alberto Moreira, "Onboard Processing for Data Volume Reduction in High-Resolution Wide-Swath SAR," IEEE Geoscience and Remote Sensing Letters, Volume 13, Issue 8, Aug. 20, 2016. pp: 1173 - 1177, DOI: 10.1109/LGRS.2016.2574886

10) Nicolas Gebert, Gerhard Krieger, Alberto. Moreira, "Digital Beamforming on Receive: Techniques and Optimization Strategies for High-Resolution Wide-Swath SAR Imaging", IEEE Transactions Aerospace. and Electronic Systems, Volume 45, No. 2, April 2009, URL: http://elib.dlr.de/54876/1/Gebert_TAES2009.pdf

11) G. Krieger, M. Younis, S. Huber, F. Bordoni, A. Patyuchenko, J. Kim, P. Laskowski, M. Villano, T. Rommel, P. Lopez-Dekker, A. Moreira, "Digital Beamforming and MIMO SAR: Review and New Concepts," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

12) M. Villano, G. Krieger, A. Moreira, "A Novel Processing Strategy for Staggered SAR," IEEE Geoscience and Remote Sensing Letters, Vol. 11, No. 11, Nov. 2014, DOI: 10.1109/LGRS.2014.2313138
 


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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