Tandem-L Interferometric Radar Mission
Earth surface albedo
Tandem-L Interferometric Radar Mission is a proposal for an innovative radar satellite mission by the German Aerospace Center (DLR). Inspired by TanDEM-X, the mission is proposed to consist of two identical L-band radar satellites used to investigate dynamic processes in the biosphere, cryosphere, geosphere and hydrosphere.
|Launch date||31 Dec 2024|
|End of life date||31 Dec 2036|
|Measurement domain||Ocean, Land, Snow & Ice|
|Measurement category||Multi-purpose imagery (ocean), Snow cover, edge and depth, Vegetation, Ocean topography/currents, Albedo and reflectance, Landscape topography, Multi-purpose imagery (land), Sea ice cover, edge and thickness|
|Measurement detailed||Earth surface albedo, Vegetation type, Ocean imagery and water leaving spectral radiance, Sea-ice type, Ocean surface currents (vector), Sea-ice cover, Glacier motion, Snow cover, Glacier cover, Land surface topography, Land surface imagery, Above Ground Biomass (AGB)|
|Instrument type||Imaging microwave radars|
|CEOS EO Handbook||See Tandem-L Interferometric Radar Mission summary|
Tandem-L satellites are planned to carry a single instrument, an L-band Synthetic Aperture Radar (L-SAR). Based on user requirements, 26 preliminary geophysical products have been set out for Tandem-L acquisition. To satisfy these challenging mission requirements, the data acquisition concept consists of a 3-D structure mode and a deformation mode for its characteristic wavelength of 236 mm. 3-D structure mode employs fully-polarimetric SAR interferometry to derive tomographic images with fine vertical and horizontal resolutions. This is required for a number of accurate measurements including 3-D forest and ice structures, and the generation of digital terrain and surface models. Employing repeat-pass interferometry, the deformation mode will be able to measure displacements on the Earth's surface with high accuracy used for monitoring deformations for investigations of earthquakes and risk analysis.
A combination of L-SAR modes will be used to complete other important mission goals including global measurement of forest biomass and its temporal variation, glacier motion and melting, near-surface soil moisture and ocean surface dynamics.
L-SAR will be developed with a resolution of 7 m azimuth and 1 m for spot images, with a 350 km swath width in single/dual polarisation mode and a 175 km swath width in quad-polarisation mode.
Tandem-L satellites are proposed to fly on a sun-synchronous dawn-dusk orbit with a repeat cycle of 16 days. Up to four global data acquisitions will be performed during each repeat cycle, with deformation measurements further supported by flying the master satellite in a closely controlled orbital tube of radius 250 m.
Space and Hardware Components
Tandem-L satellites are proposed to feature a mission lifetime of 10 years, with consumables to last 12 years. The large amount of data to be transferred to the ground will require a high-performance Ka-band downlink with a net data rate of up to 2.6 Gbit/s. For proof of mission feasibility, a first data acquisition plan has been developed and is awaiting testing.
Tandem-L Interferometric Radar Mission
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 millimetre 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 (Synthetic 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 tomographic imaging of the three-dimensional structure of vegetation and ice bodies, as well as for systematic measurement of wide-area deformations with millimetre 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)
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 scientific 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 millimetre 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.
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 are 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.
50 m (global), 30 m (regional)
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)
100 m (global), 50 m (regional)
~ 10 % (goal) (or 10 t/ha)
LOS deformation (tectonics)
High strain areas
1 mm/year (E/V), 10 mm/year
3-D deformation (tectonics)
Subsidence & landslides (PSI)
Urban & risk areas
1 mm/year (after 10 years)
LOS displacement (volcanoes)
Glacier velocity maps
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
10 m vertical resolution
Ice sheet elevation change
Ice sheets worldwide
0.5 m – 1 m
Selected Arctic regions
10 m (quad)
1 cm LOS displacement./season
0.05 – 0.1 m3/m3
20 m (16 looks, quad pol)
1 dB rad. res., NESZ ≤-28dB
Wind speed & wave height
Speed: 2 m/s, height: 0.1 m
Digital Terrain & Surface Model
~ 12 m (bare), ~ 25 m (forest)
2 m (bare), 4 m (vegetated)
Global basemap and landcover
All land surfaces
single (2/year), quad (2/year)
4 / year
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's surface with accuracies down to centimetres or even millimetres. To minimize errors from atmospheric disturbances and temporal decorrelation, special attention has been paid to maximising 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.
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 modes. 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 Terabytes 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.
231 cycles / 16 days
Orbital tube diameter
500 m (3σ)
Refers to master satellite
1 km to 18 km
Variable horizontal baselines for tomography
0 m to 600 m
Radial baselines for passive safety (Helix concept)
6 h / 18 h
Enables up to 4 global data acquisitions from different directions every 16 days
~ 8 TB/day
Ka-band downlink and ground station network
> 10 years
Consumables for 12 years
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, the 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.
Listening to Earth's Heartbeat — Greater insight into climate research with the Tandem-L satellite mission (Ref. 6)
• 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 satellites 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 their 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 is 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 millimetre 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's 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 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 benefit 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."
A spacecraft description will be provided when available.
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.
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 3). 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)
≤ 84 MHz
NESZ(Noise Equivalent Sigma Zero)
7 m (1 m spot)
5.2 m x 0.86 m
32 x 6
Incident angle range
26.3º - 47.0º
T/R (Transmit/Receive) modules
2 x 32
28.4º - 39.5º
TRM (Transmit Receive Module) power
< -25 dB
< -22 dB
right & left
4% (8% quad)
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 an 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 neighbouring 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 onboard 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 the 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 centres 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 of 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 a 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 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)
The system architecture is based on a large parabolic reflector antenna that is illuminated by a digital feed with multiple elevation channels (Figure 9). As each feed element is associated with a different secondary beam, it becomes possible to image a wide swath with high Rx gain by a time-variant combination of the feed signals in synchrony with the expected direction of arrival of the desired radar echo. The imaging capacity is further increased by using not only one but multiple elevation beams that map multiple swaths at the same time. As these swaths are separated by blind ranges, several strategies and modes have been proposed to avoid such gaps in the SAR image. Out of these modes, Tandem-L will employ a technique where the pulse repetition interval is rapidly changed from pulse to pulse. This technique, now denoted as staggered SAR, has been further analyzed and elaborated in detail in Ref. 12). In combination with an optimized Tandem-L reflector and feed system, it becomes then possible to map a 350 km wide swath with an azimuth resolution of 7 m, thereby significantly improving the imaging capacity if compared to state-of-the-art L-band SAR systems like ALOS-2 or even the C-band satellite constellation Sentinel-1A and 1B. 13)
While staggered SAR enables the acquisition of an ultra-wide image swath with high resolution, it requires also a notable oversampling in azimuth. Such an increase in the average PRF (Pulse Repetition Frequency) is mandatory to avoid a rise of azimuth ambiguities caused by missing samples along the synthetic aperture. 14) The high PRF will, however, also increase the susceptibility to range ambiguities. Range ambiguity suppression is further challenged by the required wide swath illumination, which causes multiple mutually ambiguous radar echoes to arrive at the same time from different elevation angles but with comparable magnitudes. This poses high demands on the multichannel receiver system which has to steer multiple elevation beams in real-time towards the radar echoes' expected directions of arrival. The shape of each of these receiver beams must be adjusted to maximize for each instant of time the antenna gain in the direction of the desired radar echo while minimizing the gain towards the arrival angles of the interfering range ambiguous radar echoes.
Several beamforming algorithms have been developed to address this challenge, but their performance depends crucially on the accurate knowledge of the amplitude and phase of the secondary beam patterns associated with individual feed array elements. 15) 16) While this knowledge is less important for a planar array, where the element patterns are typically pretty similar and the overall radiation performance is primarily determined by the array's geometry and excitation, it becomes mandatory for reflector SAR systems, where each feed has its own element pattern that points its secondary beam to a different direction. As it is impossible to measure the antenna patterns for a SAR system like Tandem-L with sufficient accuracy before launching on the ground, one needs an alternative strategy to obtain the required far-field pattern information. One established approach is the use of dedicated calibration targets like corner reflectors or transponders. 17)
A very large number of calibration targets would, however, be required to meet the accuracy requirements as the antenna patterns associated with individual feed elements cover different areas and are moreover characterized by a high degree of spatial variation, which is further complicated by the fact that the patterns are typically non-separable in elevation and azimuth. Another approach, which was successfully applied for TerraSAR-X [18)], is based on the use of an appropriate antenna model, but the requirements regarding the precise knowledge of the deployed reflector's attitude and shape are rather high to meet the demanding performance requirements. 19)
In-Orbit Multichannel Antenna Pattern Calibration
As a complement to the conventional antenna pattern estimation and modelling techniques discussed before, we propose here a new multichannel calibration approach that enables highly accurate in-orbit measurements of the amplitude and phase differences between the secondary far-field patterns of the feed elements/channels. The proposed technique is capable of providing this information in two dimensions without the need for dedicated calibration targets and/or a sophisticated antenna model. The core concept is to operate the SAR system over an appropriately chosen natural scene with known topography in a series of dedicated calibration modes, which are described in more detail in the following subsections. The basic idea of this technique has already been suggested in the context of the cross-elevation beam range ambiguity suppression technique CEBRAS, 20) and is here further elaborated in view of the demanding requirements for Tandem-L.
LPAC (Low PRF Antenna Calibration):
As a first calibration mode, we consider an operation with a very low pulse repetition frequency (PRF), so that only a single radar echo arrives at the radar satellite at any time. A wide swath is illuminated, and all elevation channels simultaneously record and digitize their received signals independently from each other without applying any onboard beamforming. As the total data rate of this transparency mode is likely to exceed the capacity of the input channel to the onboard memory, it is suggested to reduce the RF bandwidth in this mode. Tandem-L is, for example, capable to handle in its nominal staggered SAR mode the radar signals from five elevation beams, each recording a radar echo with an RF bandwidth of 84 MHz. A reduction to, e.g., 10 MHz allows therefore the simultaneous recording of all 35 feed signals without increasing the internal data rate.
Note: The reduced RF bandwidth is also beneficial to increase the SNR of each recorded signal. By this, even the weak echoes from the antenna sidelobes can be acquired with sufficient accuracy. The SNR could be further improved by increasing the length of each individual pulse if compared to the rather short pulses used in the staggered SAR mode.
If necessary, the full bandwidth can then be covered by subsequent measurements where the range centre frequency is appropriately varied from data take to data take. The multichannel radar data acquired in this dedicated antenna calibration mode are then transmitted to the ground, where they are further evaluated.
The evaluation starts with a range compression of the signals from the individual elevation channels. Assuming a calibration scene that is sufficiently flat and free of layover, there exists thereafter a one-to-one relation between the instant of time in the range-compressed data and the elevation angle from which the radar echoes are arriving. This relationship is easily derived from the imaging geometry and the known topography of the calibration scene. (Note: A coarse DEM with a resolution in the order of 100 m is considered sufficient.)
However, as the low PRF of the LPAC mode does not allow for azimuth focusing, radar echoes arriving at the same time from multiple azimuth angles cannot be separated from each other. A comparison of the recorded feed signals in amplitude and phase yields therefore an estimate of the relative antenna patterns integrated over azimuth.
Note: The involved azimuth angles are typically small and span for Tandem-L an angular interval in the order of 1°.
The performance simulations (below) reveal that this information is nevertheless well suited to derive beamforming coefficients for highly efficient range ambiguity suppression without detailed a priori pattern knowledge.
HPAC (High PRF Antenna Calibration):
While the LPAC model outlined in the previous section is well suited to derive appropriate weights for elevation beamforming, a more detailed knowledge of the relative azimuth antenna patterns may be of interest to further improve the SAR imaging performance and to support the end-to-end system calibration.
For this, we suggest acquiring SAR data over an appropriate scene in the high PRF antenna calibration (HPAC) mode. In contrast to the LPAC mode, only a narrow swath is illuminated by transmitting with one or a small subset of the available feed elements. The reduced swath illumination provides an efficient means to suppress range ambiguities as they are, in contrast to the nominal wide-swath staggered SAR mode, already suppressed by the Tx pattern.
Note: Residual range ambiguities could, in principle, also be resolved by blind source separation as proposed in (Ref. 20).
The calibration data for the full swath extension can then be recovered by combining the data from a series of measurements where different sub swaths are illuminated.
Note: For stability reasons, these measurements can be combined into a single data take, where the Tx pattern sequence is similar to that of a ScanSAR mode.
In comparison to the staggered SAR mode, the use of a constant PRF avoids, moreover, the loss of azimuth samples and enables therefore an excellent suppression of the azimuth ambiguities. Stated differently, a reduced PRF could be used to achieve the same azimuth ambiguity-to-signal ratio.
The HPAC data are then transmitted to the ground and further analyzed as illustrated in Figure 10. The evaluation starts again with a range compression of each individual feed signal. In contrast to the LPAC mode, the range-compressed data are then transformed to the range-Doppler domain (or decomposed into multiple azimuth looks) which provides the basis for a two-dimensional analysis of the antenna patterns (Ref. 20).
This section shows an example of how the multichannel radar data acquired in the LPAC mode can be used to improve the elevation beamforming in a reflector SAR system like Tandem-L. For this purpose, we simulate a time series of multichannel radar data, as they would be recorded by a dedicated LPAC data take. To simplify the calculations, we model the backscatter by independent and identically distributed complex Gaussian noise, but a more refined simulation could also use real radar data to better account for inhomogeneities in the backscatter statistics. After range compression and appropriate consideration of the radar imaging geometry [Note: This includes the incorporation of the known scene topography. The simulation in this paper assumes the same geometry and spherical Earth model with flat topography as in Ref. 20).], the simulated multichannel radar data are obtained for each radar channel and range line by projecting the antenna-weighted backscatter along iso-range contours (cf. green line in the upper plot of Figure 11, which shows the 2-D antenna pattern of an optimum MVDR beamformer steered towards this range). To provide a realistic weighting, we use here the results from GRASP computations that predict, for each feed element, the two-dimensional complex antenna pattern for the nominal Tandem-L antenna geometry.
In summary, a set of new antenna calibration modes and techniques is introduced for multichannel reflector SAR systems. The proposed modes are well suited to acquire information about the relative amplitude and phase of the secondary far-field patterns associated with pairs of feed elements/channels. This mutual pattern information can then be used to improve the performance of advanced real-time beamformers that maximize their gain towards the direction of arrival of the desired radar echo and suppress, at the same time, range ambiguities arriving from different directions. Such beamformers are of great benefit for advanced SAR imaging modes like staggered SAR, which employs multiple elevation beams to map a wide image swath with high resolution.
As the proposed technique does not depend on dedicated calibration targets, it can be used anywhere in the orbit, provided the scene has a known and sufficiently flat topography. To simplify the derivation of optimized beams in the LPAC mode, it is moreover advisable to use a scene with homogeneous and sufficiently strong backscatter, e.g., provided by rainforests. The 2-D calibration with the HPAC mode may, on the other hand, also benefit from inhomogeneous backscatter to obtain further information about low sidelobes. In this context, permanent scatterers may offer promising potential for advanced multichannel SAR calibration.
The proposed LPAC mode can not only provide optimized beamforming weights to suppress range ambiguities, but also offers a new opportunity to improve nadir suppression. This prospect is of high interest for advanced imaging modes like staggered SAR, which is susceptible to nadir echoes. To this aim, the LPAC data recording must include the nadir return, which is treated as an additional directional interference. This interference is then added while deriving the beamforming weights, either for all elevation beams, or at least those which may be superimposed by nadir echoes, taking into account variations in satellite height and nadir topography. To improve the strength of the nadir signal, it may even be advantageous to use extra data to take over flat scenes like calm water. The width of the nadir notch can moreover be increased by combining data from multiple LPAC measurements, each acquired with a slightly different roll angle of the satellite.
While this paper focused on multichannel antenna pattern calibration in Tandem-L and its use for advanced range ambiguity suppression, the proposed technique is neither limited to reflector antennas nor to beamforming in elevation. For example, a dedicated data take with a narrow azimuth Tx beam is well suited to derive the relative antenna patterns for a planar HRWS system with multiple azimuth channels (Ref. 13).
Concepts for Tandem-L Products
Tandem-L is a proposal for a SAR mission with two L-band satellites which aims to observe globally dynamic processes of the Earth's surface. The mission will provide, on the one hand, 12 higher-level products addressing the problematics of, for example, 3D forest structure measurements or large-scale deformation monitoring. On the other hand, it will deliver focused SAR data to generate other products, which will contribute to the understanding of e.g. ice or ocean dynamics. 21)
Some important mission goals are:
The global measurement of forest biomass and its variation in order to understand the carbon cycle, the systematic recording of Earth's surface deformations for earthquake research and risk analysis, the large-scale observation of ocean currents and sea ice drift or the quantification of glacier movements and melting processes in the Polar Regions. Inspired by the TanDEM-X mission, the Tandem-L mission concept uses two SAR satellites at L-band which fly in different formations to enable bistatic or (pursuit) monostatic acquisitions of the whole Earth's landmass up to four times per week.
A set of 24 so-called higher-level products have been defined based on user requirements. Among these, 12 products will be generated operationally during the mission. These cover the biosphere and geosphere topics but also DEMS (Digital Elevation Models). Besides that, various basic SAR and InSAR products will be delivered as a basis for higher-level products covering the hydrosphere and cryosphere matters. Table 4 summarizes all basic SAR products and Table 5 the deformation (geosphere) of higher-level products.
Single-Look Slant Range Complex;
Coregistered Bistatic SSC
Multi-look Ground Range Detected
Geocoded Ellipsoid Corrected
Geocoded Terrain Corrected
Coregistered Bistatic MGD
Coregistered Bistatic GEC
Coregistered Bistatic GTC
Stacked interferometric phases
Large scale deformation
PSI deformation database
LOS (Line-Of-Sight) Displacement maps
Volcanoes, Earthquakes, Landslides
Line-Of-Sight deformation rate maps
Large scale deformation
3D deformation rates map
Large scale deformation
These products enable to monitor large-scale deformation, urban subsidences, landslides, volcanic activity and earthquakes. Forest (biosphere) products, on the other hand, will provide global measurements of 3D forest structure and biomass.
Each of these products is generated to support different application fields which require different kinds of processing and consequently specific acquisitions conditions:
• acquisition mode (standard is Staggered StripMap)
• geometry (right/left looking, ascending/descending orbits)
• acquisition frequency (e.g. one acquisition per cycle)
• flight configuration (close formation for bistatic acquisitions or pursuit monostatic or constellation for monostatic acquisitions)
• polarization mode (single or quad pol) implying different swath widths (respectively 350 km and 175 km)
• range bandwidth (20, 40 or 84 MHz).
In this way, forest observations require bistatic quad pol 84 MHz acquisitions whereas deformation products need single pol monostatic 20 MHz SAR data. Additionally, it is reasonable to monitor these applications only for specific regions of interest (Figure 12), which may of course overlap. As a consequence, an acquisition configuration has to be preferred when the ROIs overlap and some applications may get some acquisitions with different data-taking configurations than the foreseen ones.
For example, processors for deformation products will have to deal with data with forest acquisition parameters since they are more constraining as suggested in Figure 2. It also implies a seamless transition between acquisitions to avoid gaps within the regions of interest (the instrument must thus be able to switch the acquisition mode from one instrument source packet to the next).
Altogether, an acquired (SAR raw) data volume of up to 8 TB per day has to be downlinked and processed to L1A and made available for external users or external processors but also for a good part of them systematically further processed to higher-level products.
Basic SAR Products Processing
Combined Systematic L0 annotation and L1A processing: It is foreseen to logically assemble the SAR raw data during their ingestion together with a comprehensive SAR header analysis and quality assessment of the source packets. The more complex processing is performed at the same time as the systematic L1A (SSCs and CoBiSSCs) generation in order to avoid shuffling the data several times. In this way, further processing steps are performed once all auxiliary data (orbit, calibration information, attitude, etc) are available i.e. about 3 days after acquisition. The complex products are generated systematically and are provided at a data hub for user access for a given period of time. SSCs or CoBiSSCs needed for the operational higher-level product generation are archived for the needed duration. For example, the line-of-sight deformation rate maps shall be generated based on one year of acquisitions.
The exemplary observation scenario as depicted in Figure 13 could hint, that the individual acquisitions are processed according to the foreseen applications. This means complete processing for the trigger application and possibly repeated processing for other applications and that the same raw data are possibly processed times. To avoid it and the associated handling of L1A products specifically generated for a given application, the SAR processor should be able to focus the data take-wise "in one go" by taking into account which data is available with which configuration. At transition areas, it will process for the "lower performance" i.e. when a switch occurs between 84 MHz and lower bandwidth, it extends the acquisition with the lowest bandwidth, or when a transition from quad to single pol (or inversely) happens, the length of the single polarization channel will be increased at the cost of a reduced swath width (single pol is 350 km wide whereas quad pol is only 175 km).
This SSC data stream is then cut into seamless and nonoverlapping SSC pieces, called SSC frames, following a global framing scheme. Each SSC frame is then treated as a stand-alone self-containing product with complete meta and annotation information, quicklooks, etc. Every further product (other L1 and higher-level products) is consistently generated based on these SSC frames.
Figure 14 depicts the resulting SSC frames for the forest and deformations applications for the acquisitions of cycle 9 in descending orbit of the current acquisition timeline of the close formation. In (a), acquisitions triggered for forest are in green and those for deformation in red. In (b), the resulting SSC frames for forest are plotted, since their required mode is the most constraining i.e. bistatic quad pol with 84 MHz bandwidth, no other triggered applications delivered usable data. In (c), on the opposite, it can be observed that deformation applications can accept data with almost any acquisition parameters and consequently exhibit all possible acquisition modes.
A number of reasons favour the systematic processing scheme as presented:
• Focusing may be done in a straightforward way on a data-driven basis without taking follow-on application products into consideration.
• Comprehensive annotation and quality information (about RFI, ionospheric perturbations, sync pulses, notch beam processing, etc.) is obtained as soon as needed input data are available.
• All acquired data are processed and the generated L1A products can be provided at a data hub for user download.
• All acquired data are processed and the generated L1A products can be provided at a data hub for user download.
• Different polarization channels may be assessed individually by different application processors on different time scales (weeks / months / years).
• The applied framing concept supports systematic stack processing by allowing a consistent identification, processing and reuse of L1A products for the built-up of stacks and combined stacks as needed for higher-level product generation.
Systematic and on-demand L1B and L1C processings:
A systematic generation of L1B and L1C products shall be also possible, but has to be based on more sophisticated production rules since this shall only be done on a restricted basis. The restrictions are coming from selected applications and geographical areas. An example might be the systematic generation of MGD products over given coastal areas to support ship detection.
Mostly, the L1B and L1C generations are done on-demand. The L1C products which are stacks of SSCs or CoBiSSCs are important inputs for a higher-level product generation. The activation of such higher-level products generation results then in a demand for the generation of the needed L1C inputs based on archived or reprocessed L1A. Furthermore, all level 1 products may be requested by users leading to an on-demand processing.
Higher-level Products Generation
Deformation products deal with Earth's surface large or small scale deformation monitoring. These products are listed in Table 4. LOS (Line-Of-Sight) displacement maps are designed for monitoring single events such as earthquakes, landslides or for supervising volcanic activities. The PSI (Persistent Scatterer Interferometry) deformation database is intended to study urban subsidence but also landslides. Finally, the LOS deformation rates maps and 3D deformation rates maps (based on the stacked interferometric phases) are targeting long-term monitoring of tectonic deformation in inter-seismic periods.
The deformation processor delivers most of its outputs systematically but some LOS displacement maps have to be provided on-demand. Indeed, in case of earthquakes or landslides, a displacement map has to be generated based on available acquisitions (one before and one after the event).
Figure 15 depicts the product dependencies together with the main algorithmic steps for their generation. The PSI analyses and the displacement maps generation lines are independent whereas the three last products are dependent from each other. The products generation of these higher-level products starts from the Stacked Single-look-Slant-range Complex (StaSSC). The first step is dedicated to the interferometric phase estimation. Atmospheric and ionospheric disturbances are compensated. No phase unwrapping or geocoding is therefore performed. In a second step the displacement rate maps w.r.t. the radar LOS are generated. These products present already a measurement (in mm/y) of the geophysical phenomena and they are already mapped in a geographical raster nevertheless such measurements still require some knowledge of the radar geometry as far as they represent only the part of the motion in the LOS direction. As a last step,the 3D displacement rate vectors are derived by combining data acquired from ascending and descending orbits with different incident angles as well as in right- and left-looking configurations providing a product completely independent from the radar geometry.
A general issue in the higher-level products generation framework is the update of the products from year to year. Intermediate products storage has to be considered in order to avoid huge memory consumption and data transfer. Concepts and algorithmic implementations are being studied.
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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).