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 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)
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.
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.
Table 1: Main geophysical products of Tandem-L
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.
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. 3)
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. 4)
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.
Table 2: Key parameters and performance figures of the Tandem-L satellites
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.
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)
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 . 5) Figure 5 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. 6)
Table 3: Key parameters of the L-SAR instrument
Figure 5: 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. 7)
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 .8) 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. 9) One example is the combination of displaced phase centers in azimuth with ScanSAR or TOPS mode (Figure 6 (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 6 (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 6 (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 6 (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. 5). 10)
Figure 6: 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)
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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 (firstname.lastname@example.org).