# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 3 Synthetic Aperture Radar Satellite Missions
### Medium/Large Satellite Constellations
#### 3.1.3 TanDEM-X  
The TanDEM-X (TDX, 2007) mission is an extension of the TerraSAR-X (TSX, 2010) mission flying two identical satellites in a close formation to achieve a highly flexible SAR constellation. Following the previous mission name (TerraSAR-X), TanDEM-X means \"TerraSAR-X add-on for Digital Elevation Measurement,\" both of which were implemented through a Public-Private-Partnership of the German Aerospace center (DLR) and European Aeronautic Defense and Space (EADS) Company Astrium (now Airbus Defense and Space). The SAR payload of both TDX and TSX has flight heritage dating back to Shuttle missions such as SIR-X and SRTM, which tested the single-pass SAR interferometry (InSAR) in the X band [60]. The primary objective of the mission is digital elevation model (DEM) generation, for which bistatic processing is employed [61]; both spacecraft, less than 2 km apart as represented in Figure 9 (left), orient their SAR instruments to a common target, which provides different viewing angles [62]. Both spacecraft simultaneously receive echoes of signals transmitted by one of them, which minimizes temporal decorrelation. The resultant single-pass interferometry does not suffer accuracy loss, which would affect repeat-pass (multi-pass) interferometry with a single satellite. An example of bistatic InSAR is the TanDEM-X Forest/Non-Forest Map in which the global raw data set with 50 m \(\times\) 50 m ground independent pixels were used to classify forest and non-forest areas (urban settlements or freshwater), as shown in Figure 10[63]. The second interferometric mode is the pursuit monostatic mode, whose Tx/Rx operations are decoupled in two satellites as shown in Figure 9 (right). The amount of temporal decorrelation therein is still small compared to the repeat-pass counterpart. In addition to the aforementioned multistatic (bistatic or monostatic) baseline, the TDX mission concept demonstrated the along-track interferometry (ATI) with four inter- or intra-satellite phase centers. The ATI technique can be used to resolve the velocity of ocean currents, ice drift, and other on-ground objects (e.g., transportation traffic) [64; 65; 66]. The TSX spacecraft retains its sun-synchronous dawn-dusk orbit (LTAN = 18:00) with a 515 km altitude and a revisit period of 11 days with 167 orbits, or 15 and 2/11 revolutions per day. The TDX orbit uses eccentricity-inclination separation with respect to the TSX orbit, through which the two  
Figure 10: Examples of TanDEM-X Forest/Non-Forest map: (**left**) Germany. (**middle**) South America over Amazon Rainforest. (**right**) Amazon Rainforest, State of Rondonia, Brazil (zoomed-in). Source: DLR. Figure 9: Twin-satellite measurement modes of the TanDEM-X/TerraSAR-X mission: (**left**) Multistatic SAR in a bistatic or monostatic mode for digital elevation measurement. (**right**) Along-track interferometry for object movement detection. Source: DLR. spacecraft leave a helix-shaped trajectory relative to a virtual reference in between. The procedures for eccentricity-inclination separation and the relative trajectory are depicted in Figure 11. First, the ascending node of the TDX orbit plane is shifted by a small offset angle to provide horizontal distancing with maximum distance at equator crossings, and the subsequent eccentricity offset provides vertical (radial) distancing maximized at the poles. Small changes in the argument of perigee may also be made to increase inter-satellite distance in medium latitude regions. This kind of orbit and its building procedure have been proposed for consistent separation distance, which in turn reduces collision hazards evenly along the orbit [67; 68]. The remote sensing data obtained from TSX/TDX missions have been widely shared with and studied by the scientific community including the TanDEM-X Forest/Non-Forest Map available free of charge online [25; 69]. Differencing digital elevation models taken at different moments in time can be applied to monitor climate change, for example, in polar regions.