**4. Technical features and development of spaceborne SAR parabolic phased array antenna**

TanDEM-L is a spaceborne distributed SAR satellite plan proposed by Deutsches Zentrum für Luft-und Raumfahrt (DLR). The plan consists of two L-band spaceborne SAR systems. The schematic diagram of the concept is shown in **Figure 14**. The program was developed to monitor dynamic changes on the Earth's surface such as forest biomass, millimeter-scale surface deformation, polar ice, and soil moisture. In the TanDEM-L plan, the inversion of vegetation parameters, stereo imaging, and surface deformation monitoring technology are realized through polarization interference, multi-baseline, and heavy orbit technology, which are helpful for a better understanding of the earth system, risk analysis, disaster management, and environmental monitoring are of great significance (**Figure 14**) [57].

**Figure 14.** *Preview of TanDEM-L Plan.*

**Figure 15.** *Example of Elevation multibeam reflector SAR system.*

Technologies such as parabolic reflector antenna, DBF technology (as shown in **Figure 15**), and variable PRF technology are used in the TanDEM-L satellite program. Through the combination of these technologies, TanDEM-L plans to achieve 3 m resolution, 350 km wide Earth observation, and the ability to realize a dynamic observation of the world in 6 days. Among these technologies, the combination of reflector antenna and DBF technology is used to achieve high gain acceptance through distance scanning and improve the system signal-to-noise ratio; variable PRF technology changes the position of the blind spot to achieve continuous imaging of the scene [58].
