# The Development of a Spaceborne SAR Based on a Reflector Antenna
## 4 Challenges and Development Trends in Reflector Antenna-Based SAR Technology
### Emerging Techniques for Reflector Antenna-Based SAR
#### 4.1.1 Reflector Antennas with DBF  
To achieve wide-swath imaging, spaceborne SAR systems often require reduced antenna sizes in the range direction. However, the minimum antenna area constraint [63] limits size reduction, as decreasing the antenna size also reduces transmission gain, negatively impacting the signal-to-noise ratio (SNR). Additionally, broad beam formation in the range direction exacerbates range ambiguities [64]. Traditional DBF [65] mitigates these challenges by dividing the antenna into multiple sub-apertures in the range direction. During transmission, a single aperture generates a broad beam for wide-swath coverage, while each sub-aperture independently receives echoes from different ground regions. DBF then applies time-variant weighting to the received signals, effectively emulating a dynamic high-gain narrow beam for ground echo reception--a concept known as Scan-On-Receive (SCORE) [66; 67; 68]. Compared to conventional analog beamforming methods, DBF provides greater flexibility, precision, and multi-beam processing capabilities [69; 70]. This single-transmit, multi-receive mechanism successfully achieves the goal of HRWS SAR imaging. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**SAR System (Country, Year)** & **Feed Type** & **Reflector Structure Type** & **System Characteristics** \\ \hline SAR-Lupe (Germany, 2006–2008) & Single-Beam Feed & Rigid Reflector antenna & Compact rigid antenna, military imaging \\ TECSAR (Israel, 2008) & Multi-Beam Feeds & Radial Rib Deployable & Lightweight deployable \\  & & Antenna & reflector, 3.6 m aperture \\ RISAT-2 (India, 2009) & Multi-Beam Feeds & Radial Rib Deployable & 3.6 m diameter mesh reflector, Antenna & all-weather surveillance \\ HJ-1-C (China, 2012) & Multi-Beam Feeds & Framework Deployable & Dual-polarized X-band SAR, multi-mode operation \\ QPS-SAR (Japan, 2019) & Single-Beam Feed & Dual-Reflector Antenna & 3.6 m deployable reflector, lightweight small \\ Qilu-1 (China, 2021) & Single-Beam Feed & Framework Deployable & Ku-band, high-resolution lightweight SAR, total \\ Acadia (USA, 2023) & Single-Beam Feed & Perimeter Truss Deployable & 8 m\({}^{2}\) reflector, high revisit \\  & & Antenna & rate, commercial \\ HJ-2-E/F (China, upcoming) & Phased Array Feed & Framework Deployable & Small satellite \\  & & Antenna & phased array feed and \\ Tandem-L (Germany, upcoming) & Multi-Beam Feeds & Perimeter Truss Deployable & reflector antenna \\  & & Antenna & DBF-based reflector SAR, \\  & & & used for deformation and \\  & & Antenna & biomass monitoring \\  & & & Dual-frequency (L/S-band), \\  & & Antenna & 12 m deployable mesh, \\  & & & wide-swath InSAR \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of reflector antenna configurations in representative SAR systems. In recent years, the integration of reflector antennas with DBF has become a promising trend in spaceborne SAR systems. This combination leverages the high gain and efficiency of reflector antennas while incorporating DBF's beamforming flexibility and precision control, offering a viable solution for HRWS SAR systems [15; 70; 71; 72]. However, implementing DBF with a reflector antenna requires the use of PAF. Compared to planar phased array antennas, reflector antennas provide superior gain performance [70] and enable HRWS imaging. Unlike planar phased arrays, which directly receive signals, reflector antennas capture echoes via their feed elements after signal reflection. Figure 12(a) shows a typical reflector-based DBF SAR system architecture [73]. During transmission, all feed elements are activated to generate a broad beam, which illuminates a specific ground area. In reception, different ground regions correspond to specific feed elements, forming a wide-transmit, narrow-receive configuration. This mechanism is similar to the SCORE technique, wherein high-gain narrow sub-beams track ground echoes dynamically [74; 75; 70], as depicted in Figure 12(b).