## 4 Trends and Challenges
### Geosynchronous Orbit  
To address the issues of small imaging swath width and long revisit times of low-orbit SAR satellites in imaging the observation area, the Earth-Synchronous Synthetic Aperture Radar (GEO SAR) [113] emerged. By taking advantage of the orbital position of the Earth-Synchronous Orbit, the revisit period for the same location can be increased from the sub-day level of low-orbit SAR to the hour level, and the imaging swath width can be expanded from hundreds of kilometers of low-orbit SAR to thousands of kilometers. By improving the temporal resolution of the same observation scene, high-frame-rate SAR images, namely in the high-temporal mode, can be obtained, which extends the information to three dimensions. The additional temporal dimension provides more spatio-temporal degrees of freedom. It can provide strong support for the timely and effective implementation of disaster emergency responses.  
Figure 25: Image acquired by Copernicus Sentinel-1A on 1 November 2017 over the Brittany region, France, with VV intensity image, VH intensity image, and RGB color composite. Copernicus Sentinel data (2017) processed by CLS [89].  
The concept of high-orbit SAR satellites was first proposed in 1978 [114], and it remains a research hotspot to this day. NASA of the United States, Cranfield University of the United Kingdom [115], Politecnico di Milano of Italy [116], Vega Company of Russia, China Academy of Space Technology, and other research institutions [117; 118; 119] are all conducting research on high-orbit SAR.  
China was the first country to launch the GEO satellite, demonstrating a major breakthrough in high-orbit SAR technology. On 13 August 2023, China's LT-4(01) satellite was launched, becoming the world's first Earth-synchronous orbit SAR satellite [119], as shown in Figure 26. The synthetic aperture radar payload carried by the LT-4(01) satellite has advantages such as high resolution, wide coverage, multi-mode, and being lightweight. In addition to adopting synthetic aperture radar technology, this satellite has another feature, which is that its operating orbit is exceptionally high, about 40,000 km away from the Earth. The higher it flies, the farther it can see.  
The LT-4(01) satellite operates in an inclined geosynchronous orbit at a distance of nearly 40,000 km from the Earth. Satellites operating at this orbit resemble a repeatedly drawn \"8\" shape in the sky, enabling relatively continuous observations of a fixed large area, as shown in Figure 27. The observation revisit period is short and the imaging swath is wide. Compared with low-orbit satellites and optical satellites, the LT-4(01) satellite combines the advantages of short high-orbit observation revisit periods and wide imaging swaths with the advantages of microwave observation that is not limited by weather conditions (all-weather) and not limited by lighting conditions (all-time). This can improve the recognition accuracy and efficiency of abnormal changes in disaster information and enhance the comprehensive prevention and control capabilities for natural disasters. In the development of the LT-4(01) satellite project, an innovative scheme of a large-aperture annular reflector antenna and phased array feed was proposed for the first time and implemented engineering-wise [120]. The imaging maximum swath of the high-orbit SAR satellite can reach 3000 km. It has the ability to conduct a full-basin observation of potential flood disasters in the Yangtze River and Yellow River basins. Operating in the scanning mode (50 m/3000 km), it can complete the coverage of the entire country within 3 days and form a national map, as shown in Figure 28. The same efficacy can only be achieved by networking five low-orbit SAR satellites.  
Figure 26: LT-4(01) Satellite configuration [image credit: CAST].