## 4 Trends and Challenges  
### Multidimensional  
#### 4.1.1 Multi-Satellite Networking  
Since the launch of the first satellite, Sentinel-1A, in April 2014, the Sentinel-1 series has played a significant role in the field of Earth observation. In 2016, the second satellite, Sentinel-1B, was successfully launched, further enhancing the mission's capabilities. However, Sentinel-1B ended its mission early in August 2022 due to a technical glitch that prevented it from continuing to acquire data. The third Sentinel-1 satellite (Sentinel-1C) was successfully launched on 5 December 2024 by a Vega-C rocket from the European Spaceport in French Guiana. Equipped with the most advanced C-band SAR instrument, Sentinel-1C is capable of providing high-resolution images of the Earth's surface at all times and in all weather conditions, supporting a variety of critical applications such as environmental management, disaster response, and climate change research. In addition, Sentinel-1C introduces an automatic identification system (AIS) to enhance maritime traffic detection and monitoring capabilities, enabling accurate tracking of a ship's identity, location, and direction of travel. A dual satellite constellation (consisting of two identical satellites in the  
Figure 14: Schematic diagram of GF-3 ATI mode.  
Figure 13: The measurement results of ocean surface currents by the ATI mode of the GF-3 satellite.  
same orbit but 180\({}^{\circ}\) apart) can be reformed to ensure the continuity and efficiency of Earth observation data [89].  
A large number of studies have shown that Sentinel-1 can continuously and effectively monitor surface deformation. Figure 15 shows Sentinel-1C's ability to monitor ice cover and environmental changes in harsh remote areas [90].  
In terms of China, we have progressed greatly in the constellation development of SAR satellites. The first notable example is GF-3. Following the successful launch and operation of GF-3 in 2016, GF-3B and GF-3C were launched in November 2021 and April 2022, respectively, marking the establishment of China's first SAR satellite constellation. Both GF-3B and GF-3C completed their in-orbit commissioning in February 2023 and entered the operational phase. The new satellites introduced several system upgrades, including the integration of an Automatic Identification System (AIS) receiver for ship tracking, onboard real-time data processing, additional imaging modes, an increased maximum duty cycle, and enhanced support for maritime monitoring and InSAR applications. The coordinated operation of the three satellites significantly improves data revisit frequency and response time, while also enriching data diversity, thereby providing strong support for applications such as marine environment monitoring, disaster response, and deformation mapping [18].  
Existing research has demonstrated that the coordinated operation of the GF-3 constellation exhibits improved interferometric performance. For instance, an interferometric pair between GF-3 and GF-3B with a short revisit interval of six days, despite a large perpendicular baseline, was able to produce a coherent interferogram. This capability highlights the advantages and potential of the GF-3 constellation for high-frequency, all-weather, and long-term SAR observation missions [19].  
In addition to GF-3, recently launched missions such as LT-1 and PIESAT have also been developed as constellations. The success of these missions marks the development and advancement of China's SAR satellite programs.