## 4 Trends and Challenges
### Multidimensional
#### 4.1.3 Multi-Baseline  
On a single spaceborne platform, the resolution capability of InSAR is limited. However, by having multiple spaceborne platforms work in coordination, the spatial and temporal resolution of the images can be significantly enhanced, and the formation of satellite constellations can cover broader and more detailed geographical areas. Moreover, formation interferometry technology can dynamically adjust the number of platforms and sensor layouts based on specific mission requirements, and can carry multiple sensors to achieve multi-band monitoring and meet the monitoring needs of different scenarios, thereby enriching the data. In 2019, at the IGRASS conference, the ESA announced Harmony as a candidate for the Earth Observation 10 mission [97; 98]. The Harmony project plans to launch two SAR satellites weighing less than 500 kg each, carrying only receivers, to receive echoes from Sentinel-1D. Their orbital configuration will be divided into two phases, as shown in Figure 17. In the first phase, Harmony-A and Harmony-B satellites will form a cross-track  
Figure 16: NISAR satellite [image credit: courtesy of NASA/JPL-Caltech]. baseline and maintain it for over a year to study the seasonal changes in ice layers. In the second phase, B and A satellites will be distributed approximately 250 km ahead and behind Sentinel-1D, forming two pairs of along-track interferometric pairs to achieve a spatial resolution of 1 to 4 km and a sea surface velocity measurement accuracy of 0.15 m/s. China has made substantial progress in the development of satellite interferometric observation systems, particularly in formation flying and multi-satellite InSAR technologies. On 30 April 2019, the China TIANHUI-2 satellites were launched. The TIANHUI-2 can obtain InSAR data for the ground area between 75\({}^{\circ}\) S and 82\({}^{\circ}\) N latitude on the globe, mainly for the production of high-precision DEM, DSM, and radar orthophoto products [66; 99]. The InSAR altitude measurement mode of the fly-by formation is the main working mode of TIANHUI-2, as shown in Figure 18. During a single pass, two satellites can obtain the coherent echo data of the same area on the ground in strips, and the data are transmitted to the ground for InSAR altitude measurement processing [100]. In the early stage of orbiting, the two satellites operated in the follow-up mode, with an inter-satellite distance of about 30 km. During this period, the SAR performance indicators of each satellite were tested, and on 17 June, when the Sichuan Yibin earthquake occurred, the distance between the satellites was 5 km, and the satellite follow-up mode images of this area were obtained. DSM data were processed, as shown in Figure 19.  
Figure 17: Harmony formation configuration. Figure 18: TIANHUI-2 formation configuration. (**a**) Fly-around formation InSAR; (**b**) follow-up formation InSAR. On 9 July, the system formed a fly-by formation configuration, with an effective baseline of about 500 m. After power-on, InSAR data were obtained that night. The data were transmitted to the ground system, and using self-developed software, the first DSM mapping product was successfully processed at one attempt, as shown in Figure 20. The successful launch of the TIANHUI-2 satellite enabled China's microwave remote sensing to achieve a stepwise development from single-satellite observation to dual-satellite interferometric mapping, filling the gap in the field of distributed formation radar satellites in China, making China the second country in the world to have a microwave interferometric mapping satellite, and proposing for the first time an international method to solve the absolute ambiguity problem of interferometric phase through a system design of dual-frequency imaging, completely eliminating the dependence on control data. This marked a major breakthrough in the construction of China's aerospace surveying and mapping capabilities.