## 4 Trends and Challenges
### Multidimensional
#### 4.1.3 Multi-Baseline
Its all-weather data acquisition capability, fast data processing speed, etc., allow it to overcome the limitation that optical mapping satellites cannot obtain data in areas with perennial cloudy and rainy weather. Moreover, the rapidly generated DSM data can be directly used for the production of topographic maps and orthophotos of optical mapping satellites. This will greatly accelerate the speed of China's global geographic spatial information construction [101]. The TIANHUI-2 satellite system adopts a dual-satellite inter-satellite fly-by formation and a one-launch dual-reception interferometric technology system, operating in a Sun-synchronous orbit at an altitude of 500 km. The in-orbit test results show that the system is in good operating condition, and its main performance indicators are superior to the design indicators. Its positioning accuracy is comparable to that of the German TDX system and can be used for the production of 1:50,000 scale geographic spatial information products. On 30 March 2023, China PIESAT-1 satellite was launched. It is formed of a \(1+3\) wheel interference formation. Its main tasks are global land surveying and mapping, imaging observation, deformation monitoring, and elevation measurement. The elevation measurement accuracy reaches relative elevation accuracy (CE90) \(\leq 3\) m (flat land, hilly land), 7 m  
Figure 19: DSM data in follow-up mode. (mountainous land, highland land), and absolute elevation accuracy (CE90) \(\leq\) 7 m. The mapping accuracy is better than 1:50,000, and the deformation accuracy is 3-5 mm/year. As shown in Figure 21, PIESAT Information Technology Co., Ltd., Beijing, China, provided the \"PIESAT-1\" satellite constellation remote sensing image product set in 2024 [102]. On 9 November 2024 and 17 December 2024, four satellites were launched in one rocket in the form of one satellite per group, forming a 90-degree equip phase in-orbit formation, constituting a 1 \(\times\) 4 Walker constellation system. Among them, one satellite serves as the backup satellite for the \"PIESAT-1\" main satellite and operates in the main satellite's orbit, with technical status basically consistent with the main satellite. The PIESAT-2 constellation provides single-satellite high-resolution imaging, multi-satellite high-precision interferometric measurement, on-board data processing and autonomous planning, and emergency communication and rapid response, as well as continuous and stable data production capabilities. It mainly conducts observation tasks such as global regional regular coverage, rapid revisit of specific areas, high-frequency monitoring of key targets, and emergency call for special scenarios, supporting and serving application demands in fields such as land resources, earthquake, disaster prevention and mitigation, acquisition of basic geographic information, and forestry. The satellites have played an important role in multiple emergency events such as monitoring of the rainstorm and flood disasters in the Beijing-Tianjin-Hebei region in 2023, monitoring of the breach of Dongting Lake in Huarong County, Hunan Province, monitoring of rare heavy rain in Huluado, Liaoning Province, monitoring of forest fires and damage in the Abei Forest Farm of the Inner Mongolia Forestry Industry, and monitoring of the collapse of the Meida Expressway in Taubao, Guangdong Province. As shown in Figure 22, they have supported the monitoring  
Figure 20: DSM data in fly-around mode. of various events more than a hundred times and generated over 500,000 standard image data, covering the entire country, such as Laos. Based on the DSM products, DEM products are generated. The terrain features of the DEM products are clearly visible. The terrain details of mountains, rivers, and flatlands are very distinct. Moreover, the high-precision DEM products can ensure the accuracy of terrain analysis and the precision of hydrological modeling, providing reliable data support for various industries' analyses.