# Orbit Determination Method for BDS-3 MEO Satellites Based on Multi-Source Observation Links
## 1 Introduction  
The BeiDoub-3 navigation satellite system (BDS-3) has officially provided positioning, navigation and timing (PNT) services to global users since 31 July 2020. Compared with the BDS-2 [1, 2], it has realized the leap from providing a regional service to a global service. The progress from the Beidou-2 regional service to the Beidou-3 global service is inseparable from the engineering implementation of inter-satellite link technology [3]. The BDS-3 possesses a dynamic wireless network with precision measurement and data transmission functions, which can realize the rapid distribution and return of informationthroughout the whole constellation. The inbound rate of the effective data within inter-satellite planning can reach about 95% [4], and the average age of data for the BDS-3 MEO satellite is 1.07 h [5; 6]. Due to the inter-satellite ranging error being better than 10 cm [7], it can effectively support the improvement of orbit determination and the long-time prediction of satellite clock parameters [8]. Therefore, inter-satellite links have become a significant supplement for satellite-to-ground observation links. As the concept of comprehensive PNT are proposed [9], LEO satellites are also proposed to supplement the satellite-to-ground observation links of the global navigation satellite system (GNSS). Research on the use of LEO satellites as an augmentation system started earlier in the United States. The high-integrity global positioning system (iGPS) was the earliest navigation and timing system, using both LEO satellites and MEO satellites. The MEO satellites of the global positioning system (GPS) provide high-precision timing and positioning services for Iridium satellites in low Earth orbits. The Iridium satellites carry GNSS receivers and support high-precision orbit determination and time synchronization for MEO satellites. Due to the coverage limitation of the Iridium satellites, the positioning accuracy is only 20 to 50 m. In recent years, more and more programs for LEO augmentation constellations have been proposed. The European Union has carried out research on the Kepler system, which consists of the Galileo navigation satellite system and 4-6 LEO satellites, to achieve centimeter-level augmentation positioning accuracy [10; 11]. LEO satellites equipped with GPS single-frequency receivers, such as LANDSAT-4 and LANDSAT-5, have not realized high-precision orbit determination. In 1992, the TOPEX/POSEIDON satellite, equipped with a dual-frequency GPS receiver, achieved great success, as its orbit determination accuracy was better than 4 cm [12]. Since then, various research works on LEO orbit determination were carried out for the CHAMP satellite, GRACE satellite, Jason satellite and other LEO satellites [13; 14; 15; 16], verifying that the LEO satellites initially had the conditions to be used as high-dynamic monitoring stations. With the launch of the LEO satellites in China, such as \"Feng Yun\", \"Luojia\" and \"CentiSpace Constellation\", research on the orbit augmentation determination of BDS satellites with the support of the satellite-based observation data has been gradually carried out. [PERSON]. [17] and [PERSON]. [18] conducted preliminary augmentation tests for BDS-2 satellite orbit determination, with the support of the satellite-based observation data of the Feng Yun-3C (FY3C) satellite. The test results showed that the orbit accuracy of the BDS-2 satellite could be effectively improved. [PERSON]. [19] determined that the orbit accuracy of BDS-3 satellites could be improved by 92% with the augmentation of the \"Hong Yan\" satellites. With these studies, the feasibility and value of the engineering applications for LEO satellites as satellite-based monitoring stations has been gradually verified.