## 1 Introduction  
The TH-2 satellite system, which is the first distributed interferometric synthetic aperture radar (InSAR) formation system in China, was sent into space by the Long March-4B launch vehicle on 30 April 2019 [1, 2, 3]. This formation consists of two almost identical satellites, the TH-2A and TH-2B, which both fly in sun-synchronous orbits (527 km altitude, 97.45\({}^{\circ}\) inclination) with a relative distance of 700 m in 2019, as shown in Figure 1. To accomplish the missions of topographic mapping, deformation detection, scientific research, etc., the orbit and baseline accuracy requirements of the TH-2 are 1 m and 8 mm, respectively. The precise orbit determination (POD) of the TH-2 satellites using spaceborne Global Navigation Satellite System (GNSS) observations is a prerequisite for the successful implementation of the InSAR mission. As a crucial navigation system for positioning and timing, the potential of GPS in the POD processing for low Earth orbits (LEOs) was noted early in its development. The initial accuracy of the GPS-based POD results was only dozens of meters [4]. Currently, with the increasing accuracy of force models and tracking systems, the precision of the GPS-based orbits for LEOs such as GRACE [5], SWARM [6], TerraSAR-X and TanDEM-X [7] can be better than 5 cm [8]. The spaceborne GNSS receiver on the TH-2 satellite is able to track both the GPS and the BDS signals, which provides an alternative measurement for the POD. The second and third generation BDSs (the BDS2 and BDS3, respectively) were completed in 2012 and 2020. With the completion of the BDS, and the increasing number of LEOs with BDS receivers, many researchers have investigated the contribution of the BDS to LEO orbit determination. For example, [PERSON] et.al. [9] demonstrated that the orbit error of the BDS2-based POD is smaller than 10 cm for the FengYun-3C. [PERSON] et.al. [10] analyzed the real-time orbit determination of the FengYun-3C and concluded that the precision of the GPS and BDS2 (GC) combined solutions is better than that of the GPS-only. The research on the Fengyun-3D [11] and the China Seismo-Electromagnetic Satellite (CSES) [12] proves that the inclusion of BDS2 can improve the consistency of the POD solutions of LEOs. Therefore, the orbit determination of LEOs based on multi-GNSS data is promising in scientific research and commercial tasks. The inter-system bias (ISB) is an important parameter in multi-GNSS processing. Generally, there are two strategies to handle ISB. One is to estimate the independent receiver clock offsets for each GNSS [13], and the other is to introduce one constant ISB for the interval of processing [14]. Since the ISB is receiver-dependent [15], it is necessary to consider the characteristics of the receiver during multi-GNSS processing. The ISB is usually estimated as a constant for spaceborne receivers, e.g., the instruments on the Fengyun-3C, Fengyun-3D [16], and CSES [12]. However, the spaceborne receiver of the TH-2 satellite can provide three positioning modes, namely GPS-only, BDS2-only, as well as GC combined. Due to the difference of the time tag corrections for each positioning mode, the ISB processing strategy still needs to be further studied for the GC combined POD of the TH-2 satellites. In addition, the TH-2A satellite should perform three maneuvers per day due to the requirement of formation keeping. It is necessary to handle the maneuvers properly to avoid the precision degradation of the orbits around the thrust execution periods. There are several maneuver estimation approaches to improve the orbit quality during the maneuver periods, including instantaneous velocity pulses [17], constant thrust model [18], and piecewise linear and continuous accelerations [19], etc. By using these methods to estimate the maneuvers, centimeter-level precision orbits of the LEOs with maneuvers can be obtained based on global GPS data [20; 21].