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° 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, Li et.al. [
9] demonstrated that the orbit error of the BDS2-based POD is smaller than 10 cm for the FengYun-3C. Xiong 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 maneuvering 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]. However, the maneuver estimation based on the spaceborne BDS2 data still needs to be assessed due to the unbalanced distribution of the BDS2 satellites.
In this study, we analyze the orbit precision and the maneuver estimation results based on dual-frequency GPS and BDS2 observations from the TH-2. First, we introduce the spaceborne receiver on the TH-2 and assess its tracking ability, as well as its data quality. Next, the maneuver estimation by the POD using the spaceborne GPS and BDS2 observations is assessed. Furthermore, we analyze the performance of the GPS, BDS2, and GC combined POD of the TH-2. Finally, the conclusions are presented.
3. POD Strategy
Table 1 shows the overview of the observation and dynamic models used in the POD of the TH-2. The GPS, BDS2, and GC combined POD are implemented by the reduced-dynamic orbit determination approach [
30] and the experiments are carried out in the National University of Defense Technology orbit determination toolkit (NUDTTK) software [
21,
31]. The position and velocity
of the LEOs are integrated by an 11th-order Adams–Cowell method [
32,
33] with a 10 s step-size:
where
is the initial epoch and
denotes the accelerations of the gravitational and nongravitational perturbations acting on the satellites. During the test periods, the TH-2A performed three maneuvers per day in the along-track direction, whose durations vary from 1 to 5 s. In the POD processing, maneuver accelerations
are considered as constants over the predefined thrust durations and estimated together with the initial state vector, receiver clock offsets, ambiguities, as well as dynamical relations by the batch least-square estimator. The velocity change for each maneuver can be obtained by
, where Δ
t is the thrust duration. Since irregular thruster operation times may cause the intervals of the Adam–Cowell integrator to not match with the maneuver durations, we adopt an eighth order single-step Runge–Kutta method [
34] with a 0.001 s step-size to increase the integration accuracy in the vicinities of the maneuvers. Specific mathematic formulas can be found in the strategy of the NUDTTK [
21]. Since the impact of the receiver phase center variation (PCV) is non-negligible in the POD processing [
6], we generate the PCV maps by the residual method [
35] based on in-flight data and apply them in the GPS, BDS2, as well as GC combined POD. With respect to the GC combined processing, due to the fact that the current precision of the orbit and clock products for BDS2 is weaker than GPS [
36,
37], the weights of the GPS and BDS2 observations are set to 1 and 0.5, respectively. Meanwhile, the independent epoch-wise receiver clock offsets are introduced for each system because the time tag corrections for the GPS measurements are different than those for the BDS2. Detailed explanations are illustrated in
Section 4.3.
5. Discussion
Orbital maneuvers can change the accelerations of the LEOs and therefore have an important impact on the accuracy of LEO orbits. Our results show that the maneuver handling based on the spaceborne BDS2 data can eliminate the influence of thrusts to the TH-2 orbits, and the accuracy of the BDS2-based orbits of the TH-2 is at the same level as other missions, such as the Fengyun-3C [
9], Fengyun-3D [
11], and CSES [
12]. After the correction of the pseudorange data, the RMS of the pseudorange multipath for BDS2 is less than that for GPS. Therefore, the BDS2 pseudorange measurements exhibit a smaller PC RMS than the GPS. The LC RMS of the BDS2-based POD is larger than that of the GPS for the TH-2A without maneuver estimation, which may be attributed to the lower precision of the BDS2-based orbits. By using the observations from only six BDS2 IGSOs and three MEOs, the maneuver estimations are on par with those based on global GPS. With the completion of the BDS3, more BDS3 satellites can be used for the POD of LEOs and the further investigation of maneuver estimation. Meanwhile, considering that the BDS3 orbit and clock products are significantly improved, more research is needed on the contribution of BDS3 to the POD of LEOs.
In the GPS and BDS2 combined processing, due to the differences of the time tag corrections for the GPS and BDS data, we found that it is necessary to estimate the independent receiver clock offsets for each system. The results show that the GPS and BDS2 combined SPP and POD solutions have better precision than the corresponding GPS-only results. However, the combined POD solutions are only assessed by the observation residuals and overlap comparisons, which are both internal evaluation methods. If independent external validations are performed in the future, like satellite laser ranging measurements, it would be good for orbit quality evaluation. In addition, the ISB is usually estimated as a constant in the previous combined POD of LEOs. Such a strategy is effective for GNSS receivers using both the GPS and BDS2 to perform in-orbit positioning [
16]. For the GNSS receiver under a single GNSS positioning mode, if the time tag corrections for the different system are inconsistent, the independent clock offsets for each system should be estimated in combined processing.
6. Conclusions
We analyzed the orbit determination and maneuver estimation of the TH-2 satellite system based on spaceborne GPS and BDS2 observations. The analysis of the data quality shows that the spaceborne receiver of the TH-2 has a stable data tracking ability and a good observation quality.
The assessments of the maneuver handling show that the residuals of the POD for the maneuvering TH-2A are comparable to those of the maneuver-free TH-2B. Maneuver handling can avoid the precision degradation of the GPS- and BDS2-based POD caused by thrusts. The GPS-based POD solutions have good accuracy, which are better than 0.7 cm in 3D RMS. The orbit precision of the BDS2-based POD is smaller than 8 cm compared with the GPS-based orbits. This result indicates that the precision of the BDS2-based orbits achieves the subdecimeter level and satisfies the accuracy requirement of 1 m for the TH-2. In addition, the relative error of velocity changes between the GPS- and BDS2-based POD is less than 7.0%, which indicates that the maneuver estimation by using regional BDS2 data can be on par with using global GPS data.
In the fusion of the GPS and BDS observations, we found that the independent receiver clock offsets need to be estimated for each system due to the different time tag corrections for the GPS and BDS2 data obtained by the TH-2. Compared with the GPS-only solutions, the precision of the GPS and BDS2 combined SPP solutions is improved by 12–14% when the PDOP values of GPS exceed three. For the GPS and BDS2 combined POD, the 3D RMS of the overlap comparison is 0.64–0.65 cm. Nevertheless, due to the weaker consistency of the BDS2 precise ephemerides, the orbit precision of the combined POD is only improved by 3–4% compared with the GPS-only orbits.