## 1 Introduction  
FORMAT-7/COSMIC-2 (Constellation Observing System for Meteorology, Ionosphere, and Climate-2 and Formosa Satellite Mission 7) (COSMIC-2) satellite mission was launched on 25 June 2019, started weather forecasting, monitoring and other tasks in February 2020, and completed all orbit adjustment work in May 2021 [1, 2]. This is a remote sensing satellite mission jointly developed by the National Space Organization (NSPO), Taiwan and the National Oceanic and Atmospheric Administration (NOAA), USA. It is a continuation of COSMIC-1 mainly used to collect atmospheric data for weather prediction, ionosphere and gravity research [3, 4]. COSMIC-2 constellation contains six satellites, the designed orbit inclination is 24 degrees with occultations primarily distributed from 45\({}^{\circ}\)N and 45\({}^{\circ}\)S, the designed life is 5 years, the initial orbit altitude is 750 km, and the final orbit is 525 km, and it takes about 100 min to fly around the earth per circle [1, 5]. In order to realize the scientific function of the constellation and provide the basis for scientific data, each satellite is equipped with the advanced Tri-GNSS radio occultation (RO) system (TGRS) receiver, and its two orbit determination antennas are POD + X and POD \(-\) X respectively. POD + X is located in the satellite flight direction, and POD \(-\) X is back in the satellite flight direction [6]. TGRS receiver can track the GPS signal and navigation signal of the GLONASS system, to provide data for the POD of COSMIC-2. At present, there are few studies on the POD of COSMIC-2. [PERSON] et al. (2020) evaluated the neutral atmosphere data quality of COSMIC-2 using the GPS occultation data [9]. [PERSON] (2021) realized the orbit determination of COSMIC-2 with the RD method, and its 3D orbit determination accuracy was within 10 cm [10]. [PERSON] et al. (2021) solved the orbit of COSMIC-2 with the RD method and found that the accuracy of the overlapping orbits is about 10 cm [11]. [PERSON] et al. (2014) carried out orbit determination for COSMIC-1, analyzed the satellite orbit and satellite clock offset, and the 3D orbit determination accuracy was about 10 cm [12]. [PERSON] et al. (2009) used RD and kinematics to solve the orbit and analyzed the overlapping orbit, the accuracy of the overlapping orbits of the two methods was almost at the same level, both of which were 2-3 cm [13]. [PERSON] et al. (2006) used the space-borne GPS data of a single antenna to realize the POD of COSMIC-1 by RD and kinematic methods, and the RMS of the average orbit difference between the two methods of satellite FM1-FM6 is 6 cm, and the RMS values of the overlap orbit difference and the orbit difference between the resolved and the reference are about 5 cm and 10 cm, respectively [14]. [PERSON] et al. (2008) applied GPS data of two antennas to determine the orbit of COSMIC-1, and the three-dimensional accuracy of the orbit was about 6 cm [15]. For the LEO satellite, the required orbit accuracy for COSMIC-2 occultations is about 30 cm [16]. TACC (Taiwan Analysis Center) determined the orbit of COSMIC-2 and used the overlapping orbit comparison method for orbit verification, and the satellite orbit position accuracy was below 15 cm, and the velocity accuracy of the satellite was below 0.15 mm/s ([[https://tacc.cwb.gov.tw/v2/trops_download.html](https://tacc.cwb.gov.tw/v2/trops_download.html)]([https://tacc.cwb.gov.tw/v2/trops_download.html](https://tacc.cwb.gov.tw/v2/trops_download.html))) (accessed on 12 March 2021). It can be seen that the RD method is mostly used for orbit determination of COSMIC satellite missions, and the 3D orbit accuracy is about 10 cm. There are mainly three POD methods such as kinematic, dynamic and RD methods. The kinematic method is based on PPP (precise point positioning) using GNSS data [17; 18; 19] and the dynamic one is mainly based on the dynamic models, while the RD method has combined the advantages of the first two methods. Therefore, we also apply the RD method to carry out POD for COSMIC-2.