# Precise Orbit Determination of FY-3C with Calibration of Orbit Biases in BeiDou GEO Satellites
## 1 Introduction  
Precise orbit determination (POD) of the low Earth orbiter (LEO) is the primary prerequisite for a wide variety of scientific research and engineering applications, such as estimation of Earth gravity field, ionosphere sounding, ocean altimetry, Earth magnetic field geolocation, crustal deformation monitoring, atmospheric occultation sounding, establishment and maintenance of the terrestrial reference frame, and so on. GPS has been an efficient, convenient, and widely-employed technology for LEO POD since the 1980s. It has been validated in practice through post-analysis of space-borne GPS data obtained by GRACE [1], Jason-1 [2], GOCE [3, 4], SWARM [5], and HY-2A [6] missions, etc. It has been demonstrated that centimeter level or even sub-centimeter radial orbit error is achievable by using accurate dual-frequency carrier-phase observations and satellite orbit dynamic models along with carefully-optimized empirical accelerations, which is also known as the reduced dynamic approach. In addition to GPS, a number of emerging global or regional satellite navigation systems are under development. The Chinese BeiDou navigation satellite system (BDS) has already provided excellent performance in precise positioning, estimating troposphere delay, and modelling Earth ionosphere, etc. principally in the Asian-Pacific region. As of July 2017, BeiDou consists of five geosynchronous Earth orbiters (GEO), six inclined geo-synchronization orbiters (IGSO), and three medium Earth orbiters (MEO). Furthermore, it will fully support global full-time and full-weather positioning, navigation, timing, and short message communication services until approximately 2020 [7]. The contributions of simulated and realistic onboard BDS observations on the LEO POD have already been investigated in some special aspects under different scenarios by a number of researchers. [PERSON] et al. [8; 9] presented the design overview of the navigation subsystem and the preliminary onboard performance of the space-borne BDS receiver experimental payload mounted on the LING QIAO satellite, which is the first Chinese LEO mobile communication experimental satellite. [PERSON] et al. [10] analyzed the initial basic performance of the BeiDou regional navigation system used in the LEO absolute and relative orbit determination. They simulated space-borne receiver observations with the broadcast ephemeris of the BDS first phase and fitted reference orbits of GRACE and Jason-2 with different altitudes of about 500 km and 1330 km, respectively. [PERSON] et al. [11] exhibited several qualitative results in analyzing satellite visibility, code multipath, ratio of cycle slips, post-orbit determination, single-point positioning, and real-time orbit determination, etc. by using one week of GPS and BDS tracking observations collected by the global navigation occultation sounder (GNOS) receiver equipped on the Chinese FengYun-3C (FY-3C) satellite [12; 13; 14]. They found that the differences in the average root mean square (RMS) of the 6-h orbit overlap indicated that the precision of the FY-3C orbit derived by GPS and BDS is 3.8 cm and 22.0 cm, respectively. The results of the experiment performed by [PERSON] et al. [15] also assessed and validated the POD performance of FY-3C using BDS observations during March 2015. The results indicated that the GPS derived precise orbit achieves a precision of 2.7 cm for the overlapping differences. The precision of the BDS derived orbit reaches 30.1 cm, and it can be improved to 8.4 cm by excluding the measurements from GEO satellites. [PERSON] et al. [16] also investigated the possibility and potentiality of augmenting the BeiDou precise orbit and clock products using the FY-3C onboard GPS and BDS observations. They found the precision of the FY-3C orbit using BDS observations is about 15.8 cm and is 11.7 cm with BeiDou products derived only by ground data and integrated ground and onboard data, respectively.