# Precise Orbit Determination of FY-3C with Calibration of Orbit Biases in BeiDou GEO Satellites
## 5 Conclusions  
The poor orbit accuracy of BeiDou GEO satellites is the major challenge to BDS-based LEO POD. In this study we proposed a GEO orbit bias in-flight calibration algorithm which mitigates the adverse impacts of the BeiDou GEO orbit biases on the LEO POD. When compared to the algorithm for neglecting GEO orbit biases completely and the algorithm for excluding the GEO roughly, the proposed algorithm enables the LEO POD to benefit from the BeiDou GEO observations. In our algorithm the along-track and cross-track GEO orbit biases are estimated as daily constants and the radial orbit biases are neglected. The efficiency of the algorithm is validated by one month of space-borne BDS observations from the FY-3C satellite. The FY-3C satellite allows us to conduct a highly-significant analysis of the FY-3C reduced dynamic POD using only BDS and the contribution of BeiDou GEO to POD with and without the calibration of orbit systematic errors.  
We investigated the effects and contribution of involving GEO in the POD of FY-3C by comparing three strategies: disregarding GEO orbit errors, excluding GEO, and calibrating GEO orbit biases. The obtained orbit using all observations from three types of BeiDou satellites shows an orbit precision of 0.57 m in average 3D RMS when compared to the GPS-derived orbit. Hence, it is not possible to achieve a centimeter level precise orbit with current orbit products of BeiDou GEO. A precision of 7.8 cm in 3D achieved for the FY-3C orbit demonstrates a significant improvement of 86.4% when the observations of GEO are rejected. More available observations can enhance the performance in the POD of FY-3C by including BeiDou GEO and the simultaneous calibration of systematic biases in GEO orbit products. The precision of the obtained orbit reaches an encouraging level of 6.2 cm in 3D, which represents a general improvement of 20.2% with respect to that obtained by excluding GEO. The results are further confirmed by the residual analysis according to the average daily RMS of the posteriori phase residuals are 45.8, 6.2, and 7.9 mm for the above three solutions, respectively. The RMS of DBDs in the recovered GEO orbits with daily orbit biases calibrated using FY-3C onboard BDS data are also used to assess the performance of the proposed method. If the abnormal DBDs in the calibrated orbits are removed, the average RMS of DBDs for GEO decrease to 0.29 m and 0.21 m in along-track and cross-track, respectively. The calibrated GEO orbits can be directly used to perform POD for other LEO satellites.  
The precise orbit of FY-3C is obtained using BDS data with estimation of daily systematic biases existing in GEO orbit products in this investigation. It remains a progressive achievement for the current BeiDou navigation constellation which provides service focused mainly on the Asia-Pacific region. There is no doubt that the performance of the BDS-based POD of LEO could reach a comparable level to GPS with more and more BeiDou satellites launched and gradually improving quality of BeiDou products.  
We are greatly appreciative to the GFZ and WHU analysis center of MGEX for providing BeiDou precise orbit and clock products. This research work is sponsored by the Natural Science Foundation of China (grant no. 41774035).  
[PERSON] conceived, designed, and performed the experiments; [PERSON] analyzed the onboard GPS and BDS data from FY-3C GNOS receiver; [PERSON] and [PERSON] contributed software packages for precision orbit determination of LEO satellites; and [PERSON] wrote the paper.  
The authors declare no conflict of interest.