# Precise Orbit Determination of FY-3C with Calibration of Orbit Biases in BeiDou GEO Satellites
## 2 BeiDou Orbit Error Analysis and Mitigation
### Characteristics of BeiDou GEO Orbit Errors  
The present orbits of GEO produced by using only ground tracking network always display evident systematic biases with respect to the true orbits. This is because the range errors caused by the time-invariant part of orbit errors projected onto the signal propagation direction cannot be separated completely from the one-way float phase ambiguity parameters, which are due to the nearly stationary relative geometric structures between GEO and the ground stations in one arc segment of the POD. However, it is fortunate that the systematic biases in the along-track, cross-track, and radial components of the orbital reference frame could be mostly constants if the GEO is observed continuously by evenly-distributed ground stations.  
Another practical method to determine the BeiDou orbit accuracy is by comparing orbits among various BeiDou products generated by several independent ACs of MGEX. The properties in the orbit difference time series can represent the features of BeiDou GEO orbit determination to a certain degree. Figure 1 shows the orbit difference series for BeiDou C02 between the GBM and WUM [24] products which are considered at the same precision level during July 2017. The relatively significant and separate short lines shows the orbit differences in along-track and cross-track during one day. The feature of C02 orbit differences can support the assumption that the distinguishing constant offsets with slight variations occur among single-day solutions published by GFZ, WUM, and the other ACs. All of these solutions adopt a similar strategy for BeiDou orbit determination using only ground station data.  
Daily jumps in the along-track and cross-track orbit differences are irregular and can reach a level of 10 m. The daily orbit biases and DBDs are quite limited in the radial component of GEO orbits since the radial component of the orbits is tightly constrained by the accurate Earth gravity, which focuses on the gravitational direction from the satellite to Earth center. Orbit radial differences between GBM and WUM products represent a precision at the decimeter level and are not showed as daily constant offsets. It seems that orbit errors in the radial direction are greatly limited and the orbit biases most likely do not exist. Nevertheless, obvious biases in the radial component of GEO orbit products can be demonstrated by analyzing and assessing the residuals of independent satellite laser ranging (SLR) measurements [23; 24]. Hence, the time period of POD for LEO should be limited to one daily arc. This avoids the situation where one continuous tracking arc containing the daily boundaries will be interrupted by interpolating the GEO satellite position at the time of signal sending.  
Figure 1: Orbit difference series in along-track, cross-track, and radial components with 15 min sampling interval for BeiDou C02 between the GBM and WUM orbit products during July 2017. Blue dots represent along-track, red dots represent cross-track, and yellow dots represent radial.