# Precise Orbit Determination of FY-3C with Calibration of Orbit Biases in BeiDou GEO Satellites
## 4 Discussion  
The reason that systematic biases frequently appear in BeiDou GEO orbit products can be explained by diagnosing the almost unchanged geometric observation condition in the current BeiDou POD strategy which is based purely on the ground tracking station network. Effects of constant offsets in GEO orbits can also be eliminated in the PPP of low kinematic stations in accordance with the same idea. However, the effects in the POD of LEO cannot be removed directly because of varying range errors along with the rapidly changing of line-of-sight between LEO and GEO. We attempt to estimate the orbit systematic biases of GEO in orbital reference frame as well as the receiver clock errors and the phase ambiguity parameters by using onboard BDS measurements when fixing the GPS-derived precise orbit of FY-3C. The results indicate that in-flight calibration of systematic biases in GEO orbit products is feasible and effective in view of the posteriori phase observation residuals, which reduce from decimeter level to several centimeters under the constraints of reference precise FY-3C orbit.  
Even though the results in this article exhibit a positive contribution from involving GEO on FY-3C POD using BDS observations, there are still several associated aspects that need to be studied with more detailed investigations. Characteristics of systematic biases in BeiDou GEO products should be modeled carefully using comprehensive patterns instead of the simple daily constants in along-track and cross-track currently adopted. The daily jumps in the GEO orbit systematic biases cannot be neglected since they are caused by the orbit DBDs and can reach several meters. Moreover, they should be taken into a special consideration so it is possible to realize the POD for a long arc beyond 24 h and  
Figure 10: RMS of DBDs in along-track and cross-track for each BeiDou GEO satellite of the original fitted orbits and calibrated orbits with biases correction, top part of the figure denotes the RMS of all DBDs participate in statistics, the bottom part denotes the RMS by excluding the DBDs with more than 1 m in along-track or cross-track for the calibrated orbits.  
obtain more reasonable reduced dynamic orbit solution. Estimated frequencies, priori constraints, and mathematical models, etc. of the coefficients associated with non-conservative forces and empirical accelerations deserve a thorough optimization to reach a favorable balance between the observation level with current BeiDou products and LEO orbit dynamics. Estimation of the variance component for observations from different BeiDou satellite types, such as GEO, IGSO, and GEO may probably perform an important function in the POD relative to the fully-equivalent weight for all observations. The PCV map in BeiDou signal frequencies of the positioning antenna connected to the FY-3C GNOS receiver should be calibrated using onboard BDS observations along with the POD process in order to achieve a better orbit solution.  
The well-performed processing strategy of calibrating daily orbit biases that we recommend for FY-3C POD using BDS observations does not only apply to BeiDou GEO, but also to any other satellites which have comparable systematic errors in orbits. At the same time, the method used to absorb orbit errors currently existing in BeiDou GEO is temporary at this stage because GEO products are produced using only static ground stations. Calibration of GEO orbit biases in LEO POD will be no longer needed if the GEO products are enhanced using observations collected by highly-kinematic stations such as the LEO, vessel, balloon, airship, plane, and so forth. It would be a tremendous opportunity and challenge for the integrated POD of BeiDou and other navigation satellite constellations using all available ground, space-borne, and onboard data from multiple sensors and multiple platforms. We will investigate more interesting and meaningful analysis in the not-so-distant future.