# BeiDou Geostationary Satellite Code Bias Modeling Using Fengyun-3C Onboard Measurements
## 4 Conclusions  
Besides BDS-satellite-induced biases, onboard BDS code measurements from the Fengyun-3C GNOS receiver are subject to strong local MP effects, which result in a static pattern depending on frequency and the line-of-sight direction relative to the receiver antenna. There is a decreasing trend of MP in the static pattern from the signal ascending direction to the signal descending direction; its variation is larger than 1 m. Thus, removal of the local MP effects is necessary if the Fengyun-3C measurements are used to analyze the characteristics of BDS signals. We identified that there is no dependence of the local static pattern on BDS satellite type. Therefore, we estimated an elevation- and azimuth-dependent grid model for the static pattern together with the groups of the linear piece-wise parameters of the BDS-satellite-induced biases for each of the 2 BDS frequencies. The derived linear model values for BDS IGSO and MEO code biases agreed with those of [PERSON] and [PERSON]'s model even if the elevation angle falls below 40\({}^{\circ}\). Also, the linear model parameter values of BDS GEO satellites from 0\({}^{\circ}\) to 90\({}^{\circ}\) can be obtained at the same time, which is nearly impossible using ground-fixed measurements due to small movements of the GEO satellites relative to the ground receiver. Compared with other GEO satellite B1 model values, a different behavior really existed in C05 and a new group for C05 B1 was considered. Also, the derived B2 linear model values for the GEO and IGSO groups were so similar that we suggest they should be placed in the same category. Apart from an increasing deviation when the elevation angle was above 60\({}^{\circ}\), the difference between groups in the same frequency was less than 5 cm except for the C01 to C04 B1 group.  
To analyze the effect of these code systematic biases on the orbit determination of the Fengyun-3C satellite, we generated 2 single-frequency BDS orbit determination solutions using the nearly one-month onboard data set. There was a noticeable improvement in each direction when we applied  
Figure 14: Daily RMS of orbit differences between the dual-frequency GPS solution and the single-frequency BDS solutions before (**top**) and after (**middle**) application of the receiver- and satellite-induced code bias model corrections and the improvements (**bottom**) when using corrections.  
the GNOS-receiver- and BDS-satellite-induced code bias model corrections. In particular, the mean RMS in the along-track component decreased from 135.3 to 51.2 cm, which accounted for 78.8% of the 3D improvement from 150.6 to 56.3 cm. The result shows a substantial contribution from these corrections. However, due to BDS data quantity being less available and the relatively lower precision of BDS products, the single-frequency BDS orbit determination precision is still not in competition with that of GPS.  
We are very grateful to the iGMAS for providing the BDS precise orbit and clock products. This work is sponsored by the National Natural Science Foundation of China (Grant Nos. 41574027, 41574030, 41325015) and by the Hubei Province Natural Science Foundation of China (Grant No. 2015 CFA057).  
[PERSON] and [PERSON] initial idea; [PERSON] and [PERSON] designed and performed the data analysis and wrote the manuscript; [PERSON] and [PERSON] edited the manuscript [PERSON] helped with the data processing program. All authors reviewed the manuscript.  
The authors declare no conflict of interest.