# BeiDou Geostationary Satellite Code Bias Modeling Using Fengyun-3C Onboard Measurements
## 3 Results
### The Effect on Single-Frequency BDS Orbit Determination of Fengyun-3C  
In previous sections, we analyzed the BDS MP effects based on the Fengyun-3C onboard data set, and found that the code measurements are not only subject to the BDS-satellite-induced and elevation-dependent systematic biases, but also suffer from the Fengyun-3C local near-field MP effects. Based on the analysis of the MP variations, we carefully modeled the BDS-satellite-induced biases of the corresponding group and the static GNOS-receiver-induced MP pattern. For dual-frequency precision orbit determination, the solution quality is mainly dependent on the carrier-phase observations but not the code measurement; thus the code biases can hardly affect the final orbit determination precision. However, code measurements can play an important role in single-frequency orbit determination, because in that case the graphic combination is used as a basic observation model, which enables the elimination of ionospheric delay but depends on code measurement precision [15; 16]. Based on a single-frequency GPS orbit determination, the 3D RMS of the gravity recovery and climate experiment (GRACE) is better than 0.2 m, and the Chinese HY-2A and ZY-3 satellites can reach the same level [17; 18]. In this section, to analyze the effect on the single-frequency BDS orbit determination of the Fengyun-3C satellite, we determined precise orbits with the onboard BDS data, and generated the orbit determination solutions with and without the model corrections using the positioning and navigation data analyst (PANDA) software package [19; 20] developed at the GNSS Research Center of Wuhan University. Because the Fengyun-3C satellite does not carry a satellite laser reflector, which is generally used to independently validate the accuracy of precise orbit products, we compared the results to the orbits using onboard dual-frequency GPS data provided by [PERSON] et al. [5]. The 3D RMS of overlapping orbit differences is better than 3 cm for a dual-frequency GPS solution. Therefore, the dual-frequency GPS orbit results can be seen as reference orbits to judge the accuracy of both the single-frequency solutions. Figure 13: An example of MP2 variations after application of the GNOS-receiver-induced (rec-ind) and BDS satellite-induced (sat-ind) bias corrections for different BDS satellite types. Also, we adopted the BDS precise orbit and 30-s clock products provided by Wuhan University for both BDS solutions. The qualities of BDS products were evaluated by [PERSON] et al. [21] and [PERSON] et al. [22]. The detailed strategy used for orbit determination solutions is summarized in Table 3.  
Figure 14 shows the daily RMSs of orbit differences between GPS and BDS solutions before and after application of the receiver- and satellite-induced code bias model corrections in along-track, cross-track, radial, and 3D respectively, and the improvement percentages. After applying the corrections, the RMS values are clearly improved in each direction. Particularly in the along-track component, the mean RMS is reduced from 135.3 to 51.2 cm with an improvement of 62.1%. The mean improvements in cross-track and radial components are 49.7% and 69.5% respectively. Among the improvements of the 3D mean RMS, which decrease from 150.6 to 56.3 cm, 78.8% comes from the along-track component, and 2.0% and 19.2% from the cross-track and radial respectively. The comparative analysis shows that the BDS-satellite- and GNOS-receiver-induced code biases seriously degrade the precision of Fengyun-3C orbits. However, the precision of the single-frequency BDS orbit determination is still not in competition with that of GPS. This is mainly because the available BDS data quantity is obviously less. Considering the Asia-Pacific regional service of BDS in 2015, only 3 MEO satellites were visible outside the Asia-Pacific region, and the number of GNOS positioning antenna tracking satellites is less than 4 at approximately two-thirds of the epochs, which typically contributes to poor positioning accuracy.