# BeiDou Geostationary Satellite Code Bias Modeling Using Fengyun-3C Onboard Measurements
## 3 Results
### Satellite- and Receiver-Induced Code Biases and Modeling  
To find out whether there are some other biases in the BDS code measurements, we followed the viewpoint outlined in [PERSON] et al. (2014). MP effects are due to diffuse reflections at the Fengyun-3C body, which results in a static MP pattern that depends only on the line-of-sight direction relative to the GNOS positioning antenna. We thus combined the 26-day MP time series to obtain a good observation sky coverage and estimated the average map on a grid of 2\({}^{\circ}\)\(\times\) 2\({}^{\circ}\) resolution in the satellite reference frame for each BDS satellite type. The elevation angles varied from 0\({}^{\circ}\) to 90\({}^{\circ}\), as shown in Figure 6, and for comparison, Figure 7 also shows the average MP grid maps of GPS L1 and L2 frequencies. The zero-direction of the zenith antenna pointed along the positive x axis. The satellite reference frame is defined as [5]: the origin of the coordinate system coincides with the Fengyun-3C satellite's center of mass; the x axis points along the satellite velocity direction, the z axis points toward the center of the Earth, and the y axis completes the right-hand system. In the Figures, the white areas are where there were no available measurements from the signals of the ascending satellites. It is easy to see that the distribution of MPs was not strictly symmetric in azimuth, and some near-field variations with interference fringes are clearly visible. Although it is not clear exactly what caused these interference fringes, a likely cause of the perturbations was cross-talk between the different antenna paths in the GNOS receiver; that is, the signal from one of the occultation antennas leaked into the signal from the positioning antenna. Moreover, when comparing the positive and negative directions of the x axis at the same elevation angles, particularly for elevations below 40\({}^{\circ}\), the biases in the signal of ascending satellites are much larger. In Figure 7, similar perturbations are also obvious for GPS, which is evidence that the near-field effect is not caused by biases in BDS satellites. Figure 5: Elevation-dependent MP linear piece-wise models for each BDS satellite from elevation angle 0\({}^{\circ}\) to 90\({}^{\circ}\) range. For comparison, the 2 blue lines in the IGSO and MEO panels represent [PERSON]’s model. Figure 8 shows the subtraction between different BDS satellite type grids in the same BDS frequency, i.e., that IGSO-MEO represents IGSO grid values minus these of MEO in the same frequency. Clearly, the azimuth-dependent variations are obviously eliminated or reduced after subtraction between any 2 BDS satellite type grids. This indicates that there may be some azimuth-dependent systematic biases that were unrelated to BDS satellite types in the onboard BDS measurements. Moreover, according to [PERSON] and [PERSON] [8], there should be no substantial azimuth dependence of the BDS satellite-induced biases. Hence, we consider that these biases should have been caused by local MP effects, and not by biases in BDS satellites, which has been demonstrated from the GPS grid maps in Figure 7. This also explains the aforementioned systematic inconsistency under lower elevation angles for IGSO and MEO satellites, when computing the linear piece-wise parameter values regardless of uncalibrated receiver-induced MP effects in the Fengyun-3C environment. To further investigate the static receiver-induced MP pattern in the Fengyun-3C environment, we adopted the model correction values given by [PERSON] and [PERSON] [8] to correct the BDS code biases in IGSO and MEO code measurements.