## 3 Results
### Satellite Signature Effect of BDS  
In this section, we used the method detailed in Section 2 to process the orbit products from WU to analyze the signature effect. A threshold with the value of \(\pm\)0.5 m was set to eliminate gross errors in SLR residuals. While the POD strategies of different ACs differed, the signature effect remained consistent among them. Moreover, the precise orbit products released by WU for BDS were continuous, enabling the analysis of satellite signature effect through a wide distribution of SLR residuals. In consideration of the different systematic errors of three groups of stations, Figure 3 shows the number of observations and the RMS of the SLR residuals of the 34 stations listed in Table 2. Unlike other GNSSs, CSPAD and MCP stations provide more than 90% of the SLR observations for BDS, whereas a percentage of the observations are only tracked by PMT stations. This can be explained by the observation priority of SLR stations, while PMT stations concentrated in Russia and nearby countries mainly track GLONASS satellites. The number of SLR observations for MEO satellites is approximately the same for both CSPAD and MCP stations. However, for BDS-2 IGSO satellites, the number of CSPAD observations is three times greater than that of MCP observations. Several CSPAD stations located in China, e.g., Changchun and Shanghai, provide about half of CSPAD observations. The RMS of the SLR residuals from most CSPAD and MCP stations are similar and vary from 50 to 70 mm. The RMS of the residuals from four PMT stations i.e., Altay, Komsomolsk, Hartebeesthoek, and Riga, reached the same level as CSPAD and MCP stations, while the RMS of the residuals from some other PMT stations had a value lower than 40 mm. Despite having a smaller RMS value, the PMT stations with this characteristic had a limited number of SLR observations. As a result, the overall SLR residuals for these stations exceeded 80 mm in RMS. For the sake of accurately reflecting the satellite signature effect, it is necessary to exclude stations with limited observations. Therefore, we selected 20 stations according to the total number of observations from February 2017 to March 2021 with a threshold of 200. Among multi-photon stations, only two MCP stations, Yarragadee (7090) and Matera (7941), as well as two PMT stations, Altay (1879) and Komsomolsk (1868), could track BDS-2 IGSO satellites. Furthermore, each of the other three stations, except Yarragadee, had only a few hundred observations of BDS-2 IGSO. According to the installation method of LRAs, the nadir angle of SLR stations with respect to a satellite reference frame is equivalent to the incident angle of the laser beam on BDS satellites. To investigate the signature effect of multi-photon stations tracking BDS satellites, Figure 4 shows the variation in SLR residuals with respect to the nadir angle observed by Yarragadee. As a high-energy station, Yarragadee provides the majority of observations among the MCP and PMT stations, serving as a representative for multi-photon stations. The slope of COMPASS-I3, COMPASS-I5, COMPASS-I6B, COMPASS-M3, Beidou3-M2, and Beidou3-M9 with respect to the nadir angle was \(-\)4.24, \(-\)3.54, \(-\)3.21, \(-\)0.89, \(-\)1.23, and \(-\)1.07 mm/deg, respectively. The observed SLR residuals demonstrate a clear dependence on the nadir angle, caused by the laser pulses reflected from the nearer cube of the LRAs and a shorter collection time by the detectors. This explains why the slopes are negative values. Moreover, the slopes of IGSO satellites were larger than those of MEO satellites, which is attributed to the larger LRAs and the greater number of corner cubes deployed on IGSO satellites. Figure 3: The number and RMS of BDS SLR residuals tracked by 34 stations. In order to compare the systematic error between multi-photon and single-photon stations, we selected Shanghai, which is equipped with a single-photon detector, to compare it to Yarragadee.