## 3 Quality Assessment of the BIC/B2a Signals from BDS-3 Operational Satellites
### Differential ISB estimation between BDS-3/GPS/Galileo BIC-L1-E1 and B2a-L5-E5a signals
#### 4.3.2 Test Results  
Another short baseline experiment with different receiver types (iGMAS and M300 Pro) was conducted on 15 June 2018 (from 07:00 to 21:00 GPST) to investigate the impact of ISB correction on ambiguity resolution and position estimation. The baseline is also separated by approximately 3.85 m as previous experiment that conducted from 3-4 June 2018 for ISB estimation in Section 4.2, but the involved receivers have been rebooted between data collection of the two experiments. The observations were collected with 10 s sampling interval and 10\({}^{\circ}\) elevation cut-off angle. During data processing, the position estimation and integer ambiguity estimation is conducted epoch-by-epoch. The phase and code ISBs are corrected in advance with their estimated values in Section 4.2. Figure 8: Estimated ISBs between GPS/BDS-3 L1-B1C (**a**), L5-B2a (**b**), and Galileo/BDS-3 E1-B1C (**c**), E5a-B2a (**d**) for short baseline based on different receiver types (iGMAS and M300 Pro receivers). Figure 9 shows the number of satellites tracked above 10\({}^{\circ}\) and 20\({}^{\circ}\) elevation cut-off angles together with their corresponding Position Dilution of Precision (PDOP) series. We observe that the tracked GPS Block IIF and BDS-3 operational satellites above 10\({}^{\circ}\) and 20\({}^{\circ}\) elevation cut-off angles are no less than one over the time span, and the available Galileo satellites are two to seven. The average tracked GPS, Galileo, and BDS-3 operational satellites during the day are 13.1 and 11.6, while the corresponding average PDOPs are 1.60 and 2.05, respectively. Figures 10 and 11 show the epoch-by-epoch ADOP of LCM and TCM with single-frequency (L1-E1-BIC) and dual-frequency (L1-E1-BIC/L5-E5a-B2a) observations under 10\({}^{\circ}\) elevation cut-off angle for GPS/BDS-3 and GPS/Galileo/BDS-3 solutions, respectively. We find that the single-epoch ADOP time series of TCM is obviously smaller than LCM for both GPS/BDS-3 and GPS/Galileo/BDS-3 solutions. In the case of GPS/BDS-3 combination, the mean ADOP values decrease from 0.397, 0.260 to 0.284, 0.179 cycles for single-frequency and dual-frequency, respectively, while for GPS/Galileo/BDS-3 solutions, the values decrease from 0.221, 0.128 to 0.155, 0.093 cycles. The improvement is more significant with single-frequency observations due to its relatively weaker model strength. Figure 10: Ambiguity dilution of precision (ADOP) for GPS/BDS-3 single-frequency (**a**) and dual-frequency (**b**) solutions with 10\({}^{\circ}\) elevation cut-off angle. Figure 9: Number of satellites tracked and the corresponding PDOP series above 10\({}^{\circ}\) (**top panels**) and 20\({}^{\circ}\) (**bottom panels**) elevation cut-off angles. Figure 12 shows the epoch-by-epoch ratio values for LCM and TCM with single-frequency and dual-frequency observations under 10\({}^{\circ}\) elevation cut-off angle for GPS/BDS-3 (top panels) and GPS/Galileo/BDS-3 (bottom panels), respectively. The corresponding statistics is listed in Table 4. We find that the achieved ratio values of TCM are much higher than LCM. For example, the epochs with ratio values larger than 3.0 are increased from 1087, 3005 to 2114, 4015 with GPS/BDS-3 single-frequency and dual-frequency observations, respectively, while from 3108, 4800 to 4523, 4888 with GPS/Galileo/BDS-3 observations. Such results imply a higher empirical ambiguity resolution success rate for TCM.