Consistency of BDS-3 transmit antenna phase center offsets

Information about satellite antenna phase center offsets (PCOs) is crucial for high-precision applications of global navigation satellite systems. Pre-launch manufacturer calibrations of the PCOs are available for all individual BDS satellites and three frequencies: B1, B2, and B3. With the imminent retirement of BDS-2 and the superior quality of B1C/B2a signals, the International GNSS Service (IGS) has launched a campaign for BDS-3 and QZSS phase center correction (PCC) calibration. However, only the IF combination of B1C/B2a is required. Considering the upcoming multi-GNSS and multi-frequency applications, the consistency of PCO values across different frequencies and GNSS is important.

We present the estimation of BDS-3 satellite antenna PCCs consistent with the International Terrestrial Reference Frame (ITRF2020) from the ionosphere-free linear combinations of B1I/B3I and B1C/B2a. The results demonstrate that the nadir-angle-dependent phase center variations (PCVs) of the same satellite block type have fairly good consistency. The mean horizontal PCOs of the different frequencies agree at the millimeter level. The X-PCOs show a bias of about 1 cm compared to the manufacturer calibrations, whereas the Y-PCOs are free of such bias. For Z-PCOs, some SECM satellites exhibit unexpected long-term variations and/or abrupt jumps, while all CAST satellites remain stable. The mean estimated values of B1C/B2a and B1I/B3I for BDS-3M-CAST, BDS-3M-SECM, and BDS-3I-CAST are (-15.9, -13.5), (-17.8, -3.0), and (+26.9, +24.0) cm larger than the CSNO values, respectively.

By applying the estimated PCCs instead of the CSNO values, the precise orbit precision can be improved by 3.6-12.2%, and the scale factors determined by BDS solutions exhibit good consistency with the IGS20 frame, with mean scale differences below 0.10 ppb for both frequency combinations.

Furthermore, CSNO values for SECM satellites are excluded from delivering the TRF scale by BDS-3 as they still remain questionable. Although the scale delivered by GPS and Galileo shows good consistency, with +0.84 and +0.73 ppb compared to ITRF2020, the scale difference of BDS-3 with respect to ITRF2020 is -0.84 and -0.92 for B1C/B2a and B1I/B3I. These results demonstrate the consistency of different frequencies for BDS-3 but also highlight the discrepancy within GNSS, specifically between BDS-3 and GPS/Galileo.