GPS phase center variation modeling using ambiguity-fixed carrier phase observations from low Earth orbit satellites

Precise knowledge of the antenna phase center variations (PCVs) of global navigation satellite system (GNSS) is indispensable for GNSS high-precision applications. Currently, the PCV models of most GNSS satellites are exclusively estimated from ground observations, which reveals some drawbacks such as limited nadir coverage (usually < 14°) and high correlation with tropospheric parameters. Onboard observations from plenty of low Earth orbit (LEO) satellites launched in recent years offer us a great chance to investigate the LEO-based GNSS PCV calibration. We estimate GPS PCV model using ambiguity-fixed carrier phase observations from multiple LEO satellites to obtain more reliable GPS PCVs. Onboard data of eight LEO satellites from day of year (DOY) 122, 2019 to DOY 180, 2019 are processed. We first evaluate the contribution of integer ambiguity resolution (IAR) to the LEO PCV calibration. Compared with the traditional ambiguity-float PCVs, the ambiguity-fixed PCVs can contribute to the maximum reduction of 6.2% in carrier phase residuals and 8.0% in satellite laser ranging (SLR) residuals. After that the LEO-based GPS PCV calibration using different LEO satellites is performed and the impact of different estimation strategies and datum definitions is assessed in detail. The results show that the IAR can contribute to the stability improvement in GPS PCVs by (24.5%, 41.5%, 42.4%, and 45.2%) for four BLOCKs. As the number of LEO satellites increases, GPS PCVs gain an evident stability improvement. With the inclusion of eight LEO satellites, the PCV stability is mainly within 0.3 mm with the maximum improvement of 63.8% and the PCV difference w.r.t igs14.atx is less than 1 mm. Meanwhile, we also find that the LEO-based GPS PCV calibration is hardly influenced by the datum definition which is applied to decouple GPS PCVs and LEO PCVs. Validation results indicate that our GPS PCV estimates slightly outperform PCV model from igs14.atx with the LEO kinematic orbit accuracy improvement of 5.1% and the precise point positioning (PPP) error reduction of 1.1%. The reference frame scale of our GPS PCV estimates is also consistent with that from igs14.atx.