MultiGNSS Reference Frame Consistency As Realized Through PPP

Abstract:

"Rapid" multi-GNSS (GPS, Galileo, GLONASS) orbit and clock solutions typically constrain the positions of some/all of their chosen station network to values from a specific frame, producing fiducial solutions consistent with that frame. Most fiducial orbit and clock users prefer to be in the current ITRF. Standard IGS GPS processing realizes ITRF14 through IGb14; however, IGb14 lacks a consistent set of antenna calibrations for all 3 constellations, posing a problem for multi-GNSS operations. The interim IGS repro3 (R3) frame, developed to facilitate the generation of ITRF2020, does make consistent antenna calibrations available, at least up to differential errors in factory Galileo transmit- and robot ground-antenna calibrations. Assuming the first order inconsistency with IGb14 is the Z phase center offset (PCO) of Galileo satellites, JPL adjusted their values in the IGb14 frame using 3-months of data (April-June, 2019). We tested frame repeatability with 3-months (July-September, 2019) of daily ambiguity resolved (bias-fixed) precise point positioning in: 1) the IGb14 frame, using JPL’s estimated PCOs for Galileo; 2) R3, with PCOs from its associated ANTEX file. In both cases, solutions using only GPS measurements reproduced the frame coordinates better than solutions using GPS, Galileo, and GLONASS measurements. We believe this indicates that Galileo modeling issues remain, potentially all JPL’s own, although JPL’s multi-GNSS orbit and clock products closely match other IGS MGEX analysis center’s products, with general improvement of the GPS component vs GPS-only. As a sanity check, we also adjusted the Galileo Z-PCOs in IGS R3 over the same period. On average, these differed from the R3 PCO values by 13 cm. As this value is large compared to the scatter and formal errors, we also looked at GPS-only Z-PCO estimates in IGb14 for the same 3 months; as expected, its average variation is only 2 cm. Going forward, we recommend further investigation into Galileo force modeling and antenna offsets/calibrations, and suggest that static PPP users in ITRF14 currently use only GPS measurements.