Rigorous propagation of Galileo-based terrestrial scale

Until now, the GPS and GLONASS satellite antenna phase center offsets (PCOs) used within the International GNSS Service (IGS) have been estimated based on the International Terrestrial Reference Frame (ITRF) scale provided by Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI). Therefore, the IGS products have themselves been conventionally aligned to the ITRF scale, hence could not contribute to its realization. However, the disclosure of metadata, including PCOs, for the Galileo satellites by the European GNSS Agency recently opened a unique opportunity to realize an independent GNSS-based terrestrial scale.

Before its ongoing third reprocessing campaign (repro3), the IGS thus re-evaluated the PCOs of the GPS and GLONASS satellites by fixing the PCOs of the Galileo satellites in multi-GNSS solutions. The repro3 products, based on these re-evaluated PCOs, can provide an independent Galileo-based scale, which could potentially contribute to the scale of the next ITRF2020. However, the re-evaluated GPS and GLONASS PCOs are introduced as known constant values in repro3 without realistic uncertainties. Therefore, finally no realistic uncertainty will be available for the realized terrestrial scale.

In this study, another re-evaluation of the GPS and GLONASS PCOs based on the Galileo PCOs is carried out, accounting this time for their variability and estimation errors, with the goal to obtain a more rigorous Galileo-based scale with realistic uncertainty, in particular during the pre-Galileo era. For that purpose, daily time series of GPS and GLONASS PCO estimates derived from the repro3 solutions of different IGS Analysis Centers (ACs) are first analyzed. Deterministic and stochastic models of the time series are then introduced in a global adjustment of all GPS and GLONASS PCOs based on the Galileo PCOs. The re-evaluated PCOs – together with their uncertainties – are finally re-injected into the AC terrestrial frame solutions. The analysis of the latter allows a more rigorous evaluation of the Galileo-based scale and its uncertainty and a more sound comparison to the ones realized by SLR and VLBI. The outcome of this study will provide valuable information for the final selection and realization of the ITRF2020 scale.