The Earth Rotation Parameters (ERPs) belong to the fundamental transformation parameters linking the International Celestial Reference Frame (ICRF) and the International Terrestrial Reference Frame (ITRF). The ERPs are conventionally composed of the X and Y pole coordinates (polar motion, PM), their rates, UT1-UTC, and its rate called length-of-day (LoD). The Global Navigational Satellite System (GNSS) is one of the major satellite techniques, which delivers reliable information about ERPs.
ERPs are time-variable global geodetic parameters with a purely geophysical origin. Theoretically, the estimates of these parameters should be independent of the satellite constellation used in GNSS processing. Nonetheless, clear differences in the time series of ERPs are noticed when using different GNSS constellations. In this study, GPS, GLONASS, and Galileo estimates of ERP have been tested in search of system-specific signals.
The main motivation of this work was the presumption that the newly developed European system – Galileo could help with the mitigation of the known errors in the GPS-based ERP products. In the case of GPS, the orbital period equals 11h 58m, thus, it is in 2:1 resonance with the earth rotation. The corresponding resonances for GLONASS and Galileo are much weaker and equal to 17:8 and 17:10, respectively.
Since December 2018, the Galileo system constellation is composed of 24 usable satellites and is nearly fully operational together with the legacy GPS and GLONASS systems. Moreover, the detailed metadata containing optical and geometrical properties for the In Orbit Validation (IOV) and Fully Operational Capability (FOC) satellites have been released since 2017 by the European GNSS Service Centre. The metadata brought new perspectives for the advanced approaches for the orbit modeling based on the hybrid or semi-empirical models. Finally, Galileo is certainly a perfect system to check its suitability for ERP estimation and to confirm the hitherto results based on GPS and GLONASS systems.
The GPS-based polar motion estimates are of better quality than those based on GLONASS and Galileo, which are susceptible to deficiencies in the orbit modeling. On the other hand, we observe a systematic bias of GPS-based length-of-day (LoD) with respect to the reference series with a mean offset of −22.4 µs/day. The Galileo-based solutions are almost entirely free of this issue.
The spurious signals inherently influence the Galileo-based and GLONASS-based ERPs at the frequencies which arise from the resonance between the satellite revolution period and earth rotation, e.g., 3.4 days for Galileo and 3.9 days for GLONASS. These and the draconitic signals overshadow the GNSS-based ERP estimates. Although all the system-specific solutions are affected by the artificial signals, the combination of different GNSS mitigates most of the uncertainties and improves the ERP results.
Details of our study are presented in: Zajdel, R., Sośnica, K., Bury, G. Dach R., Prange L. (2020) System-specific systematic errors in earth rotation parameters derived from GPS, GLONASS, and Galileo. GPS Solut 24, 74 (2020). https://doi.org/10.1007/s10291-020-00989-w
This work was funded by the National Science Centre, Poland (NCN) grant UMO 2018/29/B/ST10/00382 („Determination of Global Geodetic Parameters using the Galileo Satellite System”).
Number of satellites used in the multi-GNSS processing by the particular analysis centers. The gaps in data reflect the unavailability of the products