Inconsistency in Precise Point Positioning products from GPS, GLONASS and Galileo

Global Navigation Satellite Systems (GNSSs) are widely used for Earth system monitoring, e.g., solid earth and atmosphere. However, the time series of station coordinates and zenith tropospheric delay derived using GNSS are inherently affected by several technique-specific errors that influence the interpretation of geophysical processes and phenomena. GPS plays a crucial role and is most often used in interdisciplinary studies. However, the multiplicity of navigation systems, including fully operational GLONASS and Galileo, allowed us to assess system-specific high-frequency signals and inconsistencies arising from using different constellations.

This work shows that using different GNSS constellations leads to the appearance of various artificial signals with amplitudes up to several millimeters in the series of station coordinates. The presence of the GNSS system-specific artifacts and inter-system disagreements are demonstrated using the 2-year long series of station coordinates and zenith total delay parameters for 15 stations using Precise Point Positioning algorithms. Finally, we assessed the benefit of using GPS, GLONASS, and Galileo jointly.

We identified the origin of the spurious signals in orbital errors. The most dominant orbital artifacts for Galileo appear with periods of 14.08 h, 17.09 h, 34.20 h, 2.49 d, ~3.4 d. The corresponding signals for GLONASS appear with periods of 5.63 h, 7.36 h, 10.64 h, 21.26 h, 3.99 d, and ~8 d. Moreover, when estimating discrete 24-hour solutions from high-rate GNSS data, high-frequency signatures are under-sampled, resulting in long-term aliased periodic signals. The GPS orbital signals arise at the periods corresponding to the harmonics of the K₁ tide, which leads to the inconsistency of the GPS-based products with ocean tidal loading models reaching on average 12 mm for the K₂ tidal term in the Up component. The magnitude of the orbital signals varies between different site locations and depends on the GNSS observation geometry and dominant direction of satellites' flybys. For example, because of the high inclination of the GLONASS orbital planes, the stations located in absolute low latitudes observe mostly North-South satellite flybys; thus, the estimated East component of the coordinates is exposed to the orbital artifacts.

Galileo is less vulnerable to the orbital signals than GPS or GLONASS. The difference is visible mainly for the East coordinate component. The Galileo-based daily estimates are up to 55% and 36% better than those delivered from GLONASS and GPS. Finally, using a combination of GPS and Galileo increases the precision of estimates by 10% compared with the best-case Galileo-only solution and remarkably reduces the system-specific errors in station coordinate time series.