Correcting Non-Tidal Atmospheric and Oceanic Loading Displacements at the Observation, Normal Equation, and Parameter Levels in GNSS Data Analysis

Abstract

Non-tidal loading (NTL) introduces surface deformation on the Earth and increases the variability in coordinates measured by space geodetic techniques. Correcting NTL displacements in Global Navigation Satellite Systems (GNSS) analysis has been discussed extensively, commonly at the parameter level. We investigate the three levels of correcting non-tidal atmospheric and oceanic loading (NTAOL) displacements systematically in long-term analysis of GNSS global network solution, including the observation, normal equation, and parameter levels. The difference between the observation and normal equation levels lies in addressing high-frequency (sub-daily) displacements, and that between the normal equation and parameter levels concerns datum realization. Correcting NTAOL at the observation (or normal equation) level improves the station coordinate repeatability by 3%–4% horizontally and 13% vertically, slightly greater than that at the parameter level by 0.5%. Discrepancies in station coordinates between the observation (or normal equation) and the parameter level are minor but systematic, and the horizontal discrepancies can be largely reduced by Helmert transformation. These transformation parameters correspond to the datum parameters, including polar motion offsets and geocenter coordinates. High-frequency loading displacements mainly impact site-wise tropospheric parameters and especially receiver clocks, albeit with small magnitudes at the sub-mm level. Satellite orbits in the along and cross components are affected by both datum differences (between normal equation and parameter levels) and high-frequency displacements (between observation and normal equation levels), while radial component and satellite clocks are solely affected by the latter.