Observing ocean tidal loading with GNSS

Abstract

Ocean tidal loading (OTL) is the displacement of the Earth’s crust caused by the redistribution of sea water due to ocean tides. This affects geodetic measurements with Global Navigation Satellite Systems (GNSS), and can thus be observed with continuous GNSS measurements. OTL can also be modelled to remove its effect on geodetic measurements. This master thesis project aims to observe OTL with GNSS at 50 worldwide sites, and to compare these observations to four OTL models. Through investigation of disagreements between observations and models, insight into factors that negatively affect OTL models can be gained. The findings are similar in all four models. No clear, global geographical pattern was found, but there are indications of some geographical factors with negative effect on OTL models: inland sites and complex tidal conditions. It has been shown that three-year-long GNSS observations of OTL are accurate enough for use in evaluating OTL models. The disagreement between OTL models and observations is generally up to 2 mm in the radial component; larger differences indicate problems with models or observations. Observations of the solar tidal constituents proved problematic. The problem was identified as perturbing diurnal and semidiurnal signals, most likely solar driven perturbations, possibly in combination with multipath and monument stability issues. The tidal analysis method should be adjusted to eliminate these effects. There are also indications that GNSS observations of OTL are sensitive to Earth structure. It is thus possible that OTL observations with GNSS can be used to investigate Earth structure, and that Earth models can be adapted to improve OTL models.