Non-tidal ocean loading effects on geodetic GPS heights

GPS observations used in geophysical studies are not usually corrected for non-tidal ocean loading (NTOL) displacement. Here we investigate NTOL effects on 3–4 year GPS height time series from 17 sites around the southern North Sea, and compute the NTOL displacement according to two ocean models; the global ECCO model and a high resolution regional model, POLSSM, which covers the northwest European continental shelf. To assess the susceptibility of GPS height estimates to NTOL, reprocessed GIPSY PPP daily solutions are corrected for the resulting displacement, together with atmospheric pressure loading (ATML). We find that the displacements due to NTOL are comparable in size to ATML and the combined correction reduces the variance by 12–22 mm2 (20–30% reduction in RMS). We find that POLSSM outperforms ECCO, with around an 11% greater variance reduction, and hence high resolution models are recommended to correct GPS heights for NTOL