Ocean loading corrections for continuous GPS: A case study at the Canadian coastal site Holberg

The continuous GPS stations of the Western Canada Deformation Array (WCDA) are subject to movements due to the loading of the lithosphere by ocean tides. Using 4 weeks of continuous GPS data, this study examines the effects of ocean loading corrections at the coastal site HOLE. Theoretical ocean loading displacements were based on the Grenoble Global Model FES95.2, modified by detailed modeling of nearby coastal ocean cells. Positions for HOLE were obtained using the Bernese software for 3hr and 24hr solutions with hourly tropospheric delay estimates in both cases. Without ocean loading corrections, the 3hr-computed heights had daily variations as large as 8 cm that were well correlated with tidal predictions. Application of ocean loading corrections to these solutions removed the tidal correlations and reduced the variance by a factor of two. The repeatability of the 24hr vertical positions was not significantly improved by the addition of ocean loading corrections. However, for these daily solutions without loading corrections, a significant amount of the relative vertical tidal motion of the station was absorbed in the (1hr) tropospheric delay estimates. We conclude that constraining station position for HOLE, in our case to a daily average, can bias short-term estimates of zenith delay by up to 10 mm and, in turn, estimates of precipitable water vapour by several millimetres.