Evaluating the feasibility of GPS measurements of SSH on board a ship along the Portuguese West Coast

Aiming to validate coastal altimetry data, an exploratory experiment was carried out, using a survey ship with onboard GPS and motion compensation systems. The ship navigation plan followed a track as coincident as possible with the passage of two altimetry satellites, Envisat and Jason-1, along the Portuguese coast, in June 2007.GPS data were processed in order to achieve high accuracy epoch-by-epoch three dimensional coordinates of the onboard antennas. GPS ellipsoidal heights were corrected from ocean tides, solid Earth tides, vessel motion effects and vessel load variation. Satellite altimetry data were corrected using new models for ocean tide and wet tropospheric delay.The results for the comparison, without data smoothing, between the satellite altimetry and the SSH measurements, with more than 500 points compared for each satellite, shows 9 cm standard deviation for the Envisat pass and 13 cm for the Jason-1 pass. These results are within the expected values for SSH variability, which is captured in different ways by the two measurement systems. This approach is a simple and flexible data collection system, in a well-defined terrestrial reference frame that can be implemented, with some improvements, to obtain precise SSH measurements for coastal altimetry calibration or sea level monitoring.