XLIII, No 2 – 1029 SEA SURFACE TOPOGRAPHY IN THE GULF OF PATRAS AND THE SOUTHERN IONIAN SEA USING GPS Lycourghiotis S. 1 and Stiros S. 2 1 University of Patras, Department of Civil Engineering, Laboratory of Geodesy, 26500 Patras, Greece, lykourgiotis@upatras.gr 2 University of Patras, Department of Civil Engineering, Laboratory of Geodesy, 26500 Patras, Greece, Stiros@upatras.gr Abstract The topography of the sea, which is changing continuously mostly due to tides, meteorological forc- ing and climatic changes, is very poorly known in both regional and global scale. In our work we apply a new method, “GPS-on boat”, based on differential (DGPS) or kinematic GPS for the record- ing of the Sea Surface Topography (SST) in the Patras Gulf and the southern Ionian Sea. Analysis of data collected permitted to compile a first map of Sea Surface Topography in the study area. Τhe application of the methodology that we present seems also quite promising for the accurate deter- mination of the Geoid in coastal areas, in which other methodologies provide lower quality results. Key words: GPS, Sea Surface Topography (SST), Geoid, GPS-no boat, Ionian Sea. 1. Introduction Sea Surface Topography (SST) roughly coincides with a surface of constant gravity, the geoid, rep- resenting the zero level for height measurement on land. The S.S.T. is usually poorly known, espe- cially in closed gulfs and near-shore areas, which are not properly covered by satellite data merely focusing on oceans. The knowledge of the topography of the sea is important for various reasons, including geophysical (gravimetry-related) studies, meteorological models and climatic change stud- ies. In the past, models of the geoid and the mean sea level for Hellenic area have been presented (Tziavos and Andritsanos, 1999; Andritsanos el al., 2000, Pavlis and Mertikas, 2004). This paper summarizes results of a first attempt to measure SST along certain traverses in the Ion- ian Sea and the Patras Gulf using GPS, in particular the method GPS-on-boat. GPS have been widely used for geoid determination on land (Fotopoulos et al., 2000), but the on-boat application are new.The details and results of this technique, which may provide optimum results at coastal areas and semi-closed gulfs, are presented below. 2. Methodology The method of ‘GPS-on boat’ is based on kinematic GPS (Smith, 1997), the simultaneous tracking of satellites by a moving (“rover”) receiver and by one or more stable (“base”) receivers on stable ground. The apparent changes of the position of the base receiver permit corrections to the computed coordinates of the rover receiver. This technique requires that both receivers are at relatively short distances, usually up to a few or a few tens of km, so that ambient conditions are quasi-similar, and Δελτίο της Ελληνικής Γεωλογικής Εταιρίας, 2010 Bulletin of the Geological Society of Greece, 2010 Πρακτικά 12ου Διεθνούς Συνεδρίου Proceedings of the 12th International Congress Πάτρα, Μάιος 2010 Patras, May, 2010 Ψηφιακή Βιβλιοθήκη Θεόφραστος - Τμήμα Γεωλογίας. Α.Π.Θ.