Abstract: Mathematical models for GPS positionivg on sea——surface using pseudorange and/orcarrier phase observations are given. A simulated study is done by analysing a set ofdata obtained with the TI——4100 receiver on a fixed point. It is shown that the GDOPspike is climinated and the PPS's accuracy reaches±5m when additional height infor-mation is corsidered. The accuracy of the point positioning using both the carrier phaseand P——code observations can attain to ±0.5m (repeatability).