The main purpose of Otec is to correct Topex/Poseidon measurements for tidal effects, as deduced from local models, and compare the corrected heights from one cycle to the next.

The Otec operation, part of the Aviso/Quick-look project [Le Provost, 1974; Le Provost, 1981; Le Provost, 1983; Le Provost & Fornerino, 1985] uses existing in-depth knowledge of tides in the Channel. In these coastal waters, variability in sea level is mainly due to very strong semi-diurnal tides, with tidal ranges up to 12 m at St. Malo on the French coast during spring tides. The general circulation and the weather have little effect.

From the tide model for the Channel we can forecast the instantaneous level to within 10 cm [Le Provost, 1983].
The along-track differences between cycles represent the cumulative orbital error, propagation corrections and tidal corrections. In the ideal case, with these errors zero, if we assume that the ocean signal contribution is negligible, we would have a perfect overlay of the corrected heights, giving the shape of the geoid along that track.

The passes over the Channel are passes 61, 137, 146 and 222 (Figure 1). In this article we refer to pass 61 only, at the western end. The data we used are the simplified files, version 1 IGDR-Ms, distributed as part of the Aviso/Quick-look project.