Ocean Tide Determination from Satellite Altimetry
Tides in the deep ocean can be measured directly by satellite altimetry, independent of assumptions about earth tides, crustal loading, bottom topography, and dissipation. Thus, satellite altimeter techniques are free of the uncertainties which plague numerical tide models. These uncertainties can yield a one-meter error in predicted tide height in the Northeast Pacific, for example.
Several different techniques for recovering tides from satellite altimetry have been investigated, with mixed results. These techniques differ mainly in the approach to solving the satellite orbit error problem. Orbit error translates directly into tide height error and several investigators have found that the orbit error is not sufficiently random to ignore. However, for data arcs of less than 1/2-orbit in length, one can separate the longer wavelength orbit error from the shorter wavelength ocean tide effects. In such short arcs, the orbit error may be parameterized by a low order polynomial and corrected in a least squares data adjustment. Harmonic analyses can then be performed on the differences in sea height at satellite subtrack intersections (crossovers). Preliminary results yield good agreement with bottom pressure gauge measurements at several points, indicating that these altimeter tide determinations are feasible and that the orbit error problem can be solved.
A time series harmonic analysis at selected points of high crossover density in the Northeast Pacific indicates that the M2 tide changes spatially more rapidly than has been predicted. This may be due to wave trapping effects around ridge and seamount structures. Future analysis of SEASAT altimeter data is expected to yield global tide charts for the major tidal components to 5o by 5o resolution.
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