Oceanic temporal variability is calculated from satellite altimeter data by differencing sea level measurements repeated at the same location. These differences yield variations independent of any permanent undulations of sea level. If satellite groundtracks are collinear (nested), altimeter measurements are repeated in a regular fashion continuously along a repeating groundtrack, and calculation of sea level variation is straightforward. For an orbit that repeats only at intersections (crossovers) of the groundtrack, the problem is more complicated, but results equal in accuracy to those obtainable from a collinear analysis can be achieved. For both collinear and crossover analyses, long wavelength error in the altitude of the satellite must be eliminated by using a model for the orbit error and adjusting each pass of data into a reference pass in its nest for the collinear case or into a reference grid of intersecting passes for nonnested tracks.
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