The generation of crossover height differences is an iterative process,
where first the location of all crossovers and the corresponding epochs are
estimated analytically by crossing all ascending and descending tracks
during a selected period. Subsequently, crossovers that do not contain
sufficient altimeter data around the two epochs (*e.g.*, those over land) are
rejected. The positions and epochs of the remaining ones are re-estimated
at the crossing of two linear fits through four measurement locations along
either track around the estimated position. Cubic spline fits through the
relative sea heights along the tracks yield the interpolated relative sea
height at either epoch.

The difference between the respective interpolated relative sea heights (referred to as crossover height difference) is only partly due to actual sea level change between the two epochs. The remaining part is caused by errors in the applied altimeter range corrections or the computed orbit altitude, of which the latter is the largest effect for ERS-1. If these errors are different on crossing arcs, they will affect the crossover height differences. Hence, the crossover difference is primarily a measure of the non-geographically correlated radial orbit error.

Thu 16 Feb 1995