THE FILTERING EFFECT OF ORBIT CORRECTION ON GEOPOTENTIAL ERRORS

The geophysical interpretation of satellite tracking residuals general ly ignores the filtering effect of initial orbit correction on the tru e errors of the model. While the filtered information is usually regar ded as lost, knowing the spectral characteristics of the filter is a g reat aid in the detailed interpretation of residuals, especially of gl obal data sets. In this regard, we derive the filter characteristics ( admittances) of orbit correction in the presence of geopotential-cause d trajectory errors. We then apply the filter to determine the likely power of the lost radial information in crossover differences of sea h eights determined from satellite altimetry or in the latitude lumped c oefficients derived from them. For example, we find that resonant geop otential information with periods longer than the corrected orbit's ar e length is largely lost in residual crossover data. Results are given for GEOSAT, ERS-1 and TOPEX/Poseidon in their Exact Repeat Missions, using calibrated variance-covariance matrices of the harmonic geopoten tial coefficients of several recent Earth gravity models. To prove tha t filtering is important, we first employed a simple cut of all pertur bing terms with periods longer than the general tracking period (4 day s for GEOSAT and ERS-1, and 10 days for TOPEX). But the cut is too cru de a method from a theoretical viewpoint, and thus, we developed two n ew filters. A comparison of their admittances explains the differences (and sometimes anomalous behaviour) between them and the cut. Many nu merical examples (single-satellite crossover errors and latitude lumpe d coefficient errors, as projected from the variance-covariance matric es) are presented.