DUAL-SATELLITE CROSSOVER LATITUDE-LUMPED COEFFICIENTS, THEIR USE IN GEODESY AND OCEANOGRAPHY

Latitude-lumped coefficients (LLC) are defined, representing geopotent ial-orbit variations for dual-satellite crossovers (DSC). Formulae are derived for their standard errors from the covariances of geopotentia l held models. Numerical examples are presented for pairs of the altim eter-bearing satellites TOPEX/Poseidon, ERS 1, and Geosat, using the e rror matrices of recent gravity models. The DSC, connecting separate m issions, will play an increasingly important role in oceanography span ning decades only when its nonoceanographic signals are thoroughly und erstood. In general, the content of even the long-term averaged DSC is more complex then their single satellite crossover (SSC) counterpart. The LLC, as the spatial spectra for the geopotential-caused crossover effects, discriminate these source-differences sharply. Thus, the zer o-order LLC in DSC data contains zonal gravity information not present in SSC data. In addition, zero-and first-order LLC of DSC data can re veal a geocenter discrepancy between the orbit tracking of the separat e satellite missions. For example, DSC analysis from orbits computed w ith JGM 2 show that the y-axis of the geocenter for Geosat in 1986-198 8 is shifted with respect to T/P by 6-9 cm towards the eastern Pacific . Also, where the time-gap is necessarily large (as between, say, Geos at and T/P missions) oceanographic (sea-level) differences in DSC may corrupt the geopotential interpretation of the data. Most importantly, as we illustrate, media delays for the altimeter (from the ionosphere , wet troposphere and sea-state bias) are more likely sources of conta mination across two missions than in SSC analyses. Again, the LLC of z ero order best shows this contrast. Using the higher-order LLC of DSC for both Geosat-T/P and ERS 1-T/P as likely representation of geopoten tial-only error, we show by comparison with the predicted standard err ors of JGM 2 that the latter's previously calibrated covariance matrix is generally valid.