Knowledge Bank

Approximately 2000 Geos-3 altimeter arcs, supplied by the NASA Wallops Flight Center and received by September 1977, have been analyzed to improve our knowledge of the geoid and gravity field. The first step in this process was a complex editing procedure used to eliminate bad and unacceptable data. After this, an adjustment procedure was used to fit the sea surface heights (geoid undulations) implied by the altimeter data to the geoid undulations implied by the GEM 9 potential coefficients, in an adjustment process that incorporated cross-over constraints. The error model used for the fit was a one or two parameter model which was designed to remove altimeter bias and orbit error. The adjustment process was carried out in two stages: first in a primary net of global data in 854 arcs; second in six regional areas. Typical a priori crossover discrepancies were ±8 meters while a posteriori discrepancies were on the order of ±0.6 meters. The undulations on the adjusted arcs were used to produce geoid maps in 20 regions where the data was suitably dense. In addition, the adjusted data was used to derive 301 5° equal area anomalies and 9995 1° x 1° anomalies in areas where the altimeter data was most dense, using least squares collocation techniques. The average predicted accuracy of the 5° anomalies was ±3 mgals, and ±6 mgals for the 1° x 1° values. Comparisons with terrestrial data indicate these anomaly accuracies were reasonable. We also emphasized the ability of the altimeter data to imply rapid anomaly changes of up to 240 mgals in adjacent 1° x 1° blocks.