| Satellite altimeter data spanning twenty-five years from GEOS-3, SEASAT, GEOSAT, ERS-1, TOPEX, and ERS-2 have been gathered, improved, validated, and integrated. Satellite data were updated with TOPEX-level corrections where possible. Wet troposphere and ionosphere altimeter corrections were evaluated globally, along-track, and zonally. Global mean sea level (GMSL) trend adjustments were made to ERS-1 and ERS-2 to correct for radiometer drifts with respect to TOPEX, improving their GMSL comparisons. IRI-95 model ionosphere corrections were evaluated against TOPEX dual-frequency measurements. New orbits were computed with improved accuracy for GEOS-3 (20 to 30 cm), SEASAT (10 to 15 cm), and GEOSAT (7 to 9 cm). An adaptive sequential filter was utilized to remove residual one-cycle-per-revolution radial orbit error. Optimal state noise compensation parameters for the filter were determined using a genetic algorithm. The application of the filter reduced single-satellite internal crossover differences.; Relative altimeter biases between TOPEX and the other missions were determined using tide gauges. Relative biases were estimated for GEOS-3 (173 cm), SEASAT (37 and 26 cm, for 17-day and 3-day repeats), GEOSAT (7.9 and 8.9 cm, for GM and ERM), ERS-1 (−44.7 and −48.0 cm, for Phases A-F and Phase G), and ERS-2 (−9.0 cm). These biases are consistent with several other recent bias determinations and calibration campaigns. GMSL trends estimated for GEOSAT (1985–1988), ERS-1 (1991–1996), TOPEX (1993–1999), and ERS-2 (1995–1997), are −3.4 ± 2.4, 2.3 ± 1.1, 3.2 ± 0.6, and 6.0 ± 2.2 mm/year, respectively. GEOS-3 MSL exhibits a large slope (∼40 cm/year) and does not provide truly global coverage, and therefore cannot be used for global analyses. The short duration of the two SEASAT missions (July to October 1978) prevent GMSL trend analysis. GMSL from ERS-1, TOPEX, and ERS-2 were integrated into a single time series having an estimated mean sea level trend of 3.3 ± 1.3 mm/year. Including GEOSAT, the estimated fifteen-year GMSL trend is 1.8 ± 2.3 mm/year. Global and ocean basin MSL compare well to tide gauges. Climatological indices and sea surface temperatures show some positive correlations to satellite sea levels, indicating that GMSL is related to pressure changes and steric sea level rise in recent history. |
