Global sea level change from satellite altimetry

Abstract

We examine the feasibility of using satellite altimeter data to measure the long‐term change of global sea level (estimated from tide gauge data to be a rise of approximately 0.2 cm yr−1). Two and one‐half years of collinear Geosat altimeter data (1986–1989) are used together with a 17‐day set of Seasat altimetry (July–August 1978) having nearly the same ground track. A consistent set of precise orbits was used throughout, and residual orbit error was removed as a sinusoidal fit to approximately 3‐day arcs of sea level collinear differences. The globally averaged Geosat data show sea level falling at 1.2 ± 0.3 cm yr−1 over the first 2 years, even after removal of tide errors and instrument biases not accounted for in the Geosat geophysical data records. This unrealistic result is found to be due largely to long‐term error in the ionospheric model for the single‐frequency Geosat altimeter. The Geosat‐Seasat comparison, based on data 10 years apart, shows an apparent sea level rise of 1.0 cm yr−1. Assuming this result is also unrealistic, a possible explanation is a biased scale to the Doppler‐determined Geosat orbit which, unlike Seasat, did not have the benefit of laser tracking. It is also possible that the Geosat altimeter (without external in‐orbit calibration) had a bias of the order of 10 cm. We conclude that for satellite altimetry to make a fundamental contribution to monitoring global mean sea level change, both the altimeter (including its media corrections) and the orbit model which provides a geocentric reference for the ocean surface will need continuing and careful calibration with absolute standards.