Effects of islands in the Geosat footprint

Abstract

The presence of land within a radar altimeter's footprint may contaminate the altimeter's tracking performance and perturb the nadir oceanographic signal. An altimeter tracker tends to move towards the brighter, more specular land reflections in the radar return waveforms. Researchers may unintentionally include tracking data over small mid‐ocean islands in their data sets. If their goal is to obtain sea surface height (SSH) information at the subdecimeter level, such inclusion is inappropriate because islands can locally affect SSH by more than ±4 m. Altimeter signal strengths and wave height‐related measurements are also adversely affected. The tainted data should be edited. However, if editing criteria near island areas are too conservative, useful geoid, tide, current, sea state, and storm surge information will be excluded. A study has been performed which examines the effects of small mid‐ocean islands on the Geosat tracker. The surface return waveforms, as well as the altimeter measurements, have been analyzed. Open ocean waveforms have been compared with waveforms containing land reflections; they differ perceptibly. Computations could be performed, based on a combination of waveform data, altimeter signal strength, and sea surface heights, to detect contaminated SSH values for subsequent editing.