The SEASAT altimeter data and its accuracy assessment

Abstract

The SEASAT satellite, launched on June 27, 1978, carried a radar altimeter designed to measure the altitude of the satellite above the ocean surface, the surface wave height, and the ocean-surface backscatter coetficient from which wind speed can be inferred. Postlaunch engineering assessment indicated that the SEASAT altimeter met the performance specification for 10-cm precision (noise) in the altitude measurements. However, to determine the accuracy of this measurement as well as the accuracy of the wave height and wind speed measurements a significant calibration, validation, and model development effort was required. This summary describes the instrument, atmospheric, and geophysical effects which influence the radar altimeter measurement accuracies and the attendant correction models adopted for the altimeter geophysical data record. Also summarized are the activities of the SEASAT Altimeter/Precision Orbit Determination Experiment Team directed towards the validation and improvement of these models, as well as the investigations required to assess the accuracy of the altimeter measurement and the computed satellite altitude ephemeris. Finally, an accuracy assessment is made for the various altimeter measurement corrections which are included on the altimeter geophysical data record. INTRODUCTIONThe development of satellite altimetry techniques for the remote sensing of ocean surface topography is one of the fundamental objectives of the NASA Ocean Processes Program. The requirements for NASA's satellite altimetry program were formulated at the 1969 Williamstown Conference on Solid Earth and Ocean Physics [Kaula et al., 1970]. Since this conference, satellite altimetry has evolved through the Skylab [McGoogan et al., 1974], GEeS 3 [Stanley, 1979], and SEASAT missions to the extent that it can dramatically improve the way we perform many future oceanographic measurements. Characteristics unique to satellite altimetry are (1) the high accuracy with which surface topography, wave height, and wind speed can be measured and (2) the ability to collect these measurements globally over the time interval of a few days [Apel, 1980]. The satellite served as a stable platform from which the altimeter measured the distance from the antenna feed point to the instantaneous electromagnetic mean sea level. The SEASAT altimeter tracked the position of the sea surface by using a closed loop microprocessor range tracker and an automatic gain control feedback loop. For a description of the altimeter design, see MacArthur [1978]. The geophysical measurements obtained from the returned altimeter pulse are (1) altitude above the ocean surface, which is related to ocean surface topography, (2) ocean surface significant wave height, and (3) ocean surface backscatter coefficient, which is related to the wind speed. Given an independent determination of the satellite position from the orbit determination system, the ocean surface topography can be inferred from the altimeter height measurement. The oceanographic phenomena which influe...

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