Measurement of ocean wave heights with a satellite radar altimeter

The launch of Geos 3 on April 9, 1975, by the U.S. National Aeronautics and Space Administration (NASA) represented the first step toward the goal of ocean surface remote sensing from space. Microwave radars and radiometers orbiting aboard a network of unmanned satellites are expected ultimately to offer an all-weather capability for routinely monitoring sea state, ocean surface topography, temperature, currents, and (indirectly) surface winds on a global scale. The availability of such data in real time should produce a quantum increase in the quality of long-term weather forecasts, as well as provide needed information for the efficient and safe conduct of burgeoning maritime operations. Geos 3 is the first unmanned satellite to carry a radar system for observations of the sea. In particular, it carries a microwave short-pulse altimeter capable of measuring rms ocean wave height along its nadir track. Although the instrument can measure other quantities of geodetic and oceanographic interest (such as the instantaneous mean sea level from which currents and geoidal features can be extracted), we consider here only the measurement of sea wave height; this is without a doubt the most important single parameter characterizing that general but often used term, ‘sea state.’ The mathematical model and physical principles describing the sea echo have been understood for years. This paper, however, presents some of the first wave height plots made from a spacecraft radar. These wave height plots are shown versus distance along the satellite track; they agree reasonably well with ocean wave hindcasts and laser profilometer measurements of wave height, the errors being consistent with system resolution and internal and tracking loop noise.