Satellite Radar Altimeters - Present and Future Oceanographic Capabilities
In addition to providing land and ocean topography, satellite radar altimeters have the ability to provide information related to ocean wave heights, wind speed and currents. This paper will be concerned with demonstrating the present capabilities and indicating ways to increase the information content of the altimeter return through the use of wider bandwidth, higher pulse repetition frequencies (PRF), and multibeam. Through its range measurement, the SEASAT-1 radar altimeter has detected dynamic height changes of a few centimeters caused by the Gulf Stream and associated eddies (Cheney and Marsh, 1981). Through the broadening of its 3.125 nanosecond (ns) transmitted pulse, SEASAT has measured ocean wave heights to a few percent. The SEASAT 1 KHz PRF gave the resolution to observe distinct oscillatory trends in wave height both in the open ocean and in the interaction of wind and current. The magnitude of the return power has been used to determine the wind speed at nadir to a few meters/second (m/s). In future altimeters, increasing the bandwidth to 1 GHz to achieve 1 ns resolution would significantly improve the altimeters’ abilities to measure low wave heights. It could also lead to the routine determination of sea surface height distribution. By incorporation of adaptive resolution into future altimeters a 1 GHz bandwidth system could be processed to provide multiple looks of lesser resolution so that the full extent of sloping surfaces (i.e. ice, land) could be encompassed. For the 840 km altitude and 3.125 ns resolution of SEASAT, 1 KHz was approximately the maximum PRF at which the returns would be decorrelated for low wave height. However, future altimeters may have an adaptive PRF since the returns would still be uncorrelated at 4 KHz for a significant wave height (SWH) of 20 m. By using multiple feeds and separating two moderate size antennas (1.5 m) cross track by approximately 10 m to obtain interferometric fringes it should be possible to achieve better than 10 cm altimetry along ground tracks displaced ± 50 km cross track from nadir. That would significantly increase the oceanic current measuring capability.
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