Ocean wave heights measured by a high resolution pulse-limited radar altimeter

Radar measurements of wave height are compared with independent measurements made during the JONSWAP-2 experiment by Waverider and pitch-roll buoys, a shipborne wave recorder and a laser profilometer. The radar data were recorded by a Naval Research Laboratory (NRL) nanosecond-pulse X-band radar altimeter flown in a NASA C-54 aircraft at 3-km altitude under various wind and sea conditions. Averages of 800 pulses of the pulse-limited altimeter data were used to calculate maximum-likelihood estimates of significant wave height (SWH) and skewness of the sea-surface height distribution. The mean values of the radar-estimated SWH were in good agreement with the other measurements. The standard deviation of the values of the radar measurements was typically 10% of the average wave height. A two-dimensional computer simulation of the sea surface indicates that the major portion of the observed standard deviation is attributable to the relatively small sea-surface area illuminated by the radar (125 m × 900 m) rather than to instrumental error. Increasing the number of pulses averaged reduced the variance in the estimates without changing the means. The mean value of the skewness parameter was generally near zero but the standard deviation was typically 0.25. The estimate of SWH did not change when the skewness parameter was constrained to zero.