The NASA Scanning Radar Altimeter (SRA) was designed primarily to measure the energetic portion of the directional wave spectrum by generating a topographic map of the sea surface. The SRA sweeps a radar beam of 1/spl deg/ (two-way) half-power width across the aircraft ground track over a swath equal to 0.8 of the aircraft height, simultaneously measuring the backscattered power at its 36 GHz (8.3 mm) operating frequency and the range to the sea surface at 64 positions at 0.7 incidence angle intervals. The maximum scan rate was 8 Hz during the Hurricane Bonnie flights, but it is presently 10 Hz. In realtime, the slant ranges are multiplied by the cosine of the off-nadir incidence angles (including the effect of aircraft roll attitude) to determine the vertical distances from the aircraft to the sea surface. These distances are subtracted from the aircraft height to produce a sea-surface elevation map which is displayed on a monitor in the aircraft to enable realtime assessments of data quality and wave properties. On 24 August 1998, the SRA, aboard a NOAA hurricane research aircraft, provided the first documentation of the directional wave spectrum throughout a hurricane in open water when Hurricane Bonnie was about 400 km east of Abaco Island, Bahamas. On 26 August 1998, the SRA provided the first documentation of the directional wave spectrum throughout a hurricane making landfall as Bonnie was approaching Wilmington, North Carolina.
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