COMPARISON OF AIRCRAFT SYNTHETIC APERTURE RADAR AND BUOY SPECTRA DURING THE NORWEGIAN CONTINENTAL-SHELF EXPERIMENT OF 1988

Aircraft synthetic aperture radar (SAR) measurements from seven flight paths at two different altitudes were acquired over a trimodal ocean wave system off the coast of Norway on March 11, 1988. These three wave systems traveling in different directions were also measured by a directional buoy with the buoy spectra rotated into the radar coordinates for comparison with SAR spectra. Fifteen subspectra were averaged to obtain the measured SAR spectra, and sixteen spectral bands were averaged to obtain the measured buoy spectra. The seven flight paths in conjunction with the three wave systems resulted in a nearly uniform distribution of azimuth peak directions varying from 0-degrees to 83-degrees and R/V varying from 28 to 110. The comparison of the SAR- and buoy-derived peak wavelength and direction shows some scatter. We consider the scatter to be due primarily to the statistical uncertainty in the spectral estimates, the wave number resolution, and differences in the temporal and spatial averaging. Scanning distortion is shown to cause significant bias in the SAR and buoy comparison for slow-flying aircraft imaging ocean wave swell. Correction of the SAR spectra for this special radar-ocean condition gives significant improvement in the SAR-buoy comparison. We investigate the relative importance of the motion-related mechanisms (velocity bunching, velocity smearing, and scanning distortion), and the tilt contributions using four nondimensional imaging parameters.