Estimates of rms wave height and the scalar ocean wave frequency spectrum were made by inverting high‐frequency (HF) skywave radar‐measured sea‐echo Doppler spectra. Whereas low‐power surface‐wave radars can make these measurements out to approximately 100 km from the radar, coverage out to 3000 km can be obtained with skywave radars that illuminate the sea via a single ionospheric reflection. To demonstrate this capability, we used the Wide Aperture Research Faculty (WARF) HF skywave radar to measure Pacific Ocean sea backscatter near NOAA data buoy EB 20 during the passage of an at mospheric cold front. The height of the wind‐generated waves measured at the buoy doubled within a 6‐h period. Two new analysis techniques were used to derive rms wave height and the scalar ocean wave frequency spectrum from radar echoes. Estimates of rms wave height were made east and west of the front by using a power‐law relation that was derived from theoretical simulations of the sea‐echo Doppler spectrum for a wide range of wave conditions. Two of the rms wave height estimates were compared with the estimates made at EB 20 at two different times and are in agreement to within 7 and 17%, respectively. Scalar wave spectrum and rms wave height estimates were made by matching a theoretical Doppler spectrum derived in terms of a five‐parameter model of the wind‐wave spectrum to the measured Doppler spectrum. The radar and buoy estimates of the rms wave height agree within 7%. Agreement between the WARF‐derived and buoy‐derived rms wave height and wave spectra is within the combined experimental error of the buoy and the radar measurements.
Sea‐echo data from three separate narrow‐beam HF radar experiments on the Pacific Ocean are analyzed here by techniques presented in Lipa and Barrick (1980). Only those wave spectral components whose periods exceeded 10 s were included. Close agreement of radar‐deduced wave field parameters with surface observations confirms the validity of the second‐order theoretical solution for the echo Doppler spectrum, upon which this analysis is based. Depending on the particular experiment, a variety of wave parameters are extracted, including rms wave height, mean wave direction, dominant period, angular spread of the wave field, the nondirectional wave height spectrum, and higher Fourier angular coefficients versus wave frequency. The radar‐deduced wave parameters fall within the combined error bounds estimated for the radar and buoy wave observations; consequently, we contend that the primary source of error for radar data is finite sample size. Typical accuracies for specific parameters resulting from observations averaged over a 2‐hour period are ±5% for wave height, ±0.5 s for wave period, and ±7° for wave direction. Hence the utility of HF radars for long‐wave measurements has been validated.
Measurements using the SRI Wide-Aperture Research Facility (WARF) HF sky wave radar show that the radar's azimuthal beamwidth and integration time play important roles in determining the quality of sky wave (ionospherically propagated) sea echo Doppler spectra. Using as a quality index the equivalent width of a portion of the Doppler spectrum in the vicinity of the stronger Bragg line, we find a reduction of ionospheric multipath spectral contamination as beamwidth and integration time decrease. For nominal (3 dB) beamwidths of 1/2 ø, 2 ø, and 4 ø and 256-s averaging, the mean equivalent widths of 104 spectra were 0.090, 0.099, and 0.105 Hz, respectively. Experiments using 51-s integration time gave average widths of 0.061, 0.075, and 0.079 Hz for the same three beamwidths. The additional contamination observed with the larger beamwidths and the longer averaging time is often sufficient to preclude the extraction of ocean wave height from the second-order spectral structure. A geometrical optics, rough-surface model of the ionospheric reflection process explains this beamwidth dependence by relating the number and width of multipath spectral lines to the size, shape; and number of 'reflective glints' in the ionospheric area illuminated by the radar. The lateral dimension of this area increases with antenna beamwidth. The model also predicts a greater dependence of contamination on beamwidth as the integration time is reduced. We use ionospheric measurements made with CW Doppler sounders to estimate the statistical parameters of the rough surface model and the amount of contamination the model predicts for finite beamwidth and integration time. Our main conclusion is that the best way to avoid spectral contamination caused by short-term ionospheric motions is to involve the smallest possible spatial and temporal samples of the ionosphere in the radar measurement. Specifically, a successful sea state radar should have a 3-dB beamwidth of less than 2 ø and preferably as small as 1/2 ø and should use coherent integration times shorter than about 100s. ß ß ß ß _ _ øo oo o o •g$%OooooOooooO. oo A....• 00 / ' , ß o oo ß ß ß ø o ß _ ß ß ß A _ ß ß _ ß -o o
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