The effects of space and time resolution on the quality of sea echo Doppler spectra measured with HF sky wave radar

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°, 2°, and 4° 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° and preferably as small as 1/2° and should use coherent integration times shorter than about 100s.