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

Abstract

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

Surface wind speed extraction from HF sky wave radar Doppler spectra

“…In this way, some sea state information may be obtained from the Doppler spectrum in situations where a full spectral inversion would be meaningless. Similarly, Shearman et al [1981a] have shown that for various mean ionospheric disturbance parameters defined by Georges and Maresca [1979], while errors in the derived ocean wave spectrum may be significant, those in/-/(and also in derived mean wave period z) will, in general, be somewhat less so. In this case, Barrick's [1977b] inversion technique was used, and the result presumably arises through the averaging process involved in the evaluation of H and z.…”

“…More recently, Lipa and Barrick [1980] have developed an improved inversion technique for the extraction of swell wave parameters only. While a complete spectral inversion is especially relevant to the high-quality Doppler spectra produced by ground wave radars, the (often considerable) spectral contamination due to ionospheric motions encountered with sky wave radars poses special problems [Georges and Maresca, 1979]. Recognizing this, Maresca and Georges [1980] simplified the inversion procedure of Barrick [1977b] in a semiempirical way to allow the extraction of rms surface displacement H through determination of the ratio of second-to first-order spectral powers.…”

“…Thus procedures exist for the extraction of at least H and • (or fro) from the Doppler spectrum for sky wave radars, provided that ionospheric distortion remains below certain acceptable limits. (Such limits may be defined, for example, by using the spectral width parameter of Georges and Maresca [1979].) In any event, for the purpose of this study the particular procedure used to obtain H and • (or fro) is unimportant.…”

“…However, there are a number of unsatisfactory features of this approach, which is based on the idea that as wind speed increases, so more low-frequency ocean wave components are generated, thus enhancing the second-order Doppler spectrum close to the Bragg peaks and thus, provided that the spectral resolution is sufficiently coarse, in a general way 'broadening' these first-order peaks. Difficulties include (1) choice of optimum spectral resolution [e.g., Sandham and Theodoridis, 1980], (2) the fact that even minor ionospheric contamination can also cause significant broadening [Georges and Maresca, 1979], and (3) the fact that B will not be a unique function of wind speed alone [Georges, 1980]. This last point is worth further consideration.…”

The application of HBF in small aperture OTHR

“…In the reference [3,4], it refers that beam width is related to Doppler spectrum. The formula [5] is 2 2…”