Statistical study of the spectral broadening of skywave signals backscattered by the sea surface: application to RMS wave height measurement with a skywave radar

HF skywave signals backscattered by the sea surface are studied on a large set of data (more than 30 h of 64 independent signals) to identify the sources of the broadening of the first-order spectral line. Using high-quality signals reflected by sporadic E ionospheric layers, the natural broadening due to sea-scattering effects has been scaled to about 3/100 Hz. When the signals propagate via F layer, the total broadening due to ionospheric effects is similar in magnitude and can be attributed to two causes. The first, due to frequency modulation effects, which can be identified and corrected, scales on average to 1/100 Hz. The second, called unresolved ionospheric effects, scales on average to 2/100 Hz and is probably due to the spatial variation of the ionospheric Doppler within the ionospheric control volume. Since they are greatly variable with time and space, the influence of these unresolved ionospheric effects can be reduced by sorting spectra, according to the value of the equivalent spectral width, before averaging. Using such sorting and correcting the signals for the ionospheric frequency modulation, 70% of the considered set of data are usable to measure the root mean square (RMS) wave height