Statistics of HF skywave ground backscatter Doppler spectra

Cumulative distribution functions (CDF's) are presented for Doppler spectra from skywave (ionospherically propagated) ground backscatter measured on 17 October 1980. The variance of the CDF's is much greater than would be expected if ionospheric reflection coefficient and ground backscatter coefficient were both Gaussian random variables. In contrast, skywave echoes from the sea show a normalized variance between 2 and 3, in agreement with a model that assigns a unit normalized variance to both the ionospheric reflection coefficient and the sea-echo backscatter. Furthermore, the CDF's for skywave sea-echo backscatter spectra show qualitative agreement with the Hankel distribution that would be expected if both ionospheric reflection coefficient and sea-echo backscatter were Gaussian random variables. The large variance in skywave ground backscatter is probably caused by observed trends of ground backscatter coefficient with range and azimuth. In addition, a new method is demonstrated for estimating the CDF of an ensemble that has only a few samples.