Spectral characteristics of HF ground backscatter

A fixed frequency (18.415 MHz) ionospheric backscatter sounder affording coherent detection and an associated data acquisition system were used to collect data in a form suitable for computer analysis. Most of the data collected related to single hop F-layer propagation, with backscatter from both land and sea being observed. Digital techniques were used to derive the Doppler spectra characterizing the backscatter returns and to display the spectra in formats designed to delineate clearly the relevant spectral characteristics. A statistical analysis of the data revealed that, excluding periods of sunrise and sunset, typical mean daytime ionospheric Doppler shifts fell in the range + or - 0.5 Hz, while the associated spectral widths were of the order of 0.3 Hz. The presence of ionospheric disturbances limited the spectral integration periods to values much less than those employed in ground wave propagation studies.