Sky-wave radar sea-state sensing Effects of ionospheric movement and propagation geometry

In the sky-wave radar technique, the parameters of sea waves beyond the horizon are deduced from the Doppler spectra of ionospherically propagated sea echoes. Ionospheric movements and multipath can contaminate the sea echo spectrum from a given location. Measurements of die multipath and Doppler spread for a 700 km ionospheric path during MARSEN are reported. For one hop by way of the F2 layer, a typical r.m.s. Doppler shift over 10 min was 0·005 Hz and Doppler spread, to - 10 dB, was 0·08Hz. Multihop and side-scatter modes of propagation and their Doppler structure were identified. The possible use of side scatter at the sea surface is analysed.