OBSERVATIONS OF THE BROADENING AND COHERENCE OF MF LOWER HF SURFACE-RADAR OCEAN ECHOES/

Known broadening mechanisms for the first-order Bragg peaks in high-fr equency (HF: 3-30 MHz) and very-high-frequency (VHF: 30-300 MHz) Doppl er spectra of radar echoes from the sea surface are summarized, Observ ations of medium-frequency (MF: 0.3-3 MHz)/lower HF sea echoes were ma de with a surface-wave phased-antenna array transmitting Gaussian puls es of width approximate to 40 and 70 mu s at frequencies 1.98, 3.84, a nd 5.80 MHz (sensitive to sea waves of length 76, 39, and 26 m, respec tively), Some of the broadening of first-order Bragg peaks in Doppler spectra was consistent with the effects of nonuniform surface-current fields across radar footprints. At these MF frequencies, however, the dominant cause of first-order broadening was the spread in Doppler shi fts caused by the phase speeds of first-order gravity sea waves being modified by the comparatively shallow radar footprints. Second-order s ea echoes were usually not observed with good signal-to-noise ratio be cause the sea wave heights were not a sufficient fraction of the radio wavelengths. However, data were obtained in which bathymetrically amp lified, higher order sea echoes also contributed to the broadening, Ex amples of this confused data are presented to illustrate that the inte rpretation of data from a coastal surface-wave radar at MF is difficul t, hence limiting the quality of wave heights estimated by inverting s econd-order sea echoes, Furthermore, the phase shifts of gravity sea w aves caused by propagation through nonuniform surface-current and bath ymetry fields helps to explain the considerable incoherence of sea ech oes in coastal regimes. To this end, hundreds of Doppler spectra were analyzed from records taken during 2-3 day periods and across a range of frequencies and sea states, The results show that the coherence of Bragg peaks decreases with frequency and with water depth and confirm that they integrate less coherently than a stationary tone, There is a lso a suggestion that coherence depends on wave age, Consequently, on practical time scales (similar to 1000 s) incoherent (as opposed to co herent) spectral averaging is more effective at revealing spectral inf ormation otherwise buried in noise (e.g., second-order echoes containi ng sea-state information).