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Rice 1241, Barrick [I] developed an expression for the average badacattered signal
power density spectrum. This qr-ion, when integrated with respect to frequency
clearly shows that the ocean surface pmduees scatter by the Bragg mechanism, thus
supporting Crombie's hypothesis in 1955[16] and Wait's fornulation [22].
Since HF rdections From the sea surface are scattered mnsrmctively fmm ocean
wavelengths, pmportional to the radar 1~~velength, it WBS envisioned by Crombie that
a mdti-Frequency HF system mdd be wed as a swwave spectrometer. That is, the
amplitudes of the dominant spectral mmponents may be related to the waveheight of
the gravity wa- whose wavelengths interact coastructively with the radar wave. In
the 1960's, experiments were performed at Eigjn Air Force Base on the Florida coast
where an ionspheric sounder, modi6ed to radiate in the horizontal direction, war
used to mUeet the sea echo rdections in the 2 to 10 MHz band. The intent of these
experiments ara~ to mmpare the theoretical and measured Doppler shifts wer the
Lower HF band (Cromhie 1251). However, there were diserepaneies between the simple
theory that Cromhie propaned in 1955 and the actual measurements. The &st-order
spectral components were slightly shined fmm their theoreticai dues, which resuited
in a velocity discrepancy of approximately 40 cm/s. Since this war comparable with
the hstorieal magnitudes of the ocean surf- -rents off the Florida mart, he
armired that ocean surfme currents, induced by waves, winds and tides, amounted
for thir shift. That is, the whole water surface moves due to underlying currents.
Another experiment was performed at Jupiter Inlet, Florida, which utilized two
spaced receiving antennae for direction-6ndiog purposes to determine the directions
in which the sea waves were travelling. By considering the coherence function of
the amals received at the two antema=, Cmmbie 1261 showed that mnstrunive
backscatter sign& were received over a wide range of azimuths and their Doppler
shifts indicated that the current was Bowing nearly perpendicular to the mast and
in the same diretion as the wind.