Doppler Spectrum of Sea Echo at 13.56 Mc./s.

The analysis and digital signal processing of NOAA's surface current mapping system

“…There are two prominent sidebands, an advancing sideband on the right and a receding sideband on the left. One of Crombie's [2] early findings was that these sidebands were due to Bragg scatter from resonant wave trains propagating directly toward or away from the radar; he established the resonance condition L = h/2 where L is the distance between wave crests and h is the radar wavelength. The resonant backscatter, in turn, determines the Bragg lines f = k m s h o w n in Fig.…”

“…The scattering mechanism underlying CODAR and, more generally, all HF radar oceanography was discovered by Crombie [2] more than 25 years ago. He observed that most sea echo (backscatter) is a form of Bragg scatter resulting from resonant wave trains lining up to form a diffraction grating.…”

Interpretation of Backscattered HF Radio Waves from the Sea

“…Backscatter of HF radio waves has been used for many years as a remote probe of the sea surface (Crombie 1955;Ward 1969;Long and Trizna 1973;Valenzuela 1974;Barrick et aZ. 1974;Barrick 1977;Lipa 1977;Barrick et al 1977;Trizna et oZ.…”

“…It has been shown theoretically and confirmed experimentally that to a first approximation only water waves of wavelength half that of the incident radio wave contribute to the Doppler shifted spectrum leading to two dominant peaks in the echo (see above references and Barrick 1972a;Johnstone 1975;Robson 1984; we focus here on the so-called first-order effects and do not consider either mUltiple scattering or nonlinear hydrodynamic effects, which produce other distinctive features in the spectrum). On the other hand, the physical model most often cited in the above literature is the simple ruled diffraction grating model of the sea surface, with the 'lines' corresponding to crests (Crombie 1955;Dexter et aZ. 1982) or some other unspecified feature of the waves.…”

A two year comparison between HF radar and ADCP current measurements in Liverpool Bay

“…HF radar utilises Bragg scattering characteristics of the backscatter from ocean waves with half the radar transmitting wavelength, first identified by Crombie. 6 The associated frequency shift in the first-order backscatter signal determines surface currents while the relative amplitude difference from currents moving directly away and towards the radar are used for wind directions. The second-order backscatter measures wave parameters, the theoretical formulation for which was set out in the 1970s.…”