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2.2 Radial Surface Currents
2.2.1 Theory
The motion of the ocean we.- is seen by the radar ss a translation of the frequency
of the reeelved echo signal fmm that of the carrier. In kping with the Bragg scatter-
ring phenomenon, the dominant received radiation is resonant scattering h m ocean
waves trading to-& or away horn the radar and having s wavelength one half
that of the transmitted sf@. In this mode, the radar em- ao ocean wave
spstmmeter since it predominantly observes only these wavetrains. This interaction
induces on the radar Doppler frequency spectrum symmetrically s p d peaks whoae
positions are dictated by the pbase velocities of the advancing and receding wave
trains, respectively. In the HF commuaih: these are commonly referred to as Bragg
lines. In the absence of currents, these lines have well defined Doppler frequencies
If a current exists beneath the surf- waver, the radar sees this as a translation of
the co-ordinate frame of the scattering wave trains. This imparts another frequency
shift on the Bragg lines wbieh is proportional to the radial component of the current
velocity dong the radar receiving drection'.
The following short mathematical explanation of the above pmcess dl be pre-
sented next since it mneisely summarize. the above parsgraphs. For ao excellent
introduction on fundamental radar principles the reader should refer to the text
"Radar Principles" by Nadav Levanon(431.
It is well esttabhshed tbt for p~grazingineidence, the velocity, K, of the Brauwave
ss derived hom the deep water gravity wave dispersion relation is given by
I~he bmk of the mathematical briustions of the a h mnap~s and tbessartemg
a not be preented here sinsin thhh h beaed eaed f"dSmenta1 radar prhcip1ei. Inned, only the
prhcipal equations gmwnhg Dopplm ahiffa which o-te kom mwiog tars* be marldeed