Bragg-wave scattering and the narrow-vee wake

A coherent radar on a cliff overlooking the ocean was used to illuminate the ocean surface at a grazing angle of 10/spl deg/ during and after the passage of a ship through the beam. Simultaneous L-band and X-band data were collected. The display of surface reflectivity (power) showed a prominent narrow-vee feature at L-band, as was observed in earlier L-band satellite observations. A corresponding narrow-vee structure was observed in X-band, but was not as strong as other components of the wake signature. On the other hand, the Doppler-velocity displays at both radar frequencies yielded the same distinctive signature, i.e., a pair of compact features traveling outward from the centerline, each exhibiting a Doppler velocity that was the Bragg phase velocity corresponding to that radar frequency. The location of the velocity signature coincided at L-band with that of the power signature. Further, the L-band wake was persistent enough to verify that the feature was propagating at the Bragg group velocity, roughly the same as that for X-band. This result confirms an earlier hypothesis that the mechanism underlying the narrow-vee signature is the outward propagation of Bragg waves created by the ship's passage.