The modulation of the radar cross section of the ocean surface by an azimuthally traveling long gravity wave

Abstract

A new technique is used to calculate the modulation of the radar cross section of the ocean surface by a long gravity wave having a single narrow‐band spectral peak. Spectral analysis of the measured wave height time series is used to determine the position on the dominant wave illuminated during each radar sample. When we average the samples of the signal power received from the same phase position (but during different cycles) on the long wave, the phase‐interference (Rayleigh) fading is greatly reduced. The result is a measurement of the average radar cross section of the ocean surface as a function of the position on the long wave. The technique was applied to experimental runs where the radar beam was parallel to the crest of the dominant wave (cross‐wave look direction). Significant modulation was observed at both C and X bands, although there was no tilt modulation. A scenario where hydrodynamic and aerodynamic effects give rise to the modulation is discussed. “Sea spikes” occurred primarily near the front face of the long wave, with some also appearing immediately after the crest has passed, supporting the theory that they are often associated with the breaking of ripple waves riding near the top of the long wave.