Remote sensing of directional gravity wave spectra and surface currents using a microwave dual‐frequency radar

Abstract

The modulation of small‐scale centimeter (and later decimeter) water waves induced by larger‐scale 2 to 18 m gravity waves has been studied by using a coherent, dual‐frequency radar technique. Experiments using a prototype CW X‐band radar operating at 9.3 GHz have been performed. Experience with the X‐band system led to the subsequent development of a pulsed dual‐frequency L‐band radar operating at 1.2 GHz. The gravity wave modulation manifests itself as a narrow, Doppler‐shifted, resonance peak in the product power spectrum of the backscattered returns. The dispersion relation (for both deep and shallow water) of the modulation pattern matches that of gravity waves. Modulation amplitude spectra have been experimentally obtained which, after sufficient averaging, closely resemble directional gravity wave spectra simultaneously obtained from capacitance wave probe and Sea Photo Analysis measurements. Temporal stationarity of the large‐scale gravity wave structure may only be assumed for finite data acquisition times of, perhaps, the order of one hour, or less. Experiments with the X‐band system, however, have shown that high resolution, sufficiently averaged, densely sampled spectra require measurement periods much longer than one hour. To reduce the overall data acquisition time, multiplexing of the individual spectral samples has been employed. The amount of spectral averaging required was substantially reduced by developing and using a dual‐frequency L‐band radar (Bragg resonant with 12 cm short‐gravity waves) which proved to be much less sensitive to wind‐induced fluctuations in the small‐scale wave return. A method is also presented for enhancing the amplitude of the resonance peak through selective Doppler filtering. This technique was subsequently used to facilitate detection of small tidal currents flowing in the Chesapeake Bay.