Ocean Wave Detection and Direction Measurements with Microwave Radars

Abstract

Electromagnetic energy at microwave frequencies is backscattered by the short-wave component of the ocean wave spectrum. Complex scattering models, which include both Bragg resonance and Rayleigh scattering, have been postulated to describe the amplitude of radar return. The actual return is complicated by the intensity and distribution of the facets of infra-gravity waves. Furthermore, long gravity waves modulate short waves, their tilt and displacement depending on their instantaneous position relative to the long wave profile and extraneous vector quantities such as surface current gradients. As the maximum modulation is in the direction of wave motion, the discernibility of ocean waves is dependent on the radar look direction. Waves traveling or containing components in the azimuth direction (along track direction of the flight) are sensitive to defocusing due to Doppler shifts in the case of the synthetic aperture radar (SAR), introduced primarily by velocity components such as perturbations due to long wave orbital velocities, Bragg wave velocities, and surface current velocities. For a given velocity component, defocusing is greater at L-band frequencies than at X-band, resulting from the longer residence time of a moving target in the L-band aperture, given equivalent resolution. The principal uses of radar imagery for wave studies are to derive direction and length information. While length and direction are determinable from the radar signal, the accuracy with which this information can be established had to be tested against more conventional ocean instrumentation. Data obtained with a pitch-and-roll buoy and current meters were compared to X-band SAR wave imagery obtained simultaneously over a test site offshore of Florida. Analysis shows that the wave direction at the peak frequency of the wave spectrum from buoy and current meter measurements agrees with the frequency-averaged radar directions within 2 degrees. Agreement in the data is considered exceptional, given that the measurements were made in 8-12 m water depth, where wave refraction is strong. Shore-based radars have also been used to derive wave direction and length information. Comparison between the measured wave directions for the principal wave trains, as seen on aerial photographs and shore-based radar images, simultaneously obtained show agreements within 3 degrees. In the case of airborne real-aperture radars, wave length information derived from the radar is in good agreement with that determined from simultaneous laser profilometer spectra, and the direction observed is essentially the same as the local wind. With case-by-case determination of the modulation depth and focusing parameters, the SEASAT-A L-band SAR will produce much needed information for wave refraction studies on continental shelves.