Aircraft and satellite measurement of ocean wave directional spectra using scanning‐beam microwave radars

Abstract

A microwave radar technique for remotely measuring the vector wave number spectrum of the ocean surface is described. The technique, which employs short‐pulse, noncoherent radars in a conical scan mode near vertical incidence, is shown to be suitable for both aircraft and satellite application. The technique has been validated at 10 km aircraft altitude, where we have found excellent agreement between buoy and radar‐inferred absolute wave height spectra.

KuROS: A New Airborne Ku-Band Doppler Radar for Observation of Surfaces

“…The theory underlying this measurement technique was given by Jackson et al(1985a), and is briefly reviewed here. For each azimuthal direction ϕ of the antenna, positions on the mean sea surface may be defined by their horizontal local coordinates x and y, where x is horizontal distance along the antenna pointing direction, and y is along the azimuth direction.…”

“…The relatively large antenna beamwidth is required to estimate ocean wave spectra from a real-aperture radar at smallincidence (see al., 1992 or Jackson andal., 1985a). In addition, a Doppler measurement capability has been designed for KuROS.…”

“…As discussed by Jackson et al (1985a), the speckle noise of the radar technique produces a supplementary modulation of σ°, which is included in the measured modulation spectrum, and 13 this speckle noise must then be removed from the data for precise quantitative studies. In order to estimate the spectrum of the speckle noise, Hauser et al (1992) used a semi empirical method based on the comparison between data integrated over different time intervals.…”

“…In practice m(x,ϕ) is obtained by removing a low order polynomial fit from the measured radar power profile, thus eliminating the trend and retaining only the modulations. The sea wave polar-symmetric height spectrum F(k,ϕ) is then obtained from the expression (Jackson et al, 1985a;Hauser and Caudal, 1996):…”

KuROS: A new airborne Ku-band Doppler radar for observation of the ocean surface

“…Both antennae have a ±10° elevation and ±4° azimuth one-way beam-width. This relatively large aperture is required to estimate ocean wave spectra from a real-aperture radar at small-incidence (see [3], [4]). Both antennae have been tested in anechoic chambers to determine their lobe patterns.…”