Removal of clutter background limitations in dual-frequency scattering from the ocean

Abstract

Frequency agility is applied to dual-frequency scattering from the ocean using an L-band microwave system in order to reduce the clutter background which has previously limited signal detectability in such scattcrometers. Doppler spectra of the return show improvements of up to 13dB in signal-to-clutter ratio using this technique compared with the standard nonfrequency-agile technique. This improvement greatly enhances the ability of a dual-frequency scatterometer to measure ocean surface currents. Composite surface scattering theory is applied to explain the signal improvement. The clutter background is shown to be proportional to the ocean surface wave spectrum evaluated at a wavenumber corresponding to the wavenumber separation between the carrier frequencies of two transmitted pairs of frequencies. Thus, if the frequency separation between these carriers is equal to the frequency separation between the lines of each pair, the entire signal received by the system is proportional to the ocean surface wave spectrum evaluated at the common wavenumber difference. This three-frequency technique completely removes previous limitations on the measurement of ocean wave spectra by dualfrequency scatterometers by converting the clutter background into part of the desired signal. Signaldetectability is then limited only by thermal noise.

Airborne microwave Doppler measurements of ocean wave directional spectra

“…In any case, resolution will be limited by antenna dimensions (Kenney et al 1979, Walsh et al 1985. The short pulse radar system and its Fourier transform, the two-or three-frequency microwave system, have been shown to be effective techniques for the measurement of ocean wave directional spectra (Plant 1977, Alpers and Hasselmann 1978, Jackson 1981, Johnson and Weissman 1984, Schuler et al 1985, Jackson et al 1985a, b, Weissman and Johnson 1986). Both systems offer good angular resolution and are capable of being flown at spacecraft altitudes.…”

Measurements of the effects of surface winds on ocean currents with a stepped‐frequency ΔK radar system

“…For the SFDK radar, N -16, resulting in 6 dB improvement, which was experimentally measured and reported(Popstefanija and Mcintosh, submitted). Frequency agility was also implemented bySchuler et al [1985] using a slightly different algorithm. In their case, 49 frequency pairs produced a signal-to-clutter improvement of 6.5 dB.A cross-product spectral density record obtained by SFDK over a 20-hour period is shown inFigure 5.…”