An Analysis of short pulse and dual frequency radar techniques for measuring ocean wave spectra from satellites

Abstract

or alternatively, by composite surbe* theory (Brown,1978^ hasiqft, thougl4 the situation is one of specoar reflection, and the Tadar baehacatter problem on be compared to that of sun t observation in the cue whom the sun is anti-parallel to the observer's line of eight.I am concentrating here on small angle, specular backscatter for two rend t One is t. I believe that the p~ mechanisms involved in the reflectivity modulation by Marge waves are simpler than in the case of largeangle Bragg diffraction backscstter. At large angles these is, in add} tion to a purely geometrical tilting effect, a very strong sensitivity of the electmmagnetle (am) MO& ulation to the datails of the hydrodynamic modulation of the centimetric Bragg-diffraetlng wavelets by the atmospheric and large ocean wave flow fields, While the hydrodynamic contribution-to the large-angle reflectivity modulation has been treated theoretically (Alpers and Homelmann,1978;Valenzuela and Wright, 1979), theory is hard premed to deal with the hydrodynamic complexities and actual field observations (Valenzuela and Wright, 1979;Wright et aL, 1980). In the specular scatter regime, however, there is no special sensitivity to a particular small water wavelength. RMhOt, the entire ensemble of water waves is contributing to the population of specularly reflecting facets, (excluding of course those wavelets smaller than the diffraction limit). For this large ensemble of waves, an assumption of Gaussian surface statistics seems to be a reasonable one, at least in the first order of approximation.The second reason for concentrating on the near-vertical is a more practical one. It is desirable to make the nadir angle as small as posbible in order that the radius of the azimuth scan circle on the ocean surface not exceed the distance over which the wave field can be expected to be homogeneous (cf. Fig. 1).We have said that the radars need Not actually image the surface to measure wave spectra. The idea behind the non-imaging radar measurement is simple: we let the broad em phase front on the surface (i.e., broad compared to the correlation scale of the waves) function to isolate or resolve plane Fourier surface contrast waves travelling in (or contrary to) the direction of radar look. The

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

“…Both of these factors make it difficult to investigate second-order scattering effects: we may have the theory, but we will be unable to satisfactorily verify it. Without a space experiment, there is little reason to develop the theory of measurement any further; that is, to go beyond Jackson's (1981) theory.…”

“…The choice of waveforms, and the manner of detection, is however a critical one. Jackson (1981)--hereinafter referred to as J--has shown that the narrowband two-frequency technique has, inherently, a very low signal-to-noise ratio (SNR) compared to the short-pulse technique. Basically, this is because the sea-spectrum is relatively broadband whereas the two-frequency beat-wave signal is comparatively narrowband.…”

Airborne radar measurements of ocean wave spectra and wind speed during the grand banks ERS‐1 SAR wave experiment

“…The sensor, like the SRA, is primarily used to generate the 2-D gravity wave spectra (Jackson et ai., 1985a, b;Jackson, 1987), but the instrument can also provide the relative radar cross-section using the spectrometer mode (see Chapron et al, 1994, this issue), and SWH and a measure of the sea surface roughness using a separate altimeter mode. In this section, a brief review of the radar spectrometer and altimeter measurements and their data processing is presented.…”

“…The ROWS transfer function is based on the theory that for near-vertical incidence the specular backscatter from the broad spectrum of shorter-scale ocean waves dominates the measured radar cross-section and that the amplitude modulation of that backscatter is due to the tilting of shorter waves by the longer, energy-containing swell (Jackson, 1981). The theory developed shows the modulation variance spectrum Pm(k, tt) to be approximately linearly related to the surface wave slopes through the use of a constant modulation transfer function that accounts for the beam-front matching and tilting of the scatterers.…”

“…The ROWS and SRA (axai its predecessor the surface contour radar) are real aperture systems, and although they utilize very different techniques, both sensors have provided reliable long-wave spectral estimates for numerous air-sea interaction field experiments (Walsh et al, 1985, Walsh, 1991Beal, 1987;Jackson et ai., 1985a, b;Jackson, 1987Jackson, , 1991. Each technique has its own set of limitations with regard to wave field measurements, but neither suffers from the problems caused by aperture synthesis, which limit the Airborne Radar Measurements of Ocean Waves and Wind Speed I 145 ability of any SAR to detect waves travelling in the satellite's along-track (azimuth) direction.…”