Backscattering experiments at microwave frequencies were conducted off the west coast of Scotland in the summer of 1991. Using a dual-polarization, eightfrequency, X band, coherent scatterometer mounted on the bow of a boat, we measured time-resolved backscattering from ocean waves at a range of grazing angles from 10 ø to 70 ø. From the grazing-angle-dependent signals and their Doppler spectra, we are able to differentiate Bragg scattering from non-Bragg scattering and resolve "peak separation" between the vertical and horizontal polarizations. We observe instances of "super" events, i.e., instances when the horizontal polarization return power equals or exceeds the vertical polarization power at particular frequencies. We find that "super" events occur not only at low grazing angles but at any grazing angle for upwind viewing directions and obtain statistics for such occurrences as a function of grazing angle. We study the coherence properties of scatterers and find strong evidence that at low grazing angles, lifetime-dominated, non-Bragg scattering contributes noticeably to returns of both polarizations, but is dominant in providing returns for the horizontal polarization. We examine "spiking" events and find that they can be related to, but need not be limited to, breaking wave events. By comparing the data of upwind runs with cross-wind and circle runs, we obtain wind direction dependence of Doppler spectra, which further assists in the identification of scattering mechanisms. 2591 2592 LEE ET AL.' X BAND MICROWAVE BACKSCATTERING FROM OCEAN WAVESTable 1. Frequency Pairs for Microwave Scatterometer Polarization, GHz Pair Vertical Horizontal 1 9.020 9.021 2 9.170 9.171 3 9.320 9.321 4 9.470 9.471 four quadrature mixers, one for each transmitted frequency. Each mixer generates an in-phase (I) and quadrature (Q) signal with a frequency response of 0 to 1000 Hz. With complex amplitude thus generated for each frequency, there are 16 channels of output. During an experiment the 16 signals are recorded digitally on a multichannel cassette recorder.An absolute calibration of the scatterometer system [Barter et al., 1993] was conducted in a large (10 m x 10 m x 30 m) anechoic chamber using spheres and cylinders of various sizes as well as corner reflector targets. Measurements consisted of establishing the output power and receiver gain of each of the eight frequency channels, the system radiation patterns in two perpendicular planes in the forward half sphere, the range dependence of the signal power, and the cross-polarization isolation of the scatterometer system. The essential results of the calibration are as follows: total power (eight channels), -1 W; nominal receiver gain each channel, -60 dB; system radiation pattern, approximately Gaussian main lobe; azimuthal plane -3 dB beam width, 8.7 ø for VV and 10.3 ø for HH; vertical plane -3 dB beam width, 11 ø for VV and 9.6 ø for HH; average -3 dB beam width, 9.5ø; antenna gain, 26.6 dB; cross-polarization isolation, >35 dB; image rejection ratio, --45 dB. As an ex...

Results of X-band microwave backscatter from sea surfaces at a small-grazing angle for a range of wind speeds (3.8-10.3 d s ) are presented. The data indicate that the speed of faster-than-Bragg scatterers and their time-averaged polarization ratio both increase with wind speed. At a friction velocity above -21 c d s , super events (i.e., backscattering events where the horizontal-polarization power exceeds the vertical-polarization power) begin to appear. The fraction of fast scatterers which produce super events also increases with wind speed. The presence of super events indicates that in addition to scattering mechanisms, such as Bragg-resonant scattering, composite surface theory, wedge scattering, and specular reflection, other mechanisms which favor the horizontal-polarization returns must be considered.

Abstract. A collection of laboratory experiments on Bragg and non-Bragg scattering, mainly from water surfaces, are conducted using a radar system which can be operated in a frequency-chirped, range-resolved mode, or a single-frequency mode without range resolution. A Bragg wave generator is used to generate monochromatic, plane gravitycapillary water waves with which Bragg resonance and Rice's theory are examined in some detail at small grazing angles. The Bragg resonance, which is sharp for single-frequency operation, is broadened for a chirped system where the broadening is proportional to the chirp bandwidth. For single-frequency operation, Bragg resonances are found to be Lorentzian; the resonance width has been used to infer the spatial decay rate (the imaginary wave number) of the Bragg waves, and the results are in agreement with that obtained from radar RCS and wave-height probe measurements. For scattering from water surfaces, Bragg and non-Bragg scattering are distinguished by the fact that the former process yields polarization by diffraction where HH is always less than VV, while the latter process usually yields polarization by reflection where HH is usually greater than VV. Fresnel reflection, a prime example of non-Bragg scattering, is also studied using metal dihedral and labyrinth targets. We point out that although a fine range resolution is desirable, in some cases, it may lead to "phantom binning." Other physics issues related to non-Bragg scattering using chirped systems, such as the effects of multiple scattering and the dependence of frequency chirp on the dielectric constant, are also examined and discussed. IntroductionIt is well known that Bragg scattering is one of the major mechanisms which contribute to radar returns in scattering from water waves or sea surfaces fine range resolution, a large chirp bandwidth is required; however, several questions pertaining to the effects of chirp on scattering from water waves have not been addressed. For example, what is the Braggresonant water wavelength for a chirped system? In order to answer these questions, we perform experiments in a large wave tank, using a Bragg wave generator to generate monochromatic plane waves, and compare the scattering results of a single-frequency system and a chirped system. Physics issues peculiar to non-Bragg scattering, e.g., Fresnel reflection and multipath scattering using a chirped system, are also investigated and discussed. Fresnel reflection is one of the fundamental processes which give rise to the many manifestations of what is collectively called "non-Bragg" scattering from rough water surfaces [Lee et al., 1995a]. The difference between non-Bragg scattering and Bragg scattering is that the latter process is due to coherent, constructive interference from periodic structures, which is basically diffraction, while the former process is not. A list of possible mechanisms to explain non-Bragg backscatter was 1725

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In this paper, we describe TRW's latest version of the scanning laser slope gauge (SLSG) which was used to characterize the sea surface in a recent ocean experiment. The SLSG, capable of measuring the spatial distribution and temporal evolution of the surface slopes of a patch of ocean, provides ground truth data which form a quantitative basis for the understanding of ocean wind-wave interactions and the development and validation of radar scattering models relevant to ocean remote sensing.