Abstract. Experimental data obtained in the past several years have provided conclusive evidence that non-Bragg scattering plays a major role in X-band microwave backscatter from water wave surfaces, and non-Bragg scattering events are especially noticeable at small grazing angles, for wind-roughened surfaces, in the presence of breaking waves. We have conducted scattering experiments under a variety of wind wave conditions in an attempt to determine the different mechanisms which contribute to non-Bragg scattering. At small grazing angles we find that non-Bragg scattering is due to fast scatterers generated by the wave breaking, and with increasing wave steepness and surface roughness, mechanisms of multiple scattering and multipath interference become increasingly important. -1 m) and the temporal radar backscatter was correlated with video images. They observed single-bounce backscatter from specular facets and gave quantitative results of the measured distribution of facet size. Closer scrutiny of their data reveals that the specular reflection they observed was not due to fast scatterers but rather to the slowest scatterers and was due to backscatter from the smoother patches in the troughs of larger waves. However, the notion of "specular reflection," which produces signals with HH = VV, was quickly adopted by theoreticians and modelers, and it was assumed that single-bounce reflection from specular facets was 123

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