Experiments on Bragg and non-Bragg scattering using single-frequency and chirped radars

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 gravity-capillary 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.