EXPERIMENTS ON BRAGG AND NON-BRAGG SCATTERING USING SINGLE-FREQUENCY AND CHIRPED RADARS

A collection of laboratory experiments on Bragg and non-Bragg scatteri ng, mainly from water surfaces, are conducted using a radar system whi ch can be operated in a frequency-chirped, range-resolved mode, or a s ingle-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 det ail at small grazing angles. The Bragg resonance, which is sharp for s ingle-frequency operation, is broadened for a chirped system where the broadening is proportional to the chirp bandwidth. For single-frequen cy operation, Bragg resonances are found to be Lorentzian; the resonan ce 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 dis tinguished 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 o ut that although a fine range resolution is desirable, in some cases, it may lead to ''phantom binning.'' Other physics issues related to no n-Bragg scattering using chirped systems, such as the effects of multi ple scattering and the dependence of frequency chirp on the dielectric constant, are also examined and discussed.