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

Abstract

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

What are the mechanisms for non‐Bragg scattering from water wave surfaces?

“…The experimental facility, the X-band pulsechirped radar (PCR) that was used as the principal instrument in this experiment, the operating modes of the PCR, and the gate numbering correspondence to the PCR line-of-sight range have been described elsewhere [Lee et al, 1997a]. The optical specular event detector (OSED) [Barter and Lee, 1996] is an imaging detector at visible wavelengths which measures two-dimensionally resolved plane polarized returns.…”

Crosswind ocean radar backscatter and two‐scale scattering model at low grazing angles

“…Many experimental studies report that at low grazing angles, pulsed enhancements appear mostly at horizontal polarization [Kalmykov and Pustovoytenko, 1976;Trizna et al, 1991;McLaughlin et al, 1995;Lee et al, 1995Lee et al, , 1997. These enhancements, known as ''sea-spikes,'' are observed within a narrow polar diagram, which produces the spikes, and in narrow space-time bands.…”

Electromagnetic Scattering from Multiple Scale Geometries

“…In addition the maximum of the Doppler spectra (power New experimental results obtained during the early to mid 90's sparked a renewed interest in appropriate scattering models to interpret polarized radar data. For example, experimental observations with the TRW X-band radar mounted on the bow of a boat in a Loch Lihnne experiment [Lee et al, 1995[Lee et al, , 1996 as well as complementary laboratory experiments [Lee et al, 1997a[Lee et al, , 1997b showed that for low grazing angles the horizontal returns could be larger than the vertical returns. Further, the regions where these events or spikes (HH/W >1) occurred were of an intermittent nature and the result of very low scattering returns, of the order of -60 to -100 dB range (See Figure 2 below).…”

“…Indeed, the power spectrum of the polarized backscattered returns, also known as the Doppler spectrum, is routinely used in experiments for diagnostic and analysis of the distribution and speed of the scatterers on the moving surface, cf. [Lee et al, 1997a;Rozenberg et al, 1996;Lee et al, 1996;Lee et al, 1997b;Lee et al, 1995;Ja et al, 2001;Duncan et al, 1999]. …”