Abstract
The influence of a surface-active substance (SAS) film on the Doppler spectrum width at small incidence angles is theoretically investigated for the first time for microwave radars with narrow-beam and knife-beam antenna patterns. It is shown that the requirements specified for the antenna system depend on the radar motion velocity. A narrow-beam antenna pattern should be used to detect slicks by an immobile radar, whereas radar with a knife-beam antenna pattern is needed for diagnostics from a moving platform. The study has revealed that the slick contrast in the Doppler spectrum width increases as the radar wavelength diminishes, thus it is preferable to utilize wavelengths not larger than 2 cm for solving diagnostic problems. The contrast in the Doppler spectrum width is generally weaker than that in the radar backscattering cross section; however, spatial and temporal fluctuations of the Doppler spectrum width are much weaker than those of the reflected signal power. This enables one to consider the Doppler spectrum as a promising indicator of slicks on water surface.
Highlights
The detection and recognition of sea surface slicks by means of remote facilities grow more and more urgent because of aggravating environmental situation and increasing sea surface pollution by human activity products
The dependence of the Doppler spectrum width on the measurement conditions at small incidence angles is analyzed
In fig. 8 we present the dependence of the Doppler spectrum width at nadir viewing for a knife-beam antenna pattern (1o х 22o) and the wavelength 0.021 m on the wind speed in measurement from a helicopter for the clean water case and for the slick E = 20 mN/m
Summary
The detection and recognition of sea surface slicks by means of remote facilities grow more and more urgent because of aggravating environmental situation and increasing sea surface pollution by human activity products. A SAS film occurring on water surface leads to ripple damping (slick formation) and the reflected radar signal power (the radar backscattering cross section) reduces. The above-mentioned effects impede the detection of slicks produced by a SAS film on the surface using the reflected signal power in the region of middle and large incidence angles. Our previous investigations devoted to the development of the algorithms of roughness parameter retrieval employing a microwave Doppler radar at small incidence angles [7,8,9,10] stimulated the study of the possibility of detecting films on water surface by the Doppler spectrum width. Numerical estimates of the influence of sea surface slicks on the Doppler spectrum width at small incidence angles for radars with narrow-beam (1o х 1o) and knifebeam (1o х 22o) antenna patterns are presented. The expressions for the viscous damping decrement and the wind increment are given below [1315]:
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