Imaging properties of monostatic and bistatic troposcatter radars

Abstract

Three tropospheric scatter systems, featuring antenna-beam angular discrimination, or a combination of both antenna-beam angular discrimination and signal-delay temporal discrimination, are compared for sensitivity and spatial resolution as determined by theoretical calculations. Two of the radars used are bistatic; the third is monostatic or quasi-monostatic. Whenever practical, parameters are chosen equal for all three radars. The linear dimensions of the scatter volume and its size as a function of position in space are determined. The imaging properties of each radar are analyzed after the scatter integral (received power level) is numerically evaluated using a tropospheric multiple-layer model of turbulent refractive-index structure. Results are presented for a 500-km baselength in the bistatic case. Double layers of 100-m vertical width and 500-m vertical separation are displayed as seen with the angular beam resolution of 0.15 ø (between 3-db points) and 40-nsec temporal resolution corresponding to a 25-MHz system bandwidth. INTRODUCTIONElectromagnetic scatter systems have long proved their worth in remotely analyzing refractive-index structures of the troposphere. Weather radars responding to matter that is a condensate (cloud and precipitation particles in the liquid or frozen phase) are in common use. The scattering of gaseous inhomogeneities in the air, which at radio frequencies is due mainly to the water-vapor content and the temperature and pressure composition of the air, is the subject of extensive research that has already resulted in a better understanding of meteorologic processes [Hardy and Katz, 1969;Konrad, 1968; Ottersten, 1969; Richter, 1969] and will ultimately allow detection of such essential parameters as hazardous velocities and turbulences in the air.The performance of a scatter system in analyzing the gaseous refractive-index structure of the air is measured by three criteria' temporal resolution, spatial resolution, and sensitivity. This report deals with the latter two characteristics with reference to static models of the troposphere.Evaluation of the scatter integral for volume scattering from a refractively turbulent medium is quite involved, particularly in the bistatic case, due to the complicated shape of the scatter volume and the Copyright ¸ 1973 by the American Geophysical Union. 89 variability of most factors contributing to the scattering process. In the past, generally, simplifying assumptions were made about these parameters and were justified, for example, when calculating received-power levels in widebeam transhorizon communication links [Du Castel, 1966]. A statistical approach [Atlas et al., 1968] has given a somewhat refined analysis of forward-scattered power in a highresolution system. For a detailed evaluation of the mapping capabilities of scatter radars, however, such approximations are not satisfactory. The results presented in this paper are accurate, except for the constraint that narrowbeam antennas were used. The integration techniques, particularly t...