Enhancement of radar signals by double bounce circular polarization

On the basis of available radar data, most of it concerning linear polarization, it had been postulated that although in the worst conditions of rain a circularly polarized signal may return distorted into an elipse, it would still be distinguishable from other polarizations and could be used to discriminate targets from ground clutter. Since a circularly polarized signal which was reflected from the ground to a target with vertical enhancement and then back to the radar antenna would return with a screw sense opposite that of single reflection return signals, and since ellipticity could be preserved, a system could be constructed which would effectively discriminate between background clutter from rain and the target return. In the experiment constructed to verify the preliminary hypothesis, the angle of incidence between the transmitting antenna, the target, and the ground was varied from 6 to 15 degrees. Over this range, linear polarization shows at least 5 db greater return for any angle in fair weather than does circular polarization with double bounce reflection. However, double bounce circular polarization (DBCP) reduces clutter from terrain background by from 5 to 10 db in any weather, and the portions of the experiment conducted in heavy rain showed that under those conditions the advantage of DBCP over linear was as much as 30 db. DBCP can pick up radar targets in heavy precipitation when linear radars are totally ineffective. It offers distinct possibilities as an all weather radar capability which could be added to conventional radars at very little cost either in money or in complexity. In conjunction with a system of passive reflectors on the ground it could provide a low cost bad weather navigational aid In this paper a method is discussed for radar air to surface target identification in heavy precipitation by the use of a circularly polarized signal reflected from the ground to the target, then back to the source. A reversal in screw sense allows d- scrimination between background clutter and target return. Curves are presented to show relative effectiveness variations with angle of incidence, degree of precipitation, and type of terrain. Applications are suggested for use of the system as an all weather adjunct to conventional radar systems and for foul weather navigation with a system of passive ground reflectors.