Popagation and Backscattering Challenges for Planar Polarimetric Phased Array Radars

Estimating the polarimetric variables with planar phased array radars requires calibration at every pointing direction. In the principal planes calibration is relatively simple because to first order the transmitted fields from the two ports that determine the polarization are orthogonal hence there is no coupling through propagation and backscattering by rain. This means that separate calibration of each channel can be made. If the pointing direction is outside the principal planes, the polarization of the transmitted wave in general has both H and V components, even if only the H or V port is excited. In that case, the estimates of the polarimetric variables would incur a geometrically induced bias which is affected by transmitted wave characteristics, propagation, and backscattering. The fundamental challenge is to devise a planar polarimetric PAR that can overcome this geometrically induced bias. Design alternatives that mitigate this bias are presented herein. These are: a) Antennas for which the V radiating element is an electric dipole and the horizontal one is a magnetic diploe. b) Combined use of the antenna ports so that the transmitted field is composed of equal horizontal and vertical component. c) Constrained measurements within a narrow range of directions close to the principal planes. d) Alternate transmission but simultaneous reception through the two ports. e) Phase coding of the transmitted signals in each port. The maturity of these alternatives as well as the relative merits will be discussed.