A "venitian blind" polarization grid reflector lens for dual-beam 95-GHz airborne radar measurements

The Microwave Remote Sensing Laboratory (MIRSL) at the University of Massachusetts (UMass) and the University of Wyoming (UWyo) Atmospheric Science Department constructed a low profile polarization grid lens. The purpose of the lens is to provide the 3-mm wavelength UWyo Cloud Radar (WCR) dual-beam measurement capability on-board the UWyo KingAir aircraft. The concept of the lens is based on conventional polarization grid reflectors which have the unique property of being almost perfectly transparent to incident electromagnetic fields when the electric field lines are perpendicular to the grid lines, but reflective when are parallel. The WCR can electronically switch (from pulse-to-pulse) the polarization of the transmitted field between vertical and horizontal, so a conventional polarization grid reflector plate could be used to switch the radar beam between 0 and 45 deg when placed in front of the antenna at a steep, 22.5 deg, angle. The problem, however, with using a conventional grid plate in this geometry is that with the 12" diameter WCR antenna, the reflector would extend over 3 ft out the side of the aircraft. Such a large object cannot be mounted on the King Air so as an alternative, a collapsed reflector plate was designed with the concept of the venetian blind structure.

A prototype lens was constructed and tested with the WCR in Laramie using a pole mounted trihedral corner reflector. The thickness of the 12" diameter lens turned out to be 4'' - much thinner than the height of 36" of a conventional grid reflector for the same application. This reduction in size, however, was achieved at the expense of performance. The lens reduced the sensitivity of the radar by about 5 dB and the isolation between the deflected and the direct beams was only 3 dB when the deflected beam was transmitted, making the lens a beam splitter rather than a beam switch. During airborne measurements, however, the difference in Doppler shift increases the isolation between the two beams to above 20 dB, making dual beam Doppler and reflectivity measurements still possible from an aircraft.

In November 2000 the lens was mounted in the KingAir and during two flights dual beam radar data were collected from ice clouds. The lens split the radar beam into the "side" and " side-aft" (beam pointed 45 deg aft in the horizontal plane) beams. The Doppler shifted returns from the two beams were distinct and clearly identifiable as the three times folded side-aft beam peak appeared near -13 m/s (indicating a radial Doppler velocity of -65.7 m/s), while the side beam peak was around -4 m/s. For 2-D Doppler velocity field analysis the centroid of each peak was tracked and for the estimation of the attenuation field using Stereorad processing, the power of the peaks was calculated.

This paper will describe the design and performance of the lens and present airborne dual Doppler and 95 GHz radar reflectivity measurements from ice clouds.