Calibrating differential reflectivity on the WSR-88D

On polarimetric radars which transmit simultaneous horizontal and vertical polarizations three polarimetric variables must be calibrated. These are reflectivity factor, differential reflectivity, and differential phase. Reflectivity factor remains the most difficult and elusive. Differential phase is not calibrated per se; rather the system differential phase must be known to determine the beginning of the phase unwrapping interval. This can be done easily using ground clutter. Differential reflectivity does need to be calibrated and this paper suggests how to do it on the WSR-88D network. The essence is to separate bias into a constant part and time varying part. Time varying part is caused by difference in receiver's gains. Measurements of constant bias are made at the time of installation and the variations in receiver gains is tracked by automatic calibrations at end of volume scans, much like current calibration of automatic gain control circuits. Sun scan is used at time of installation to measure the differences in antenna gains and parts of microware circuits not accessible to instruments. Existing internal signal source and internal noise generator are required to account for the differences in gains and bandwidths. Most measurements are differential so that undesirable effects cancel out. The procedure is repetitive with an rms error and bias under 0.1 dB.