Calibration of Z_dr for NEXRAD

NCAR is conducting an experiment for OS&T of NOAA to evaluate Z _dr calibration techniques for the WSR-88Ds using S-Pol, NCAR's S-band polari-metric radar. Three techniques for Z_dr calibration were investigated: 1) vertical pointing data in light rain, 2) engineering calibration and 3) the crosspolar power technique. Measurement and analyses were performed in order to quantify the uncertainty of the estimated calibration numbers and the measurement procedures that yield such uncertainty. The uncertainty of measurements can be separated into two categories: 1) systematic and 2) random. Though our work with S-Pol has had a large focus on systematic errors, this paper concentrates on the quantification of the random errors. Vertical pointing (VP) measurements in light rain are widely regarded as the most accepted way to calibrate Z_dr. Measurements with S-Pol thus far show excellent agreement between the Z_dr bias found via VP measurements and the crosspolar power techniques. For the experimental data used here, both techniques yielded uncertainties well within the desired limit of 0.1 dB. Additionally, it was shown that the crosspolar power technique can successfully be employed on radar systems that achieve dual polarization measurement via simultaneous transmission of H and V polarizations as NEXRAD will do. In this case, a slow waveguide switch is used to gather alternate PPIs of transmit only H and transmit only V data. Using indexed beams, the crosspolar powers from the alternate PPIs was equated. This was done for ground clutter since the backscatter cross sections of many clutter targets should remain constant over periods of several minutes. Thus, these results showed that the crosspolar technique could be used with NEXRAD type radars. Further evaluation of the engineering calibration technique awaits the completion of the Automated Test Equipment (ATE) system. The success of the engineering calibration technique will depend in part on precise measurement of the specifications for the waveguide couplers. The impedance mis-matches between the various connections will also need to be measured very accurately if Z_dr is to be calibrated to within 0.1 dB uncertainty.