CALIBRATION OF RADARS USING POLARIMETRIC TECHNIQUES

Absolute calibration of a weather radar is critical for quantitative estimation of rainfall. This is a very difficult measurement especially for beam-filled targets like precipitation. In this paper we discuss a method that uses the properties of rain medium itself to obtain accurate system gain calibration. This technique is based on the principle that the rainfall rate measured using absolute reflectivity (Z) and differential reflectivity (Z(DR)) is the same as that obtained from specific differential phase The measurements required for this technique are Z, Z(DR), and K(DP). We compare the rainfall rate estimates R(DR) obtained from Z and Z(DR) with the estimates R(DP) obtained from K(DP). The scatter plot between the two rainfall estimates should lie close to a 1:1 line and any systematic deviation from this line can then be removed by appropriately adjusting the system gain. It is noted here that Z(DR) can be calibrated accurately because it is a differential power measurement and K(DP) is obtained from differential phase measurement which is unaffected by system calibration. The sensitivity and accuracy of this technique are studied in this paper. We present theoretical and simulation results evaluating the accuracy of this technique for C-band frequencies. Our analysis indicates that by sampling over few data sets of rain, the calibration can be done to an accuracy of 0.6 dB.