Polarimetric Weather Radar Calibration Using Solar Scans

The US NEXRAD WSR-88D (Weather Surveillance Radar – 1988 Doppler) was recently upgraded to polarimetric capability. This upgrade permits identification of precipitation characteristics and type; thus, providing the potential to significantly enhance the accuracy of radar estimated rainfall. However, optimal benefits are only achieved if errors induced by the radar hardware are properly accounted for through calibration. Hardware calibration is a critical element in delivering accurate meteorological information to the forecast and warning community. The calibration process must precisely measure the gain of the antenna, the polarimetric bias of the antenna, and the overall gain and bias of the receive path. The absolute power measurement must be accurate to within 1 dB and the bias between the polarimetric channels must be known to within 0.1 dB. These requirements drive a need for precise measurement of antenna characteristics.

Engineers and scientists with the NEXRAD program employ solar scanning techniques to ascertain the absolute gain and bias of the 8.53 m center-fed parabolic reflector antenna enclosed within a radome. They are implementing use of daily serendipitous interference strobes from the sun to monitor differential reflectivity calibration. The sun is also used to adjust the antenna gain and pedestal pointing accuracy. This paper reviews the methods in place and under development and identifies some of the challenges in achieving the necessary calibration accuracies.