Absolute dual-polarization radar calibration: temperature dependence and stability with focus on antenna-mounted receivers and noise source generated reference signal

After two years of operational atmospheric monitoring, MeteoSwiss is satisfied with the AMR system concept choice, also in view of the necessary constrains related to maintenance and temperature control inside the radome. Nevertheless, absolute radar calibration is indeed a challenging topic. Radar manufacturers typically provide stable continuous wave RF generators known as internal test signal generators (ITSG) as an absolute reference signal to calibrate the dBZ value for the computation of the weather radar equation with a known dBm value at the calibration reference point. MeteoSwiss decided to use a noise source as absolute reference for calibrating their radar systems. Most important benefits of the realized calibration solution are the high temperature stability of the noise source, online monitoring of TR Limiter losses, online noise figure measurements, Hi-Low Calibration for the Dynrex [1] solution (dynamic range extension) and the online absolute calibration of the high sensitivity channel, since most of the metrological echoes are weaker than -50 dBm, which is the transition point between the high and low sensitivity channel. The term “online” describes a calibration performed twice during the 5min volume scan by injecting the noise signal in defined range gates where no weather echoes are present. This publication describes the most important parts of the antenna- mounted receiver (AMR) concept and in particular the inherent radar calibration concept by focusing on the temperature-dependence on the horizontal and vertical receiver chain and the calibration noise source. The receiver temperature dependencies for reasonable day/night cycles derived by field measurement on all Swiss radar sites are verified with lab measurements in a climate chamber. The bias depending on temperature for differential polarimetric measurements (ZDR) is analyzed and discussed. A formula was developed for Selex AMR- radar users to calculate the absolute receiver stability for the elected calibration interval. Finally a recommendation about temporal calibration cycles for AMR systems in European regions is given to assure a continuous accurate absolute radar calibration.