Polarimetric Weather Radar Calibration with a Target Simulator

An accurate estimate of the weather radar derived rain rate depends on a thoroughly calibrated radar. However, the continuous determination of the exact value of the radar constant is difficult for a variety of reasons (i.e., wet radome conditions, gain fluctuations etc). For the purpose of accurately calibrating weather radars, Palindrome Remote Sensing is developing a dual-polarization X-band radar target simulator system, which should facilitate the assessment of measurement errors such as reflectivity and ZDR biases as well as errors in Doppler speed, differential phase shift and antenna pointing.

The Palindrome radar target simulator (PaRTS) is an external transponder system that is set-up within the coverage of the radar. It receives the radar pulses, applies a predefined Doppler shift and a time delay and sends them back to the radar with a fraction of the received signal power, where this fraction corresponds to a specific radar cross section (RCS). If the fraction of the re-emitted power is sufficiently accurate, the virtual target that appears in the radar display at the distance that corresponds to the applied time delay has a known RCS and can therefore be used to estimate the radar constant.

In order to specifically address the needs of the weather radar community, the focus in the design of PaRTS is put on the accuracy of the size and speed of the virtual target, which is achieved by a series of internal calibration procedures, which will finally allow to generate a target with a radar cross section whose accuracy is on the order of 0.5 dB. Thanks to the dual-polarization capability, targets with pre-defined ZDR and differential phase shift can be generated, which allows to thoroughly test common dual-polarization weather radars.

First tests with a prototype RTS instrument and two different X-band polarimetric radars showed that targets with high temporal stability can be generated. The accuracy of the Doppler velocity of the virtual target was found to be on the order of 0.1 m/s. Die Figure shows the virtual radar target which was generated at a distance of 91.5 km with an RCS that corresponds to a reflectivity of 64.5 dBZ and a Doppler speed of 2.5 m/s. The target was scanned with a sector scan whose angular resolution in azimuth and elevation was set to 0.2° The scan was repeated every 2 minutes.