Fast Calibration of Weather Radar Systems for Multi Polarisation Radar Measurements

Polarimetric weather radars, which are now becoming operational, are using a very limited method to retrieve the polarimetric products, called the STAR mode. It uses robust and low cost hardware but cannot measure the full scattering matrix. The Germany Aerospace Center (DLR) in Oberpfaffenhofen, Germany operates a research weather radar for 25 years now with can receive and transmit arbitrary polarizations for each radar pulse to gather the full scattering matrix. The latest development on that radar makes it possible to perform multi polarization measurements. This also enables the radar to estimate weather targets properties using different polarimetric measurement schemes simultaneously within one scan and without any time lag. By this, a direct comparison of different measurement schemes become possible and a characterization of systematic estimation offsets can be performed. Furthermore system effects which influence the polarization, like additional phase shifts on transmit in the STAR mode, can be introduced and studied. The full power of the system can be used by scanning with several polarizations like horizontal/vertical, left/right hand circular and 45/135 degree linear polarization to estimate precipitation parameters. Besides using the optimal combination to polarization pairs for the retrieval of precipitation properties, faster and better scans become possible by using the polarimetric decorrelation to get more independent samples from the target. All these measurements have to be combined with new calibration efforts on the radar to make meaningful measurements. An end to end calibration of weather radar is always very challenging. As the used radar is not reciprocal and different polarizations are to be calibrated, a completely new calibration system, called polarimetric active radar calibrator (PARC), was designed and build. It is made to measure the polarization purity at least down to -35dB. It can handle the non reciprocal behavior of the radar and is fast enough to measure every single radar pulse transmitted. The measurements are performed in the far field of the radar antenna. Therefore the PARC system can be remotely controlled by the radar. This makes a self calibration of the radar feasible. Due to its modular concept the PARC can be easily adapted to other radars as well.