Evaluation of the refractivity measurement feasibility with a C band radar equipped with a magnetron transmitter

Precipitation radars are mainly dedicated to rainfall measurement, but the capability to use ground echo returns to measure the refractive index near the surface has recently been demonstrated (Fabry, 2004). This measurement is derived from phase variation between the radar and the ground echo, assumed to be a fixed target. As demonstrated during the IHOP project, refractivity measurements lead to near-surface moisture estimations and, once implemented on operational radar networks, could potentially be very useful for meteorological prediction. Radars equipped with klyston transmitters, like Nexrad radars, have very well defined transmitted waveform in frequency and phase, and they can be used for refractivity measurements. However, due to the cost problems, most of the operational European networks are equipped with magnetrons for which the transmitted frequency and phase are not precisely known and the refractivity measurement with a magnetron radar is still an open issue. In this paper, we present a signal processing method to correct, in the spectral domain, the received signal from the small variations of the transmitted frequency and phase due to the magnetron transmitting tube. This correction is evaluated on ground echo returns during dry clear air propagation and the refractive index measurement accuracy with an operational C band magnetron radar is discussed.