Weather Radar Calibration and Testing using the Moon as Reference Target

The calibration of weather radar systems is a permanent subject of research and development. Since there is no “reference rain” a weather radar must be calibrated indirectly. Two methods are used so far: the ground truth method and the engineering calibration. The ground truth method uses an accurate ground sensor like a disdrometer and relates the measurement of this sensor to the radar measurement of the volume above the sensor. The results presented in this paper are related to the engineering calibration method. This method is based on the calibration of factors representing the features of radar subsystems in the meteorological radar equation. Since the accuracy of the calibration can be improved if as many factors as possible are calibrated as a product with one measurement (provided that the accuracy of this measurement is at least as accurate as the accuracy of the individual calibration of the factors) methods are investigated which allow the calibration of products of factors of the radar equation. The most prominent of these methods is the calibration using the sun as reference target. This method allows the measurement of the antenna beam width and the ZDR offset of the complete receive channel. If the actual sun radiation power measured by a sun observatory is used as reference even the gain of the receive channel including the antenna gain can be measured. However the sun calibration has the drawback of not including the transmit path of the radar. Therefore other methods have been tried. The most promising methods are the transponder calibration and the balloon calibration. The transponder calibration is based on an active or passive reference target which is placed in the Fraunhofer zone of the antenna. The target must be calibrated itself with a high degree of accuracy. Since it is usually placed relatively close to the ground, multipath propagation and obstruction must be avoided. The balloon calibration avoids the problems related to ground reflections and obstructions because the reference target (which can be the balloon itself) is carried to a high elevation. But it is difficult to keep the target in a stable position which makes the measurement quite noisy. Moreover the logistic effort required to conduct such a calibration is enormous. In order to find a method which calibrates the complete system by avoiding the drawbacks of the known methods we studied the moon as a potential calibration target. The movement trajectories of the moon and the earth and its distance are exactly known which is important for calibration purposes. The radar signature of the moon and its polarimetric properties are also well known from studies conducted in the 60ties preparing the Apollo moon missions. Since the moon always turns the same side to earth its signature does not change. For our measurement we used a METEOR 1600SDP10 radar. This is a fully coherent polarimetric Doppler weather radar. The measurement was conducted during daytime. The position and trajectory of the moon was taken from a public-domain astronomical data base. We got a clear power and also Doppler signal with the highest possible integration rate of the radar. It was even possible to recognize some of the characteristics of the radar signature which were reported in the earlier publications. Due to the large distance the measurement is a very sensitive check of the range calibration of the radar. With a suited algorithm for the extraction of the radial movement of the moon relative to earth it is possible to test the velocity measurement. The measurement did also reveal that a higher integration rate is required in order to increase the signal-to-noise ratio.