Clutter analysis to monitor the stabillity of a portable X-band mini weather radar

Quantitative precipitation estimation and rainfall monitoring based on meteorological data, potentially provided as continuous, high space/time resolution and large coverage data, are of high practical use. For efficiently monitoring intense rainy phenomena events over crucial basins of interest (urban areas for example), an unrealistic extremely dense rain gauge network should be needed. Alternatively, it is possible to use another tool able to provide an overview of the weather situation: the weather radar. More common C-band and S-band ground-based weather radar are able to monitor rain fields over wide areas, but they are normally characterized by limited space and time resolutions, not enough when they are used over urban areas where it is important to monitor small and intense precipitation cells in a very precise way, to avoid for example floods in the streets. Short-range, portable X-band radar systems are a more recent solution to monitor and observe rain events with good resolution in space and time. They can be used standalone as well as integrated with other already existing networks. The Remote Sensing Group at the Politecnico di Torino started to design and develop its first prototype of mini weather X-band radar in 2004. This system is a non-coherent, pulsed, one polarization only, non-Doppler radar system (10 KW peak power, 600 Hz PRF, maximum range 30 km), equipped with a fixed elevation parabolic antenna (34 dB gain, 3.6° Half Power Beam Width). Several progresses and modifications were made in particular on the on-board software, actually leading to some fully operative installations. In particular two radars have been installed in heavily populated urban area: in the Turin town, and near the Foggia town. To achive good performances in detection and measurement of rain, it is important to control the stability of the overall radar system components. In fact, these systems may suffer some degradations due both to external factors (e.g. temperature fluctuations, humidity) and to equipment failure (e.g. frequency drift of the magnetron, de-tuning of the receiver filter). Therefore some techniques and periodic procedures for the radar stability control are necessary. This work presents some studies performed to control the radar signal stability through a statistical analysis of clutter echoes coming from a high number of ground targets during clear sky days. In these conditions, when the radar is calibrated, the daily average clutter echoes have been found to be almost stable from day to day. They vary significantly only after serious system ”degradation”. To monitor the radar stability has therefore been necessary to evaluate the variation in the intensity of clutter echoes acquired during clear sky days. In order to have a simple “tool” which give an immediate indication of the radar stability, different cumulative distribution functions (CDFs) of clutter reflectivity are computed operatively on a daily basis, considering the overall observing area and different circular rings around the radar, in order to take into account of different types of ground clutter. Comparing and analyzing the behavior of the CDFs obtained it is possible to control the radar stability and to know if it is necessary to set up a calibration procedure. Some indicators have been computed considering the CDF in order to find out which is the best one to detect modifications on radar systems capabilities.