Extending the solar signal analysis into monitoring of the radar signal processing and antenna system performance

The solar signal has been used for operational monitoring of the weather radar antenna pointing and receiver stability at Finnish Meteorological Institute and the Royal Netherlands Meteorological Institute since 2007, and has been taken into use by several other meteorological services since then. The method is entirely based on the analysis of sun signals in the polar volume data produced during operational scanning of weather radars. Basically a consistent reflectivity signal, i.e., signal from a continuous radio-frequency source, at long ranges (>100 km) is searched along radials in the operational scan data. Data below 1 deg elevation are discarded to avoid severe refraction and to ensure that the long-range observations are above precipitation areas. Uncorrected reflectivity data are used for this analysis as (time-domain) Doppler clutter filters can attenuate the solar signal by several dBs. Depending on the hardware of the radar, the volume coverage pattern, the season, and the latitude of the radar, several tens of sun hits are found per day. The method has been extended to monitoring of the calibration level, and, for polarimetric radars, to monitoring of the differential reflectivity.

In the original method the image width in azimuth and elevation was fixed to improve the stability of the fitting. We have now extended the method into monitoring of the width of the sun image. We hence fit a two dimensional polynomial function to the data (in dBm) which gives estimates of the received power, estimates antenna offsets in azimuth and elevation, and estimates of the width of the image in the two directions as well. The width depends on the apparent size of the radio sun (~0.57 deg), on the beam width of the antenna, and in the azimuth direction also depends on the averaging of samples. We show results both in the elevation and azimuth directions, and show by examples how the results can be used for monitoring the antenna system stability, for checking the working of the signal processing, and for detecting signal processing features which are difficult to locate in weather data alone.