# Pointing Accuracy of an Operational Polarimetric Weather Radar
###### Abstract  
Exact navigation of detected radar signals is crucial for usage of radar data in meteorological applications. The antenna pointing accuracy in azimuth and elevation of a polarimetric weather research radar depending on position of the sun is assessed using dedicated solar boxscans in a sequence of 10 min. The research radar of the German Meteorological Service (Deutscher Wetterdienst, DWD) is located at the meteorological observatory Hohenpeissenberg. It is identical to the 17 weather radars of the German weather radar network. A non-linear azimuthal variation of azimuthal pointing bias of up to 0.1\({}^{\circ}\) is found, which is significant as this is commonly viewed as the target pointing accuracy. This azimuthal variation can be attributed to the mechanical design of the drive train with the angle encoder. This includes the inherent backlash of the gear-drive assembly. The pointing bias estimates based on over 1000 boxscans from 26 days show a small case by case variability, which indicates that dedicated solar boxscans from one day are sufficient to characterize the pointing performance of a particular system. An azimuth and elevation range that is covered with this approach is limited and dependent on the time of the year. At Hohenpeissenberg, an azimuth range up to 50-300\({}^{\circ}\) was covered around summer solstice and about 90 boxscans were acquired. It is shown that the pointing bias based on solar boxscan data are consistent with results from the operational assessment of pointing bias using solar hits from operational scanning if we take into account the fact that the DWD operational scan definition has only a maximum elevation of 25\({}^{\circ}\). The analysis of a full diurnal cycle of boxscans from four operational radar system shows that the azimuthal dependence of azimuth bias needs to be evaluated individually for each system. For one of the systems, the azimuthal variation of the pointing bias of about 0.2\({}^{\circ}\) seems related to the bull gear. A difference of the pointing bias for the horizontal and vertical polarization is an indication of beam squint and, eventually, that of a feed misalignment. Beam squint and, as such, the quality of the antenna assembly can easily be monitored with this method during the life-time of a weather radar. antenna pointing; solar monitoring; beam squint 11115 10.3390/rs11091115 10.  
position of the sun based on the radar data determines the pointing accuracy of the radar. The difference between solar position and the radar indicated solar position is commonly compensated applying azimuth and elevation offsets. This approach can be considered as an end-to-end method as it considers not only the angle precision relative to the mechanical gear, but also the pointing characteristics of the antenna dish feed assembly [2]. One has to be aware of a number of caveats when interpreting the results from solar boxscans [3]. This involves pedestal leveling, the angle nonlinearity of the gears as well as the encoders, and the backlash of the gear drive system. Furthermore, the interpretation of a solar signal and the position of the sun are dependent on the vertical variation of the refractive index at low sun elevation [4; 5]. Naturally, depending on the latitude and season, a limited range of azimuth and elevation angles are available so that methods solely relying on the sun cannot fully characterize the pointing accuracy of the antenna drive system. This implies that methods based on the sun need to be complemented by other engineering methods to obtain a full characterization of the antenna-pointing accuracy. The monitoring of the pointing accuracy in weather radar networks can be achieved by extracting solar hits during operational scanning. Based on the pioneering work of [4; 6], this information can be used to determine the pointing accuracy and the receiver sensitivity. This method is now commonly used by weather services operating radar networks [7; 8; 9].