# Pointing Accuracy of an Operational Polarimetric Weather Radar
## 10 Conclusions  
In this work, we have investigated the navigation performance of a weather radar as one of the essential basic performance characteristics. We use the Hohenpeissenberg research radar for this thorough analysis by employing a large number of consecutive solar boxscans. To our knowledge,  
Figure 14: Azimuth pointing bias based on the sun in comparison to the difference of the readings from the absolute and relative encoder for BOO (upper panel, data from 27 June 2018) and HNR (lower panel, see also Figure 13, data from 25 July 2018). Those readings were gathered using a dedicated maintenance scan, which was carried out on 8 May 2018 (BOO) and on 5 December 2018 (HNR). this is the first study where the position accuracy is assessed at this level of detail. With a solar boxscan, we scan the sun with high angular resolution. The Hohenpeissenberg radar is identical to the 17 other weather radar systems of the DWD weather radar network. By using the sun as a reference, those measurements represent what we call an end-to-end assessment of radar navigation performance of the microwave beam as we consider all relevant elements, such as motor, bull gear, antenna assembly and radome for the quantification of the antenna pointing accuracy. For the Hohenpeissenberg radar, we find that the pointing accuracy varies up to 0.1\({}^{\circ}\) depending on the azimuth angle (with this data set we were able to cover an azimuth range from 60\({}^{\circ}\) to 300\({}^{\circ}\)). Most of this variation is attributed to the asymmetry of the bull gear, which is never precisely circular. Manufacturers are aware of this since they evaluate the high and low point of a bull gear in their quality assurance process. The azimuth dependence of the azimuth bias is consistent throughout the whole data sample which covers measurements from over 26 full diurnal cycles of boxscans. The elevation bias as a function azimuth angle indicates a linear trend in the course of the day which is indicative for a tilt of the pedestal. Elevation bias estimates in the morning and in the evening may be biased depending on the true refractive index profile compared to the employed profile based on the assumption of a standard atmosphere. However, it is concluded that the assumption of a standard atmosphere is not critical for the computation of the elevation position of the sun, at least for this data set. If a deviation from a standard atmosphere were significant, this should be reflected by a larger spread of bias estimates in the morning, and less so in the evening, considering a typical diurnal evolution of the atmospheric boundary layer. The angle acquisition of the DWD radar design is such that the operational angle source through an absolute encoder can be substituted by motor encoder angles in case of a failure. This feature allows for a characterization of the backlash between bull gear and the absolute encoder, and the backlash between bull gear and the motor. It is shown that the backlash between the absolute encoder and the bull gear is smaller 0.01\({}^{\circ}\). The backlash between motor and bull gear varies with azimuth angle and has a magnitude up to 0.05\({}^{\circ}\). During maintenance latter backlash is quantified and adjusted if the value is larger than 0.04\({}^{\circ}\). We have analyzed results of azimuth and elevation biases from four radar systems of the German weather radar network. The contribution of the absolute encoder to the overall azimuth bias is in particular apparent for the radar Boostedt which is revealed by the modulation of the azimuth bias at a frequency which corresponds to the gear ratio of the absolute encoder. The amplitude of this modulation is about 0.05\({}^{\circ}\). For the three other radar systems we find also azimuthal variations of azimuth bias with similar magnitudes except for the Hannover system where the larger variation of azimuth bias (0.2\({}^{\circ}\)) is attributed to issues with the bull gear.