# Pointing Accuracy of an Operational Polarimetric Weather Radar
## 6 Pointing Bias from Operational Solar Monitoring Compared to Solar Boxscan Results  
The bias estimates based on the solar boxscans are now compared to the bias estimate based on the evaluation of solar hits extracted from operational scanning [7]. The results are shown in Figure 9. We are considering the results from horizontal polarization, on 3 June 2018. For azimuth pointing bias, the operational analysis indicates a bias of 0.02\({}^{\circ}\) (constant red line in Figure 9; there is only one bias estimate per day from operational monitoring). This value has to be compared to the bias estimates from boxscans taken at an sun elevation lower than 25\({}^{\circ}\), which corresponds to the highest elevation of DWD's operational scan pattern. This elevation limit is given by the contant black horizontal line in Figure 9. So for elevations below 25\({}^{\circ}\), the azimuth bias in the morning is around 0.07\({}^{\circ}\) and 0.04\({}^{\circ}\) in the evening, which agrees well with the operational estimate of 0.02\({}^{\circ}\).  
Similarly, the elevation bias is computed to be 0.01\({}^{\circ}\) from the operational analysis compared to \(-0.02^{\circ}\) in the morning and \(-0.01^{\circ}\) in the evening. So the elevation bias estimates are within 0.03\({}^{\circ}\). Aside from the fact that the operational results are obviously not from the same day as the boxscans, differences may be explained by the smaller sample size from which the po  
Figure 8: Azimuth pointing bias (upper panel) and elevation bias (lower panel) based on over 1300 solar boxscans taken between June and August 2018 (upper panel). Also shown are the median bias estimates and associated 1 st and 3 rd quartile of the distribution from non-overlapping windows of 3\({}^{\circ}\) width of sun azimuth angles.  
using operational data (about 30 hits (rays) per day can be expected) compared to about 200 samples (rays) from each boxscan which resolve the main beam. The ray width from the operational scan is \(1^{\circ}\) (with typically 32 pulses at a antenna speed of typically \(18^{\circ}\)s\({}^{-1}\)) compared to that from a box scan with a ray width of \(0.32^{\circ}\) (with 128 pulses at \(2^{\circ}\)s\({}^{-1}\) antenna speed). So the difference in sample size and the different ray widths may explain the difference of pointing bias values. This can also be compared to the variability of the azimuth and elevation bias from July 2017 until September 2018 shown in Figure 10. The day to day variations of azimuth and elevation bias from the operational data are comparable to the difference between the bias estimates from the boxscans and the operational data. We also note, that the day to day elevation bias variations appear smaller than the azimuth bias variation, similar to the findings shown for 3 June 2018 (Figure 9).  
Within the DWD radar network, we employ solar monitoring to adjust angle offsets to correct for pointing bias. An adjustment is initiated if the pointing error based on solar monitoring exceeds  
Figure 9: Azimuth pointing bias based on the solar boxscans from 3 June 2018 (upper panel). The results are shown for the horizontal polarization H only and are compared to the bias estimate based on the collected solar hits from operational scanning (constant red line; there is only one bias estimate per day). In addition a horizontal line is drawn at \(25^{\circ}\) elevation indicating the maximum elevation of the operational scan. The corresponding pointing bias in elevation is shown in the lower panel.  
\(\pm\)0.1\({}^{\circ}\). This involves an adjustment of angle offsets in the radar configuration. In order to make sure that the proper bias correction has been applied, we point the radar to the expected position of the sun, and note the measured solar SNR before and after applying the new angle offset. An increase of SNR serves as a cross check for a correct angle offset adjustment.