# Pointing Accuracy of an Operational Polarimetric Weather Radar
## 9 Examples from the Radar Network  
Previous results show that the pointing error of a weather radar is a function of azimuth and elevation and as such cannot be viewed as constant. In azimuth, the pointing error has a variation of 0.1\({}^{\circ}\) for the Hohenpeissenberg radar (e.g., Figure 8). The question is, to what extent are those results representative for the other 17 radars in the network. This will be investigated in this section. During 2018, we gathered boxscans during a full diurnal cycle from sunrise to sunset from four different operational radar stations (radars Flechtdorf, FLD (27 June 2018), Neuheilenbach, NHB (24 July 2018), Boostedt, BOO (27 June 2018) and Hannover, HNR (24 July 2018)). The diurnal cycle of azimuth and elevation bias are shown in Figure 13. As expected we find for all sites an azimuthal dependence of pointing bias consistent with the findings of the Hohenpeissenberg radar data. However, the azimuthal dependence of azimuth pointing bias is different for all radar systems. The largest variation of azimuth bias is 0.2\({}^{\circ}\) which is found for the Hannover radar (HNR). This system also appears to have a larger variation of the elevation bias as a function of azimuth compared to the other three systems. An increase or decrease of elevation bias as a function of azimuth may be attributed to a tilt of the pedestal. For the Hannover (HNR) radar, the increase of elevation bias amounts to about 0.04\({}^{\circ}\). The value of 0.04\({}^{\circ}\) has been verified independently using readings from an inclinometer mounted on the antenna. The HNR variation of azimuth bias is considered to be too large. The on-site analysis of the angle acquisition, which involved the replacement of the absolute encoder, suggests that the source of azimuth pointing error can be attributed to the bull gear. Angle acquisition tests (where motor angles are compared to the absolute encoder angles, see e.g., Figure 11 as an example) before and after the replacement of the absolute encoder revealed similar results which are consistent with the azimuth pointing bias using the sun as reference (Figure 13). Therefore, the replacement of the Hannover bull gear needs to considered. At the radar Boostedt (BOO), we can observe a more or less significant modulation of the azimuth bias at a wave length of about 60\({}^{\circ}\). This modulation can be linked to the absolute encoder with its gear ratio of 1:6. For the Boostedt radar the modulation has an amplitude of 0.05\({}^{\circ}\). If we require a maximum error in azimuth positioning of 0.1\({}^{\circ}\), the absolute encoder adds already 50% to the overall  
Figure 12: Azimuth bias based on re-sampled boxscan data where only data are used if the antenna is moving in clockwise (cw) or in counter-clockwise direction (ccw). The difference between the bias estimate based on data of cw and ccw movement for a given angle source (absolute or relative encoder) defines the backlash between the given encoder and the bull gear. Shown are results of the Hohenpeissenberg radar. allowed error. This suggests that the encoder adjustment needs to be improved in order to minimize the error contribution from that device. In contrast, the Hannover example shows in fact a very good absolute encoder since we do not observe a modulation similar to the results from Boostedt. The results from the radars in Neuheilenbach (NHB) and Flechtdorf (FLD) are comparable to the results from the BOO radar. The BOO and HNR results based on the analysis of solar boxscans are compared to the results of the maintenance scan (Figure 14). The azimuthal variation has a striking similarity which is in support of the methodology of using solar boxscans as presented in this note. Note, that an exact agreement between the two data sources is not expected as the azimuthal variation of the pointing bias is compared to an angle difference which does not represent a pointing bias. The difference between the relative encoder (motor angles) and the absolute encoder reflects a non-linearity of the drive train.