# Pointing Accuracy of an Operational Polarimetric Weather Radar
## 5 Pointing Accuracy and Beam Squint from Boxscans  
We first discuss the time series of pointing accuracy in azimuth and elevation on 3 and 15 June 2018. The time series of azimuth and elevation bias as a function of solar azimuth are shown in Figure 6. Only data from solar elevations larger than about 10\({}^{\circ}\) are shown. This is done in order to minimize the influence of possible strong refractive index gradients at low elevations and the influence of surface  
Figure 4: Operational 2-D surface fit to solar power measurements to determine antenna misalignment and radar measured peak solar power. The red data points represent the sampled solar power at a given time during operational scanning. The 2-D surface is fitted to the solar power sample. The surface fit provides the maximum peak solar power based on radar data and its position relative to the true sun position. This approach is also applied when analyzing data from solar boxscans, where many more samples are available due to the dedicated boxscan. Figure 5: Solar beam plot from 15 June 2018, 17:22 UTC. Shown is the normalized SNRh relative to the peak SNRh. clutter. For the horizontal polarization the azimuth bias varies between 0.05\({}^{\circ}\) and 0.12\({}^{\circ}\). The azimuth bias for the vertical polarization follows a similar pattern, but is separated by a constant offset of about \(-0.02^{\circ}\). The elevation bias varies between \(-0.005^{\circ}\) to 0.0\({}^{\circ}\) while the results for the two polarizations are essentially identical. We observe a quasi linear trend in elevation bias, which, though small, is indicative of a slightly tilted pedestal. With respect to the target accuracy of 0.1\({}^{\circ}\), the elevation bias is within acceptable limits. The azimuth bias reaches the limits of 0.1\({}^{\circ}\) depending on azimuth position which means that an adjustment may have to be considered. An important aspect is, that the pointing bias is not independent of azimuth position. A perfect gear drive assembly would assume a constant bias as a function of azimuth position. Mechanically this is difficult to achieve, and dependent on the accuracy requirements, a bias correction as a function of azimuth and elevation may be introduced [3]. This is commonly done in the field of satellite communication where substantially stricter pointing accuracies must be achieved. Figure 6: Azimuth pointing bias based on the solar boxscans from 3 and 15 June 2018 (upper panel). The results are shown for both the horizontal and vertical polarization H and V. Boxscans are acquired every 10 min. In addition, we show the corresponding azimuth and elevation of the sun on 3 June 2018. A very similar solar azimuth and elevation readings were present on 15 June (not shown). The corresponding pointing bias in elevation is shown in the lower panel. The difference in azimuth bias in H and V denotes the beam squint (see Equation (1)). The beam squint as a function of solar azimuth for 3 and 15 June 2018 are shown in Figure 7. Our antenna specification asks for a beams quint smaller \(0.06^{\circ}\)[10]. For this dish the beams quint is smaller \(0.03^{\circ}\) and is well within the specifications. The two cases used here are within the overall bias estimates we have computed from over 1300 boxscans between June and August 2018 (Figure 8). The majority of scans were acquired between 10 June and 8 July 2018. Additional scans were performed on 27 and 28 August 2018. Those two days stick out in the plot of elevation bias estimates as they cover a narrower solar azimuth range as compared to the June data. Nevertheless, the August data show a azimuth dependence of elevation pointing bias that is consistent with the rest of the data. At low elevations (lower than \(10^{\circ}\), compare with Figure 6), there is an increase of elevation bias that may be in part due to refractive index gradients. When determining the position of the sun, a profile of refractive index for a standard atmosphere is used which always is an approximation of a real refractive index profile [11].