# Pointing Accuracy of an Operational Polarimetric Weather Radar
## 4 Determining Pointing Accuracy from Operational Scanning and Box Scans  
The approach to determine the pointing accuracy based on solar hits during operational scanning is described in [6]. The position accuracy in elevation and azimuth is specified to be better than 0.1\({}^{\circ}\)[1]. From a practical point of view this number for example is relevant when optimizing the configuration of the DWD precipitation scan. There, the elevation is azimuth dependent in order to follow the orography as close as possible without being affected by surface clutter. Elevation steps of 0.1\({}^{\circ}\) are often sufficient to minimize the clutter signal. Here we summarize the basic aspects of the approach which is also applied to the computation of the pointing error from a solar boxscans. During regular operations there are typically 30 solar interferences with the sun during the day. This is already a sufficient large number of samples to fit a 2-d surface of a Gaussian type to the radar measured solar power in terms of the difference between the antenna position and the solar coordinates, i.e., \(\Delta az_{h,v}=az_{radar,h,v}-az_{sun},\Delta el_{h,v}=el_{radar,h,v}-els_{sun}\) (Figure 4). The peak solar power \(p_{h,h,m}\) is determined from the fitted surface which then can be compared to the independent solar flux measurement [7]. This analysis is carried out independently for both the horizontal and vertical channel and the peak solar power is determined from the fitted 2-d surface (Figure 4). Perfect antenna positioning is found if \(\Delta az_{h,v}=0\) and \(\Delta el_{h,v}=0\). A nonzero value of \(\Delta az_{h,v}\) and \(\Delta el_{h,v}\) quantifies the antenna pointing error. A difference in positioning error between H and V polarization is an indicator for an antenna mis-alignment of the feed in H and V. We define this as the antenna beam squint:  
\[BSQ=\sqrt{(\Delta az_{h}-\Delta az_{v})^{2}+(\Delta el_{h}-\Delta el_{v})^{2}} \tag{1}\]  
The methodology to characterize pointing accuracy and beam characteristics is applied to both operational data and data from solar boxscans. When processing solar boxscan data, only a range larger 50 km are considered. By doing so it is avoided that clutter may bias the data. However, boxscan domains at low elevation may be contaminated by clutter. Therefore, SNR beamplots have been inspected for spatial SNR fluctuations larger about 1 dB before further analysis. Prior the fitting, a mean SNR is computed from each ray. Each ray is tagged with a synchronized time which is used to calculate the position of the sun. We compensate for the movement of the sun during the boxscan by taking the difference of each ray angle and the corresponding computed solar angle based on the ray time stamp. We then perform the fitting as described above and determine the pointing bias. An example of a solar beam plot of SNRh based on a solar box scan is shown in Figure 5. There, peak SNRh is about 18 dB above noise. Using the aforementioned procedure we have computed thepointing bias based on the location of the peak SNRh. In this example, there is only a small bias which is illustrated by the slight mismatch of the 0.5\({}^{\circ}\) circle relative to the circle centered at a relative azimuth and elevation angle of 0\({}^{\circ}\), which denotes expected position of the sun. The azimuth bias amounts here to 0.047\({}^{\circ}\) and in elevation to \(-\)0.005\({}^{\circ}\).