# Pointing Accuracy of an Operational Polarimetric Weather Radar
###### Abstract
If we consider the sun as a reference, we propose the use of dedicated solar boxscans that are scheduled at a temporal resolution of about 10 min which allows to investigate the azimuthal and elevation dependence of the pointing bias at a angular resolution of about 3 degrees. From those data, we can determine the pointing accuracy of the antenna as a function of azimuth and elevation. Here, we employ the methodology proposed in [4] and [6]. Radar maintenance software which includes a sun track module to determine the pointing accuracy, generally does not offer a automated sequence of sun tracks. In principle, solar boxscans could be integrated in the operational scheduling when there is no precipitation present. Such an event-based adjustment of the scan schedule is not feasible within the DWD weather radar network where spatially and temporal homogeneous data availability across the network is essential. No echo in the data is a valuable source of information to the user as compared to no data at all. Radar data are now used by a multitude of algorithms for e.g., warning management and by numerical weather prediction. Therefore, dedicated diurnal sequences of solar boxscans are proposed as a maintenance procedure every other year or after modifications to the mechanics of the radar system. For the data set here, up to 90 boxscans are available per day which cover a solar azimuth range of about 50-300\({}^{\circ}\) and an elevation range of 0-65\({}^{\circ}\) at a angular resolution of about 3\({}^{\circ}\). In total, over 1000 solar box scans are used in the analysis. The large number of azimuth and elevation bias estimates provides information about the quality of drive-gear assembly and the accuracy of the angle encoding in relation to the position of the sun. In fact, it provides an end-to-end view on the pointing performance as it includes the antenna assembly because this method uses the same signal path as the normal data acquisition. On the other hand, it can be only viewed as a partial assessment of the gear/drive assembly because of the limited angle range that can be covered by the sun. The pointing bias results are compared to the operational monitoring of pointing accuracy using the sun hits extracted from the operational scan volumes based on [6]. We also discuss pointing accuracy results from four operational radars of the DWD weather radar network. This paper is a unique investigation as we are able to perform dedicated scans outside operational scanning by using the DWD research radar which is technically identical to the 17 operational radar systems of the DWD network. This provides insight into the positioning accuracy of a state-of-the-art weather radar system. This work was triggered during the acceptance tests of our radar where tests had to be devised to verify the pointing accuracy specification and to understand the system performance. The paper is structured as follows:  
We first introduce elements of a radar system that influence the positioning accuracy of a radar system. This comprises the radar tower, the pedestal and the antenna assembly. Important elements of a radar system are angle encoders which provide the angle reading in elevation and azimuth that are used to tag each transmitted pulse. We discuss the principle encoder characteristics (there are two angle encoder types and therefore two angle sources built into the DWD radar system), the angle data stream and the involved signal processing. These aspects are needed to interpret and understand the results based on the solar box scans. This is followed by an introduction of the solar boxscan scan strategy and the methodology to determine the pointing accuracy using the position of the sun based on [4]. This methodology is both used for the dedicated solar boxscans and the operational monitoring of the radar based on solar hits which are collected from operational data. We also introduce the beam squint definition which quantifies the pointing mismatch between the horizontal and vertical polarized beam.