# Correcting for Mobile X-Band Weather Radar Tilt Using Solar Interference
## 1 Introduction  
Weather surveillance radars are an important tool for atmospheric scientists and hydrologists and are deployed both operationally in national networks and to support specific research objectives [1; 2]. One of the most fundamental considerations for the use of weather radar data is having precise (within \(0.1^{\circ}\)[3]) knowledge of the pointing position of the electromagnetic beam. At the broad scale, antenna pointing determines the accurate geolocation of the observed radar echoes, but it is also critical for mitigating several common sources of uncertainty in weather radar observations, including partial beam blockage, where the beam blockage fraction can be computed from terrain models when the beam position is known accurately, and the \"bright-band\", where integration of external data relies on accurate knowledge of position [4; 5]. These impacts are more pronounced in areas of complex orography where partial beam blockage is more severe, potentially impacting multiple elevation angles, and the melting layer is typically closer to the surface and is observed more frequently at the lowest usable radar beam elevation [6]. As an electromagnetic source with a precisely known position in time, the sun has become an often-employed tool for accurately assessing the pointing accuracy of weather radars [7; 8; 9]. Any variation between the known position and radar-observed position of the sun can be considered to be representative of a pointing offset in the radar system and compensated for. The positional offset value can include variations due to position encoder errors, pedestal tilt and offsets of the electromagnetic axis of the antenna assembly relative to its mechanical position [10; 11]. The most precise assessment of position offsetis typically achieved through the use of solar box (i.e., raster) scans, where the known position of the sun is slowly scanned by the radar system at high resolution, recording power observations with the transmitter turned off. However, it is known that position offset can vary as a function of the actual azimuth and elevation angle of the radar system (due to pedestal tilt or encoder error, for example), requiring multiple box scans to be undertaken at a range of azimuths and elevations to fully characterise the pointing accuracy of the system. As these box scans are often manual interventions in the radar system, they are typically not performed on a routine basis and thus are not able to fully monitor the pointing accuracy of the radar system through time. Instead, the most commonly employed technique for ongoing monitoring is that developed by [PERSON] and [PERSON] [12], where opportune instances of solar interference from repetitive volume scans (as typically implemented in national weather radar networks) are analysed to determine pointing accuracy. Comparison of the box scan and solar interference techniques shows good agreement between the two, especially where the weather radar system is capable of obtaining multiple opportune solar hits each day [13]. The interference technique has been widely applied to large operational networks [14; 15]; however, it is less commonly used for research radars. Research radars often have shorter deployments and transmit at higher frequencies, such that the solar interference is closer to the noise floor of the radar receivers [7]. The sun has also been used to assess whether a radar system is tilting relative to the horizontal plane [11; 13; 16]. Recently, [PERSON] et al. [16] demonstrated the possibility of inferring pedestal tilt for operational C-band and S-band radar systems using solar interference, with tilt being identifiable even when analysing relatively short (16 week) time windows. Such a short time window raises the possibility of applying the methodology to non-permanent deployments of research radars. The research presented here focuses on the application of online solar monitoring to an X-band research radar (now referred to as NXPol-1 [17]).