# An RTK UAV-Based Method for Radial Velocity Validation of Weather Radar
## 1 Introduction  
Weather radar plays a crucial role in weather monitoring and disaster prevention, providing critical data for predicting and responding to extreme events [1]. In order to deal with severe convective weather that develops rapidly and is potentially disastrous, the China Meteorological Administration (CMA) has constructed a national weather radar network consisting of hundreds of multi-band radar systems. However, the key devices in a radar system may all introduce systematic errors, which increase the uncertainty of the target measurement results [2]. With the continuous development of weather radar, the data application scenarios and demands are moving beyond the traditional qualitative representation and gradually developing toward quantitative monitoring. Radar calibration and validation are the first steps toward improving the quality of the observation data, which is crucial for improving the monitoring and warning capabilities of meteorological targets. Thus, velocity measurement is one of the key parameters of quantization. Due to the complexity of atmospheric conditions and the specificity of the radar system itself, radar radial velocity data may be affected by a variety of errors and disturbances, such as ground clutter, doppler spectrum folding, systematic deviation, etc. In particular, the monitoring of severe convective weather systems, such as tornadoes and thunderstorms, has put forward new requirements for radar velocity measurement capability. Traditional radial velocity calibration methods for weather radar systems typically use high-precision instruments to measure the internal and external radar system, which can calibrate the receive channel amplitude [3], phase, etc. Existing methods cannot assess the accuracy of moving target velocity measurements under real-world radar operating conditions. Current simulated moving targets mostly use flying platforms mounted with external reference sources, such as tethered balloons, kites, and unmanned aerial vehicles (UAVs) [4]. Compared with other passive carriers, UAVs have powerful flight control systems and real-time kinematic (RTK) systems and can meet the needs of a variety of application scenarios through reasonable route design. Moreover, they are flexible in flight, easy to operate, and can obtain accurate coordinate information in real time [5], which provides a new way of achieving end-to-end calibration and validation for radar systems. Several researchers have conducted experiments on radar antenna patterns, radar constants, reflectivity factors, differential reflectivity, and radial velocity using \"UAV+\" and have explored the possibility of using a UAV to calibrate radar systems. [PERSON] et al. constructed a complete antenna pattern using a UAV system, which included the ground clutter, radome, temperature, and other human-introduced external degradation [6]. [PERSON] et al. constructed a measurement method that utilizes a dual-polarized antenna system based on a UAV and a detection antenna, which achieves the cross-polarization isolation of \(-40\) dB [7; 8; 9]. [PERSON] et al. used a UAV combined with the Global Navigation Satellite System, a real-time single-frequency precise point positioning system, to determine the position of the metal ball mounted on the UAV. Through the tracking and scanning of the metal ball by the radar system, the calibration of the weather radar antenna pointing, antenna pattern, and high-precision measurement of the radar constants were realized, and the calculation accuracy of the radar constants was improved [2]. [PERSON] et al. introduced the principle and steps for undertaking the external calibration of a meteorological radar system based on a UAV-mounted metal ball, and the Ku/Ka radar before and after calibration was used to carry out the monitoring of cloud and rain processes and mutual verification of the reflectivity factor [10]. [PERSON] et al.