UAV-Based Absolute Radar Calibration

Weather radar is well recognized as one effective tool for obtaining the information of atmospheric phenomena, capable of both high spatial and temporal resolution. Radar calibration is one of the most important prerequisites for achieving accurate observations. Current calibration methods are either a metal sphere hanging underneath a tethered balloon or a trihedral corner reflector locating on the top of a tower or mast. However, there are some problems with these methods: (1) They are location bound. The calibrator should be placed in the far-field, which seems impossible for some radars locating on the top of high buildings or towers. (2) It is relatively costly for tower setup or helium balloon purchase. (3) It is not easy to repeat the calibration process for mobile radars, especially for some fieldwork campaigns in complex terrain. (4) For vertically pointing cloud radars, the current methods are impossible to calibrate them. Therefore we propose a portable, cost-effective and repeatable solution to replace the current calibration techniques. An industrial-grade UAV serves as our stable aerial platform carrying a metal sphere, flying over the radar illumination areas to complete the calibration process. To retrieve the position of the sphere, the real-time single-frequency Precise Point Positioning (PPP) type GNSS solution is developed. Both receiver, microcontroller, and antenna are mounted on an additional platform above the sphere. The radar constant is calculated in the range-Doppler domain, and only these data which metal sphere separates from the ground clutter and UAV are selected. The spectral-polarization features are studied to further verify the recognition of metal sphere. The S-band polarimetric Doppler weather radar Transportable Atmospheric Radar (TARA) is used in the calibration campaign. The preliminary results show the effectiveness of the proposed radar calibration technique.