# A Circularly Polarized Millimeter Wave Radar for Wind Turbine Sensing
## 1 Introduction  
Wind energy is a promising alternative for solving the challenges posed by fossil fuels and environmental contamination. To maximize the generation of energy, wind turbines are manufactured with higher towers and longer blades. However, as shown in Figure 1, the shape of thick blade roots and narrow tips causes inconsistent deformation under stress when the turbine rotates at high speed, which leads to the blade tip bending inward, increasing the risk of collision with the tower. To prevent collisions, sensors can be installed to detect the clearance between the blade tip and the tower accurately. When the clearance is too small to operate safely, the wind turbine's control system can respond by pitching or shutting down the turbine as necessary for safety. The wind turbine industry currently employs several methods for detecting blade clearance, including laser, camera, and millimeter-wave radar (mmWave radar) sensors. One study [1] uses a laser sensor installed in the middle of the tower to monitor blade clearance with high accuracy, but this method requires damaging the tower barrel, which could compromise the turbine's reliability. Another study [2] uses a ground-mountedcamera sensor to measure clearance, but this technique is not effective for measuring blade-tip clearance in all yaw azimuths when the blades are rotating day and night under the wind turbine's yaw control. More importantly, both LiDAR and camera sensors struggle to perform in challenging weather conditions (such as cloudy, overcast, light rain, and thundershower days, etc.) due to poor data quality. Millimeter-wave radar (mmWave radar) overcomes these drawbacks with its strong anti-interference capability and all-weather operability. According to a recent report [3], the installation of up to eight mmWave radars in the center of a tower presents significant challenges in terms of power supply and is prohibitively expensive. In this paper, only one mmWave radar is required to be installed on the top of the tower regardless of turbine direction. However, there is a tradeoff in terms of a low SNR caused by a small-RCS (radar cross-section) target obliquely incident from a distance of 70 m, so high-gain antennas and SIMO radar configuration are designed to increase the P\({}_{\text{t}}\) (transmitting power) of the RF link. In addition, antennas of mmWave radar are typically linearly polarized, but circularly polarization antennas offer numerous advantages over linearly polarization antennas, such as minimizing polarization losses caused by any misalignment between transmitting and receiving antennas, reducing interference between direct and reflected signal due to multipath propagation [4; 5; 6]. More importantly, wind turbines are usually located in areas with abundant wind resources, such as offshore or mountainous areas. There is more water vapor in the ocean area, and the mountains are more conducive to the formation of rain and fog, thus they inevitably attenuate electromagnetic signals. The circularly polarized radar array has circle orthogonality, which can reduce the influence of rain and fog in the environment, and thus improve the signal-to-noise ratio of the link [7; 8; 9], making it feasible to work in severe weather conditions such as cloudy, overcast, light rain, and thundershower. [10; 11; 12; 13; 14; 15]. In conclusion, this paper presents a method of employing a 79 GHz FMCW mmWave (millimeter-wave) radar with circularly polarization on the top of the tower to monitor the blade clearance in harsh weather conditions. To mitigate the low channel SNR (signal-to-noise ratio) caused by a small-RCS (radar cross-section) blade obliquely incident from more than 70 m and a high level of weather interference from unfriendly environments, several  
Figure 1: Wind turbine blade bending.