# Research Progress and Prospects of Agricultural Aero-Bionic Technology in China
## 2 Biological Perception and Agricultural Aviation
### Echolocation and Obstacle Avoidance  
In 1944, [PERSON] [15] successfully recorded the ultrasound emitted by bats, and named the process of acquiring information about the surrounding environment by bats emitting ultrasound as echolocation. At present, related studies have found that in addition to bats, the rodents of the genus Typhlomys [16], toothed whales [17], and birds [18] all have echolocation capability. Bat echolocation has multiple functions, such as target detection,information exchange, and environment recognition [19]. Accurate target information can be obtained by adjusting the frequency, intensity, and sound rate of echolocation sound waves. Ultrasonic detection has the characteristics of high resolution, strong directionality, wide application range, and large coverage area in short-range distance measurement [20]. It can detect harsh environments such as dark environment, electromagnetic interference, smoke, etc. [21], but temperature, humidity, gas density, etc., will affect the ultrasonic propagation speed [22], and the sound waves will gradually turn into heat during the propagation process, causing excess attenuation [23], so the accuracy of detecting long-distance targets is poor.  
The development of ultrasonic sensor technology is relatively mature, and it is mainly used for ultrasonic detection and obstacle avoidance in agricultural aviation. At present, the research of ultrasonic detection technology is mainly focused on improving the accuracy and robustness of ultrasonic detectors. [PERSON] [24] proposed a segmented fitting algorithm, which combines the MCU main control chip, ultrasonic radar, collision warning, and other modules to form a collision avoidance warning system, which calculates the control function of each flight distance separately, and finally, summarized the optimal route to realize the safe flight of the aircraft. [PERSON] [25] proposed an idea to process the data obtained by the ultrasonic sensor and binocular vision through the Federal Kalman filter fusion algorithm to process and fuse the image information taken by the ultrasonic binocular, so as to improve the obstacle avoidance perception accuracy of UAVs (unmanned aerial vehicles). Aiming at the fact that multi-channel ultrasonic sensors occupy a lot of system hardware resources and control system software resources in robot obstacle avoidance, [PERSON] [26] and others developed the RS485 bus to expand multiple ultrasonic ranging slave stations, which improves the reliability of data communication and high-speed long-distance transmission performance, and saves the hardware and software resources of the robot control system.  
Ultrasonic sensors are low in cost, small in size, and simple in structure [27]. They are widely used in obstacle avoidance and detection of plant protection UAVs, and are usually used in fixed altitude flights and the down-looking system of agricultural UAVs [28]. Since ultrasound's accuracy will be affected due to complex environmental obstacles, air temperature, humidity, etc., it cannot be used alone. Therefore, fusing it with multiple sensors is a future research trend [29].