Landing aid of drones is a function of drones, it can test the distance between the bottom of wifi smart drone
and landing area to judge whether it is safe to land or not, at last, landing to the ground slowly. In the process of this, ultrasonic sense is the main and the most accurate gist for drones. Most of drones have hovering and ground tracking pattern for catching sequence and land navigation, ultrasonic sensor is helpful for keeping drones
higher than the constant altitude of ground.
Ultrasonic can pass through various medium( gas, liquid, solid) to test acoustic resistance for resisting unmatched object. Sound velocity is the per unit of time distance that sound wave spreads in elastic medium. For example, in20°C (68°F) dry air, the speed of sound is 343 meters every second(1.125 feet every second). The decrease of ultrasonic in air is increasing with the increase of frequency and humidity. Therefore, because of excessive path loss or absorption, the frequency of air coupled ultrasonic is always controlled below500kHz.
Ultrasonic ToF is the same as many ultrasonic senor application, landing aid system of drone is using ToF principle. ToF is launched from sensor to target object and estimated from the time of ultrasonic that object reflects back to sensor.
Distance refers to the distance that is between ultrasonic sensor of drone and ground or object, ToF（t）is the previous definition, speed of sound is speed sound through medium. ToF（t）×Speed of Sound（v）and divides two, that is the time ultrasonic go back and forth object.
Ultrasonic sense can detect distance, cost of program and stability of different surface when wifi smart drone
lands. The common requirement of drone ground tracking and landing is testing the distance that is 5 meters higher than ground. If signal adjustment and dispose is correct, ultrasonic sensor in40-60kHz can satisfy this range.