Ultrasonic sensor HC-SR04 for drone - Swiss Online shop
Amazon.com: Radiolink SUI04 Ultrasonic Sensor Range Finder Obstacle Avoidance, Compatible with RadioLink Flight Controller PIXHAWK, Mini Pix, for Racing Drone, Robots, Quadcopter, Multi-Copter : Toys & Games
Pixhawk Loiter Performance with HC-SR04 Sonar - YouTube