Ultrasonic sensors are being investigated as a potential solution to the problem of landing UAVs, particularly multirotors. GPS accuracy without augmentation systems like the US' WAAS have 10 meters horizontal accuracy, or 30 meters vertical accuracy, and that error appears in complex sweeping paths. This is nowhere near sufficient to, say, initialize and land on a windowsill or a pickup truck reliably. Barometric altimeter measurements (relative to initialization point) smoothed out by an accelerometer provide the reference altitude measurement in most multirotors - but even with augmentation, GPS requires a flat area a few meters wide, and a slow controlled descent. Without a digital elevation map (DEM), the multirotor can't even guess whether it can land at some other spot - it just has to slowly descend without feedback. As an emergency dead battery mode, this is somewhat lacking.
An ultrasonic sensor allows the multirotor to descend quickly until it detects ground reflections, generally between 2 meters and 6 meters AGL, and then switch into slow, controlled descent mode to land safely. It also lets one fly a controlled-elevation ground-hugging path in a treeless area without any chance of crashing, something that's very difficult on either manual or GPS mode.
As of 2012, the standard multirotor control loadout involves a GPS, a 9DoF IMU, and a barometric altimeter. Ultrasonic rangefinders are available optionally on only some flight control boards, but along with video computer vision, optical flow sensors, laser rangefinders, LIDAR, and depth cameras, they're looked upon as one of the places that advances are likely to take place
Ultrasonic sensors are also used to supplement Laser Rangefinders for indoor navigation and obstacle avoidance, particularly in MAV competition. In the longer term, UAVs designed to actively interact with their environment need accurate close-range sensors.
- A Sensitive DIY Ultrasonic Range Sensor, Kerry D Wong