Small unmanned aircraft systems (UAS) are rapidly proliferating for a variety of commercial and civil applications. Regulators cite detect-and-avoid of manned aircraft as the most significant challenge to integrating small UAS in the National Airspace System. However, most emerging small UAS use cases call for flight within immediate reach of the ground, airspace manned aircraft rarely occupy except on approach or departure. This paper presents a low-altitude geofencing capability for small UAS as a safe alternative to detect-and-avoid. The low-altitude small UAS can be flown manually or through any commercially-available autopilot with the independent geofence unit as a low-cost add-on. The geofencing unit passively monitors the flight, activating as a guidance system override on approach to an altitude or lateral fence boundary. Once active, the geofence turns the UAS back from the border then returns to its monitoring mode. This paper implements geofencing on a small quadrotor flown indoors to eliminate regulatory concerns. While GPS would be a primary means of navigating outdoors, a dual camera, inertial measurement unit, and height sensor package is used to geofence indoors. A Kalman filter integrates sensor data, and a geofence boundary controller overrides the nominal pilot/autopilot guidance inputs to drive the system back within the fenced region. Results show that the fence can be maintained, and that the control authority switch can be done in a stable manner so long as the operator understands when the fence activates and deactivates. Work is ongoing to flight test geofencing outdoors with both multicopter and fixed-wing platforms.

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