  Redundant navigation systems are critical for safe operation of UAVs in
high-risk environments. Since most commercial UAVs almost wholly rely on GPS,
jamming, interference and multi-pathing are real concerns that usually limit
their operations to low-risk environments and Visual Line-Of-Sight. This paper
presents a vision-based route-following system for the autonomous, safe return
of UAVs under primary navigation failure such as GPS jamming. Using a Visual
Teach & Repeat framework to build a visual map of the environment during an
outbound flight, we show the autonomous return of the UAV by visually
localising the live view to this map when a simulated GPS failure occurs,
controlling the vehicle to follow the safe outbound path back to the launch
point. Using gimbal-stabilised stereo vision alone, without reliance on
external infrastructure or inertial sensing, visual odometry and localisation
are achieved at altitudes of 5-25 m and flight speeds up to 55 km/h. We examine
the performance of the visual localisation algorithm under a variety of
conditions and also demonstrate closed-loop autonomy along a complicated 450 m
path.
