  Small, low-speed fixed-wing Unmanned Aerial Vehicles (UAVs) operating
autonomously, beyond-visual-line-of-sight (BVLOS) will inevitably encounter
winds rising to levels near or exceeding the vehicles' nominal airspeed. In
this paper, we develop a nonlinear lateral-directional path following guidance
law with explicit consideration of online wind estimates. Energy efficient
airspeed reference compensation logic is developed for excess wind scenarios
(i.e. when the wind speed rises above the airspeed), enabling either
mitigation, prevention, or over-powering of excess wind induced run-away from a
given path. The developed guidance law is demonstrated on a representative
small, low-speed test UAV in two flight experiments conducted in mountainous
regions of Switzerland with strong, turbulent wind conditions, gusts reaching
up to 13 meters per second. We demonstrate track-keeping errors of less than 1
meter consistently maintained during a representative duration of gusting,
excess winds and a mean ground speed undershoot of 0.5 meters per second from
the commanded minimum forward ground speed demonstrated in over 5 minutes of
the showcased flight results.
