  The widespread use of unmanned aerial vehicles (UAVs) by the military,
commercial companies, and academia continues to push research for autonomous
vehicle navigation, particularly in varying environmental conditions and
beyond-line-of-sight (BLOS) applications. This article addresses trajectory
generation for UAVs operating in extreme environments where the wind
disturbances may exceed the vehicle's closed-loop stability bounds. To do this,
a controller is developed that has two modes of operation: (1) normal mode, and
(2) drift mode. In the normal mode the vehicle's thrust and sensor limitations
are not exceeded by environmental conditions, whereas in the drift mode they
are. In the drift mode, a drift frame that moves with the prevailing wind is
established in which the vehicle maintains control authority to generate and
track trajectories. The vehicle maintains control authority by relaxing the
inertial frame trajectory tracking requirement and re-planning the trajectory
in the drift frame. Guarantees are established to ensure tracking of the
trajectory, collision avoidance, and respecting the vehicle thrust and sensor
limitations. Simulation results demonstrate the algorithm properties through
two scenarios. First, the performance of two quadrotors is compared where one
utilizes the drift mode and the other does not. Second, multiple vehicles
navigate through two narrow openings between protected and windy environments
to demonstrate on-board updates to navigation parameters based on environmental
conditions.
