  A ground-to-air free-space optical link is studied for a hovering unmanned
aerial vehicle (UAV) having multiple rotors. For this UAV, a four-quadrant
array of photodetectors is used at the optical receiver to alleviate the
adverse effect of hovering fluctuations by enlarging the receiver
field-of-view. Extensive mathematical analysis is conducted to evaluate the
beam tracking performance under the random effects of hovering fluctuations.
The accuracy of the derived analytical expressions is corroborated by
performing Monte-Carlo simulations. It is shown that the performance of such
links depends heavily on the random fluctuations of hovering UAV, and, for each
level of instability there is an optimal size for the array that minimizes the
tracking error probability
