The Cyclic-Routing UAV Problem is PSPACE-Complete
  Consider a finite set of targets, with each target assigned a relative deadline, and each pair of targets assigned a fixed transit flight time. Given a flock of identical UAVs, can one ensure that every target is repeatedly visited by some UAV at intervals of duration at most the target's relative deadline? The Cyclic-Routing UAV Problem (CR-UAV) is the question of whether this task has a solution.   This problem can straightforwardly be solved in PSPACE by modelling it as a network of timed automata. The special case of there being a single UAV is claimed to be NP-complete in the literature. In this paper, we show that the CR-UAV Problem is in fact PSPACE-complete even in the single-UAV case. 
Hsi-Ming Ho and Joel Ouaknine
cs.LO cs.FL