Optimal persistent surveillance using coordinated soaring

Coordinated soaring by a flock of small unmanned aerial vehicles (sUAVs) provides a means of conserving fuel while performing aerial tasks. The ability to exploit thermal columns in the atmospheric boundary layer allows sUAVs to remain airborne without expending any onboard sources of energy, i.e., soaring flight. This paper presents an analysis of the cruising phase during coordinated soaring where a fock of sUAVs relies on thermal exploitation to maximize endurance for monitoring-type missions. To this end, a maneuver is investigated that involves each sUAV repeating a round-trip between a thermal and a monitoring/surveillance target so as to maintain continuous monitoring of the target. The focus is on minimizing the number of agents required to maintain continuous, persistent surveillance of the target for given atmospheric conditions (thermal strength and distance between the thermal and monitoring target) and on maximizing a free parameter (time or distance) when the number of agents is specified. It will be shown that the optimal cruising speed for maximizing the endurance of monitoring-type missions varies between the best L=D speed and the MacCready speed and depends on the "aggregate thermal strength" of a given cycle, or equivalently, the ratio of the time that one sUAV spends away from the target to the time that it spends at the target. An examination of multiple-thermal exploitation is then presented, followed by an evaluation of the flight simulations used to support the results.