AirborneRelaybased Algorithm for Locating Crashed UAVs in GPSDenied Environments We propose a location algorithm to promptly search for crashed unmanned aerial vehicles (UAVs), using an airborne communication relay UAV (ACRU) in global positioning system (GPS)denied environments. Conventional UAV based location methods require at least four UAVs to search for the crashed one. However, such methods are subject to significant errors according to the distance between the crashed UAV and the others. In the proposed algorithm, only one ACRU is required to locate the crashed UAV. The ACRU relays its own position and the signals from the crashed UAV in realtime to a ground control station (GCS), which then estimates the position of the crashed UAV based on time difference of arrival (TDOA) processing using the signals relayed from the ACRU. According to the estimated position of the crashed UAV, the ACRU then flies in the direction of the crashed UAV by applying a weighting algorithm to effectively estimate the distance to it. The performance of the proposed algorithm is verified by computer simulations by considering the conditions of an army battalion battlefield. We improve the accuracy of the proposed location algorithm by applying a weighting algorithm to the TDOA data, and we demonstrate that the location algorithm can be used to efficiently locate crashed UAVs in GPS denied environments.

Three Dimensional UAV Positioning for Dynamic UAVtoCar Communications In areas with limited infrastructure, Unmanned Aerial Vehicles (UAVs) can come in handy as relays for cartocar communications. Since UAVs are able to fully explore a threedimensional environment while flying, communications that involve them can be affected by the irregularity of the terrains, that in turn can cause path loss by acting as obstacles. Accounting for this phenomenon, we propose a UAV positioning technique that relies on optimization algorithms to improve the support for vehicular communications. Simulation results show that the best position of the UAV can be timely determined considering the dynamic movement of the cars. Our technique takes into account the current flight altitude, the position of the cars on the ground, and the existing flight restrictions.