Bilateration-based Position Estimation of Sensor Nodes for UAV-assisted Wireless Power Transfer Systems

In this paper, we present a navigation algorithm for unmanned aerial vehicles (UAVs) for battery-less agricultural sensor systems. In order to efficiently power sensor nodes from UAVs using wireless power transfer, an appropriate UAV navigation algorithm is necessary. UAVs can locate a certain sensor roughly, but they cannot find the exact location. In our proposed method, a UAV draws circles using each point as a center and each distance as a radius. Then, it counts the number of intersections of the circles and quickly approximates the exact location of the sensor with the densest intersections as a near-optimal point. There have been several studies of battery-less wireless sensor networks (WSNs) for agricultural sensing, and some recent studies have used optimal tracking methods for UAVs to wirelessly power sensor nodes. However, these proposed systems were designed without considering the time needed to complete the wireless power transfer to each node. Because a UAV's battery capacity is limited, the area coverable by a UAV is determined by the amount of time needed to power all the sensor nodes in the field. Thus, if a navigation algorithm can enable UAVs to reach the near-optimal point more quickly, fewer flight tasks will be needed to cover the fields, which will reduce system operating costs. We compared our proposed method to a conventional hill-climbing algorithm in terms of navigation accuracy and time needed to power the sensor nodes. The results showed that the proposed algorithm performed 30% better than the conventional algorithm.