Cooperative navigation for an UAV tandem in GNSS denied environments

Unmanned Aerial Vehicles (UAVs) have the potential to substitute or support conventional methods for inspection and monitoring of natural or artificial structures in challenging scenarios. However, open challenges in the autonomous navigation and guidance remain if the Global Navigation Satellite Systems (GNSS) can not be employed because the reception of the signals is denied or unreliable. This often happens in cluttered natural environments, such as gorges and steep valleys, or near to tall civil engineering structures such as dams and bridges. In these cases, the position fix is intermittent, can be biased because of multi-path effects, and the change of the constellation of visible satellites can cause sudden, undesirable shifts in the position solution. In this work we propose a method to solve the position control of an UAV in such scenarios, exploiting the fact that open sky is available above the object of interest. The system is composed of two UAVs: the first one flies higher in the open sky, where an optimal GNSS constellation is available. The second one performs the close range inspection without relying on any GNSS receiver. Instead, its position is determined by the first one by optical tracking and it is sent to the second one in real-time and with low latency to close the position control loops. We present a detailed description of the system and discuss its first experimental evaluation.