A vision-only relative distance calculation method for multi-UAV systems

Multiple unmanned aerial vehicles (multi-UAV) systems have been applied in many scenes to improve the flexibility and effectiveness of specific tasks. To reduce dependence on wireless communication and enhance stability during cooperative control of systems, a vision-only and universal method for relative distance calculating is presented in this paper. First, key components of the UAV were detected by a two-step method. Then, a novel feature encoding method was presented to establish the 2D-3D correspondence with the known 3D structure of UAVs and solve the Perspective-n-Point (PnP) problem. To mitigate misdetections in air-to-air scenarios, a robust auto-weighting Levenberg-Marquardt (AWLM) algorithm was integrated into pose estimation. Flight experiments of a two-UAV system in wireless-denied environments have been conducted to verify the performance of the proposed approach. The results show that the calculation error is less than 8 meters at a relative distance of up to 160 meters at the speed of 45.94 ms per frame, which means the highest precision and fastest processing speed among several similar methods. Besides, the AWLM algorithm exhibits superiority over other optimization methods with fewer outliers (below 0.67%) and a smaller error bound (2.899 meters).