To overcome the shortcoming about the lack of payload and endurance

the autonomous aerial refueling technology is an effective solution for unmanned aircraft to extend flight duration

which results in exponentially enhancing of the aircraft fighting efficiency

and it can also reduce the refueling operation risk for manned aircraft. Relative position sensing between the receiver aircraft and the refueling drogue is the first and necessary step in the autonomous aerial refueling procedure

and the vision-based method is the mostly used for relative positioning. We propose a circular feature-based drogue detecting and tracking method by using the internal refueling port of drogue as the matching feature. The drogue tracking is the final step which outputs the final image positioning results even in the drogue detecting process. In drogue tracking

the rectangle of interest (ROI) of current frame image is determined and extracted depending on the positioning result in the last frame

and then the Canny algorithm is applied to obtain the ROI edge image. With the edge pixel points

the row and column scanning method is used to obtain the inside edge points of circular objects

which can exclude the irrelevant image pixel points

and then the precise elliptical shape parameters can be solved through an iterative least-squares fitting method. In drogue detecting

a multi-direction closest point searching method is proposed to extract all possible drogue image areas by inspecting the circular feature when scanning the multiple directions of the interested pixel point

and final result with precise matching can be decided by the drogue tracking method mentioned before. Experiments are designed and done with a drogue model which has almost the same size of the real refueling drogue. The Hough-based circle detecting method is treated as the comparison method

and simultaneously a laser rangefinder is used to provide the reference positions in the experimental procedure. Experimental results show that

the proposed method has a success rate of 94.71% in precise relative distance positioning of drogue

better than the Hough-based circle detecting method. In real-time performance

the Hough-based circle detecting method has an average time consumption of 59.52 ms. When using the proposed method

drogue detecting takes less than 500 ms

and drogue tracking has a time consumption of maximum 21.59 ms and average 4.18 ms

which can well work with the camera having a frame rate of 25 frame/s. Vision-based navigation method is a potential and available solution for probe-drogue aerial refueling docking

and the complex background of the tanker aircraft makes the image detecting and tracking of drogue difficult and easily disturbed. We use the internal circular refueling port as the image detecting feature

and propose a new drogue image detecting and tracking method. The installation of additional optical marks on the refueling drogue is not required

and the real-time position and size of drogue in the image plane can be accurately determined. The proposed method is able to meet the requirements of the accuracy

the real-time performance

and the reliability for image processing in the autonomous aerial refueling procedure.