目的 针对自主空中加油过程中锥套图像目标识别与定位问题，以锥套内部加油口作为匹配特征，提出一种基于圆形特征的锥套检测与跟踪方法。方法 锥套跟踪采用行列扫描法获取锥套内部边缘，并通过迭代最小二乘拟合确定精确的椭圆形状参数，而锥套检测采用多方位最近点区域搜索法提取锥套所在的所有可能图像区域，并采用锥套跟踪进行精匹配和最终决策。结果 实验结果表明，该方法下锥套精确定位的成功率高达94.71%，锥套检测耗时小于500 ms，锥套跟踪的最大耗时为21.59 ms，平均耗时4.18 ms。结论 本文方法无须在锥套上额外安装光学标记，且能够实时、精确地确定锥套所在的图像位置和大小。
Drogue image detecting and tracking based on iterative least squares ellipse fitting
Objective 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. Method 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. Result 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. Conclusion 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.