As Earth-based radio tracking navigation is severely limited because of communications constraints and low relative navigation accuracy, autonomous optical navigation capabilities are essential for both robotic and manned deep-space exploration missions. Image processing is considered one of the key technologies for autonomous optical navigation to extract highprecision navigation observables from a raw image. New image processing algorithms for deep-space autonomous optical navigation are developed in this paper. First, multiple image pre-processing and the Canny edge detection algorithm are adopted to identify the edges of target celestial bodies and simultaneously remove the potential false edges. Secondly, two new limb profile fitting algorithms are proposed based on the Least Squares method and the Levenberg-Marquardt algorithm, respectively, with the assumption that the perspective projection of a target celestial body on the image plane will form an ellipse. Next, the line-ofsight (LOS) vector from the spacecraft to the centroid of the observed object is obtained. This is taken as the navigation measurement observable and input to the navigation filter algorithm. Finally, the image processing algorithms developed in this paper are validated using both synthetic simulated images and real flight images from the MESSENGER mission. K E Y WO R D S 1. Autonomous optical navigation.2. Image processing. 3. Ellipse fitting. 4. Centroid extracting.
In this study, a novel droop control method for ac microgrids is proposed to enhance the performance of power regulation, which is composed of three parts. The angle droop and the frequency droop are adopted to control the active power in coordination, while the modified voltage droop is used to control the reactive power. There are several improvements in this proposed triple droop control method compared with the conventional droop control. First, the dynamics of active power regulation are enhanced because of the angle droop. Second, the stability of the system is upgraded and the droop coefficients can vary in greater range. Third, the accuracy of reactive power regulation is improved, especially in the grid-connected mode, the DGs can output the specified reactive power with the modified voltage droop, which benefits the economic dispatch of the microgrid. Through these three combined droop control strategies, the performance of the whole system can be improved a lot whether in the grid-connected mode or the islanded mode. Furthermore, the stability of the proposed triple droop control is analysed, which shows that the stability of the system is upgraded. The effectiveness of the proposed control method is verified by the hardware-in-loop tests.
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