Research On Stereo Vision And Visual Navigation For A Lunar Rover

According to China's lunar exploration program, a lunar satellite will be launch to execute an elementary survey of the moon. In the second step an unmanned lunar spaceship will be landed on lunar surface and a lunar rover will carry out a detailed research in a rather large range. These explorations will lay the foundation for manned lunar exploration and constructing lunar base. Lunar rover is the platform of scientific instruments and its safe roaming is the basis for subsequent exploration. The vision and navigation system, as the theme of this paper, provides the terrain information surrounding the rover and the position information of the rover, which is indispensable for lunar rover's safe roaming. This paper deals with these two systems.In lunar rover's vision system a fairly large field of view is needed and thus a wide-angle lens is need in a CCD camera. As the same time the measurement precision in a large distance needs to be guaranteed for safety of the lunar rover. These two requirements constraint the camera calibration technique to adopt a camera model with distortion. In this respect existing linear methods do not accommodate distorted model and nonlinear optimization methods are usually difficult to realize and dependent on initial parameter values. This paper inherits previous research and presents an optimized calibration algorithm for distorted camera modal. The algorithm works in an iterative manner and is easy to implement as it avoids nonlinear optimization procedures. It can improve calibration precision effectively and can be extended easily to include more or less distortion term for different applications without increasing computation complexity.Stereo matching is a critical step in stereo vision. In the surroundings of lunar surface, few structured objects and many areas lacking in texture exist. This condition presents large difficulty to correlation-based stereo matching algorithm. In this paper image rectification is executed for stereo pairs in the first step. Then based on the fairly-planar property of lunar surface a disparity linearity constraint is introduced and a multi-stage matching algorithm is proposed. In the algorithm points with large intensity gradient are matched first and their matching results are used to assist in matching of other points based on the constraint. In a point's matching procedure, a multi-criterion matching technque including intensity, x gradient and gradient orientation is utilized to improve matching accuracy. The algorithm exhibits nice adaptability to various types of natural terrain and demonstrates advantage over traditional correlation-based matching algorithm both in precision and in efficiency.In reconstruction section image points are reconstructed to obtain the 3D coordinates of corresponding object points. In order to acquire the digital elevation map needed for path planning and obstacle avoiding, the isolated reconstructed object points are removed at first. Then the elevation at grid points are computed by interpolation both in original object points and grid points. As a result a fairly complete scene surface is formed with occlusions behind large rocks reserved.As to visual navigation, this paper realized the visual odometry method which is based on feature detection and tracking and analyzed the main factors affecting navigation precision. Then we proposed a compact terrain matching algorithm to position the lunar rover based on the terrain from lunar rover and orbiter or lander. These two algorithms make further use of the reconstruction digital elevation map.