Space Vision Marker System (SVMS)

Visual targets are used in space proximity operations to help in alignment of mechanical interfaces during, for example, spacecraft rendezvous and docking, and in robotic servicing of space structures. Distinctive and precisely placed target features facilitate unambiguous and reliable detection, and accurate estimation of relative position and orientation (pose) by the human operators or automatic vision systems. Existing targets used by vision systems typically do not allow for identification or for multiple targets to be visible in the field of view. The targets are detected in the first image and are tracked in successive frames, which introduce a potential failure condition. This paper describes a novel concept of a Space Vision Marker System (SVMS). Inspired by past designs of space targets and terrestrial marker systems, SVMS combines positive features of both and extends them to space applications. SVMS includes: the design of a family of markers, preferred imaging arrangement (camera, light, marker material), and software algorithms to detect the markers. The markers are reliably detected under a wide range of viewing distances and angles, and illumination (including direct sun light and shadows); encoded redundant features allow identification even with a partial data loss. Three dimensional structures of the markers allow accurate pose estimation. The detection algorithms operate on each image separately and in real time, which does not require tracking between frames and allows instant recovery from failures due to, e.g., marker occlusions. Results of laboratory tests performed under conditions representative to proximity operations in lower earth orbit are also presented.