Tightly-coupled ultra-wideband-aided monocular visual SLAM with degenerate anchor configurations

This paper proposes an enhanced tightly-coupled sensor fusion scheme using a monocular camera and ultra-wideband (UWB) ranging sensors for the task of simultaneous localization and mapping. By leveraging UWB data, the method can achieve metric-scale, drift-reduced odometry and a map consisting of visual landmarks and UWB anchors without knowing the anchor positions. Firstly, the UWB configuration accommodates any degenerate cases with an insufficient number of anchors for 3D triangulation ( N <= 3 and no height data). Secondly, a practical model for UWB measurement is used, ensuring more accurate estimates for all the states. Thirdly, selected prior range measurements including the anchor-world origin and anchor-anchor ranges are utilized to alleviate the requirement of good initial guesses for anchor position. Lastly, a monitoring scheme is introduced to appropriately fix the scale factor to maintain a smooth trajectory as well as the UWB anchor position to fuse camera and UWB measurement in the bundle adjustment. Extensive experiments are carried out to showcase the effectiveness of the proposed system.