Abstract-Indoor localization is a challenging problem addressed extensively by both robotics and computer vision communities. Most existing approaches focus on using either cameras or laser scanners as the primary sensor for pose estimation. In laser scan matching based localization, finding scan point correspondences across scans is a challenging problem as individual scan points lack unique attributes. In camera based localization, one has to deal with images with little or no visual features as well as scale factor ambiguities to recover absolute distances. In this paper, we develop a multimodal approach for indoor localization by fusing a camera and a laser scanner in order to alleviate the drawbacks of each individual modality. Specifically, we use structure from motion to estimate the pose of a moving camera-laser rig which is subsequently used to compute scan correspondence estimates which are refined using a window based search method for scan point projections on the images. We have demonstrated our proposed system, consisting of a laser scanner and a camera, to result in a 0.4% loop closure error for a 60m loop around the interior corridor of a building.
I. INTRODUCTIONocalization in environments with limited global positioning information remains a challenging problem. Indoor localization is a particularly important problem with a number of applications such as indoor modeling, and human operator localization in unknown environments. Localization has been primarily studied by the robotics and computer vision research communities. In robotics, the focus has been on estimating the joint posterior over the robot's location and the map of the environment using sensors such as wheel encoders, laser scanners and Inertial Measurement Units (IMUs). This problem is typically referred to as Simultaneous Localization and Mapping (SLAM) [5]. To localize a wheeled robot, simple 2D maps are typically generated using 2D horizontal laser scanners which serve to both localize the robot and measure depth to obstacles directly. Laser scan matching based localization approaches involve computing the most likely alignment between two sets of slightly displaced laser scans. The Iterative Closest Point (ICP) algorithm [1] is the most extensively used scan correlation algorithm in robotics. The open loop nature of the pose integration from ICP and wheel odometry tends to introduce large drifts in the navigation estimates.This research project is supported by the ARO grant W911NF-07-1-0471 and the HSN MURI grant W911NF-06-1-0076.These estimates can be improved by using a dynamic motion model for the robot, and by applying probabilistic methods to estimate the robot's location and the map. Thrun et. al.[7] use an expectation maximization approach to solve the SLAM problem on robots with wheel encoders and laser scanners. Other approaches to SLAM tend to rely heavily on Extended Kalman Filters (EKFs) [6], since the process of tracking the robot's pose and position of the geometric features in the scene can be elegantly repres...