Coarse-to-Fine Loosely-Coupled LiDAR-Inertial Odometry for Urban Positioning and Mapping

Abstract

Accurate positioning and mapping are significant for autonomous systems with navigation requirements. In this paper, a coarse-to-fine loosely-coupled (LC) LiDAR-inertial odometry (LC-LIO) that could explore the complementariness of LiDAR and inertial measurement unit (IMU) was proposed for the real-time and accurate pose estimation of a ground vehicle in urban environments. Different from the existing tightly-coupled (TC) LiDAR-inertial fusion schemes which directly use all the considered ranges and inertial measurements to optimize the vehicle pose, the method proposed in this paper performs loosely-couped integrated optimization with the high-frequency motion prediction, which was produced by IMU integration based on optimized results, employed as the initial guess of LiDAR odometry to approach the optimality of LiDAR scan-to-map registration. As one of the prominent contributions, thorough studies were conducted on the performance upper bound of the TC LiDAR-inertial fusion schemes and LC ones, respectively. Furthermore, the experimental verification was performed on the proposition that the proposed pipeline can fully relax the potential of the LiDAR measurements (centimeter-level ranging accuracy) in a coarse-to-fine way without being disturbed by the unexpected IMU bias. Moreover, an adaptive covariance estimation method employed during LC optimization was proposed to explain the uncertainty of LiDAR scan-to-map registration in dynamic scenarios. Furthermore, the effectiveness of the proposed system was val-idated on challenging real-world datasets. Meanwhile, the process that the proposed pipelines realized the coarse-to-fine LiDAR scan-to-map registration was presented in detail. Comparing with the existing state-of-the-art TC-LIO, the focus of this study would be placed on that the proposed LC-LIO work could achieve similar or better accuracy with a reduced computational expense.

Problem Addressed

Tightly coupled LiDAR-inertial fusion optimizes all range and inertial measurements jointly, so unexpected IMU bias can disturb pose estimation and limit the potential of centimeter-level LiDAR ranging. A loosely coupled alternative that fully exploits LiDAR accuracy is underexplored.

Key Contributions

• Proposes a coarse-to-fine loosely coupled LiDAR-inertial odometry using IMU prediction as scan-matching initial guess
• Studies the performance upper bounds of tightly coupled and loosely coupled LiDAR-inertial fusion schemes
• Introduces adaptive covariance estimation to model scan-to-map registration uncertainty in dynamic scenarios
• Validates the system on challenging real-world urban datasets

Conclusions

The proposed loosely coupled LiDAR-inertial odometry fully relaxes the potential of LiDAR measurements in a coarse-to-fine manner without being disturbed by IMU bias. It achieves similar or better accuracy than existing state-of-the-art tightly coupled LiDAR-inertial odometry with reduced computational expense on challenging real-world datasets.