  Multiple lidars are prevalently used on mobile vehicles for rendering a broad
view to enhance the performance of localization and perception systems.
However, precise calibration of multiple lidars is challenging since the
feature correspondences in scan points cannot always provide enough
constraints. To address this problem, the existing methods require fixed
calibration targets in scenes or rely exclusively on additional sensors. In
this paper, we present a novel method that enables automatic lidar calibration
without these restrictions. Three linearly independent planar surfaces
appearing in surroundings is utilized to find correspondences. Two components
are developed to ensure the extrinsic parameters to be found: a closed-form
solver for initialization and an optimizer for refinement by minimizing a
nonlinear cost function. Simulation and experimental results demonstrate the
high accuracy of our calibration approach with the rotation and translation
errors smaller than 0.05rad and 0.1m respectively.
