Encoder-Camera-Ground Penetrating Radar Sensor Fusion: Bimodal Calibration and Subsurface Mapping

In this article, we report system and algorithmic developments for a sensing suite comprising a camera and a ground penetrating radar (GPR) with a wheel encoder designed for both surface and subsurface infrastructure inspection, which is a multimodal mapping task. To fuse different sensor modalities properly, we solve a novel GPR-camera calibration problem and a synchronization-challenged sensor fusion problem. First, we design a calibration rig, model the GPR imaging process, introduce a mirror to obtain the joint coverage between the camera and the GPR, and employ the maximum-likelihood estimator to estimate the relative pose between the camera and the GPR with error analysis. Second, we propose a data collection scheme using the customized artificial landmarks to synchronize camera images (temporally evenly spaced) and GPR/encoder data (spatially evenly spaced). We also employ pose graph optimization with location discrepancy as penalty functions to perform data fusion for 3-D reconstruction. We have tested our system in physical experiments. The results show that our system successfully fuses encoder-camera-GPR sensory data and accomplishes a metric 3-D reconstruction. Moreover, our sensor fusion approach reduces the end-to-end distance error from 6.4 to 0.7 cm in a real bridge inspection experiment if comparing to the counterpart that only uses encoder measurements.