Extrinsic Calibration between an Optical Camera and an Imaging Sonar

In this paper, we present an open-source tool for calculating the extrinsic calibration between an optical camera and an imaging sonar. Precise determination of the relative 3D location of two sensors is a prerequisite to combining their data into a unified representation of the scene. Optical cameras are commonly used in many robotic domains due to their rich data, low price point, and small form factor. However, optical imagery and the resulting reconstruction degrade quickly in the presence of turbidity and marine snow. Conversely, imaging sonars resolve feature locations in range and azimuth, are largely unaffected by turbidity, but have an inherent elevation ambiguity due to their wide vertical beam pattern. The complementary strengths and weaknesses of these sensors make it appealing to combine them when developing a perception system for use in underwater reconstruction and manipulation tasks. As a first step, an extrinsic calibration describing the relative position of the two sensors must be determined before any data integration is possible. This paper presents a technique that uses a readily-available calibration target and simple hardware to construct a target with both optical and acoustic features and then introduces an open source toolbox for computing the extrinsic calibration.