Through-water Stereo SLAM with Refraction Correction for AUV Localization

Abstract

In this letter, we propose a novel method for underwater localization using natural visual landmarks above the water surface. High-accuracy, drift-free pose estimates are necessary for inspection tasks in underwater indoor environments, such as nuclear spent pools. Inaccuracies in robot localization degrade the quality of its obtained map. Our framework uses sparse features obtained via an onboard upward-facing stereo camera to build a global ceiling feature map. However, adopting the pinhole camera model without explicitly modeling light refraction at the water-air interface contributes to a systematic error in observations. Therefore, we use refraction-corrected projection and triangulation functions to obtain true landmark estimates. The SLAM framework jointly optimizes vehicle odometry and point landmarks in a global factor graph using an incremental smoothing and mapping backend. To the best of our knowledge, this is the first method that observes in-air landmarks through water for underwater localization. We evaluate our method via both simulation and real-world experiments in a test-tank environment. The results show accurate localization across various challenging scenarios.