I. Introduction
Estimating the 6D pose, i.e., the 3D rotation and 3D translation, is essential for underwater localization, navigation and cooperative operations between unmanned underwater vehicles (UUVs), such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs). The vast majority of previous work about multi-robot relative localization focus on terrestrial autonomous driving, on-orbit servicing and unmanned aerial vehicle [1], [2], [3], [4], [5]. Although researches on 6D pose estimation are proliferating in recent years, some important environments have not received significant attention thus far, such as underwater.