Determining a position of a submersible vehicle within a body of water

Methods and apparatus determining a position of a submersible vehicle within a body of water are provided. A method comprises determining an initial position of the vehicle while the vehicle is at or near a water surface. The method further comprises coupling the vehicle and a base node to a weight and determining a position of the base node once the base node and vehicle have reached the floor of the body of water using acoustic modems of the vehicle and a surface vessel to aid in calculating the position as they descend, and acoustically transferring the at rest position on the seafloor from the vehicle to the node. The method further comprises decoupling the vehicle from the node and weight and determining a position of the vehicle based on the position of the base node and acoustic signals exchanged between acoustic modems of the vehicle and the base node.