  This paper presents a scalable acoustic navigation approach for the unified
command, control and coordination of multiple autonomous underwater vehicles
(AUVs). Existing multi-AUV operations typically achieve coordination manually,
by programming individual vehicles on the surface via radio communications,
which becomes impractical with large vehicle numbers; or they require
bi-directional inter-vehicle acoustic communications to achieve limited
coordination when submerged, with limited scalability due to the physical
properties of the acoustic channel. Our approach utilizes a single,
periodically-broadcasting beacon acting as a navigation reference for the group
of AUVs, each of which carries a chip-scale atomic clock (CSAC) and fixed
ultra-short baseline (USBL) array of acoustic receivers. One-way travel-time
(OWTT) from synchronized clocks and time-delays between signals received by
each array element allows any number of vehicles within receive distance to
determine range, angle, and thus determine their relative position to the
beacon. The operator can command different vehicle behaviors by selecting
between broadcast signals from a predetermined set, while coordination between
AUVs is achieved without inter-vehicle communication, by defining individual
vehicle behaviors within the context of the group. Vehicle behaviors are
designed within a beacon-centric moving frame of reference, allowing the
operator to control the absolute position of the AUV group by re-positioning
the navigation beacon to survey the area of interest. Multiple deployments with
a fleet of three miniature, low-cost SandShark AUVs performing closed-loop
acoustic navigation in real-time provide experimental results validated against
a secondary long-baseline (LBL) positioning system, demonstrating the
capabilities and robustness of our approach with real-world data.
