Acoustic communications for deep-ocean observatories: Results of initial testing at the MBARI MARS node

A prototype system for connecting remote instruments to a cabled observatory is described. The system consists of base-station modems and a data acquisition system attached to the MBARI MARS observatory in 900 m water off Monterey, California, plus remote modems used for link testing. The objective of the system is to provide a drop-in capability to connect sensors on the sea-floor or on sub-sea moorings to a cabled node that would otherwise require an ROV to make a hard-wired connection. While ROV installations of sensor-to-node cables are practical for many instruments, the acoustic connection is a way of lowering the barrier to general access of cabled nodes. The system designed for demonstration purposes is a combination of commercial components and custom hardware and software. While the system is a prototype, considerable effort was expended to make the sub-sea portion as simple and reliable as possible so as to minimize the potential for failure and the reduce the associated costs of recovering and re-deploying the system. While the system uses real-time modems (the WHOI Micro-Modem) in the sub-sea portion, a data acquisition system and streaming feed of the acoustic array allows the use of one or more software modems in the future, which can be vendor independent or conform to an established standard. The system was deployed in February 2010 and has been undergoing testing since. This paper reports on initial results obtained to-date. Several types of tests have been performed to establish performance over both short (24 hours) and medium (two week) time scales. At 2 km burst data rates of 5000 bps have been achieved with near-perfect reliability using a carrier frequency of 25 kHz and 5 kHz of bandwidth. Temporal variability is low, and the largest impacts on performance are the delayed surface bounce from the surface and acoustic interference observed during occasional ROV operations in the area.