Networked Acoustic Modems for Real-Time Data Delivery from Distributed Subsurface Instruments in the Coastal Ocean: Initial System Development and Performance

Abstract

Results are reported from field tests of networked acoustic modems used for wireless real-time delivery of oceanographic measurements from a distributed array of subsurface instruments in coastal waters. The network demonstrated consists of sensor nodes, repeater nodes, gateway nodes, and a shore-based control center. Sensors are oceanographic instruments interfaced with acoustic modems, deployed in trawl-resistant bottom frames with azimuthally omnidirectional acoustic signaling needed for flexible network rerouting. Repeaters are individual acoustic modems to relay data so the array covers a larger area; only these relatively low-cost nodes are suited for deployment unprotected from trawlers. Gateways are buoys with acoustic modems interfaced to cellular telephone modems for communication between the underwater network and the shore. The experiment site is the inner continental shelf off Montauk Point, New York, and Block Island, Rhode Island, with a U.S. Coast Guard navigation buoy equipped as a gateway. Conditions span a variety of sound-speed profiles, water depths (?25?50 m), and seasons. Long-term average rates of successful transmissions fall to about 50% at a range of 3?4 km in the typically adverse shallow-water acoustic channel. This is adequate for networked acoustic modems to be cost effective in providing quantities of data typically required for data assimilative modeling of coastal oceanographic processes. Modem range degrades in association with increased winds; numerically modeled rays indicate that direct paths between nodes commonly do not exist. Networking functions demonstrated include handshaking protocols, receive-all gateway mode, and rerouting of data pathways from shore in response to a repeater node that is trawled out.