Networked Acoustic Modems for Real-Time Data Telemetry from Distributed Subsurface Instruments in the Coastal Ocean: Application to Array of Bottom-Mounted ADCPs

Abstract

Through the winter and spring of 2002, networked acoustic modems demonstrated real-time wireless data telemetry from an array of bottom-mounted acoustic Doppler current profilers (ADCPs) on the inner continental shelf 20?60 m deep off of Montauk Point, New York. To achieve typical temporal and spatial sampling needs for data assimilative numerical modeling, the array spanned 10 km ? 10 km and transmitted data each ?2 h. Network nodes included five sensors, each an ADCP with acoustic modem housed in a trawl-resistant bottom frame; five repeaters that are individual acoustic modems on near-bottom taut-wire moorings; and two gateways, each a buoy with a subsurface acoustic modem and topside cellular modem allowing for two-way communication with the shore. Deliveries from an ADCP adjacent to the gateway buoy were more than 97% successful through both winter and spring. Deliveries from ADCPs 5 km from the gateway averaged 25% (86%) reliability in winter (spring). Winter performance degrades because of upward-refracting sound speed profiles that limit direct acoustic paths, and strong winds that disrupt sea surface reflectivity and increase ambient noise. Reliability improved up to 36% due to the receive-all gateway mode, and more than doubled for certain node pairs due to a handshake protocol incorporating an automatic repeat request. Shore-based network control demonstrated adaptive sampling by changing ADCP vertical and temporal resolution, and network data path rerouting in response to unplanned events, such as trawling impacts. Networked acoustic modems are well suited for coastal ocean-observing systems, particularly at sites such as this where seafloor cables and surface buoys are vulnerable to fishing and shipping activities.