The network of acoustic modems consists of three types of nodes: sensors, repeaters, and gateways. The gateways are the only components of the system that are exposed to the difficult environment of the sea surface, as all others reside near the seafloor.

          Each sensor node consists of a moored oceanographic instrument physically interfaced to an acoustic modem. The acoustic modem periodically transmits data as it is collected by the instrument. This figure shows a trawl-resistant bottom frame with acoustic Doppler current profiler (ADCP) and acoustic modem forming a sensor node that requires no elements to be exposed to the water-column or sea surface:

          Each repeater node is an individual acoustic modem deployed singly at a position between sensor nodes with the sole purpose of relaying data. The repeaters enable a larger spatial area of the coastal ocean to be sampled by the sensor nodes. Repeaters are deployed near the seafloor and, like the ADCP sensor nodes, do not include exposure of any elements to the sea surface. This figure shows a repeater node mooring, which extends several meters up above the bottom, a small fraction of the water depth:

          Each gateway node is a surface buoy on which an acoustic modem is interfaced to a cellular modem that can send and receive data from the data center on shore at UConn Avery Point. This figure shows the USCG MP buoy engineered as a gateway node:

 

At regular intervals, data originating at a sensor node is transmitted acoustically to a gateway node via subsurface acoustic paths among repeater nodes, and/or other sensor nodes that may also act as repeaters. From the gateway, the data is delivered to shore via the cellular telephone network.

An example. Sensor node A12, at the southernmost point in the array, is an acoustic Doppler current profiler (ADCP) moored on the seafloor in a trawl-resistant bottom frame. When the ADCP has completed a measurement of the vertical profile of water velocities, the data is passed through a short cable to an acoustic modem deployed with the ADCP in the bottom frame. The acoustic modem then transmits the data as a message to the repeater node R10 about 3 km away, which relays the message an additional 3 km to repeater node R8, from which it is sent (nearly vertically) up to the acoustic modem on gateway node G2 near the center of the array. On the buoy G2, the gateway acoustic modem passes the data through an interface cable to a cellular modem, which is accessed via the internet and the cellular telephone network to deliver the data to shore.

Gaps in the timeseries stickplots. In the plots of real-time currents, you will see periods of time when there are missing data. This is because acoustic communications between nodes can be degraded, during certain unpredictable time intervals, by a number of rapidly fluctuating conditions that govern underwater propagation of sound. These include water temperature and salinity distributions, wind strength, and wave conditions. As the nodes located far from the gateway must transmit their data across larger distances and more acoustic links, their data stream is more susceptible to degradation than nodes closer to the gateway.  For example, data from node A12 will have more gaps in its record than will A7, which is located very close to the gateway G2.

Re-routing and adaptive sampling. An advantage of the system is that from shore, commands can be sent to any network node in real time to modify its behavior. For example, the particular node-to-node route that data takes from a sensor to the gateway nodes can be changed. This has been made necessary as a result of loss of a repeater node to fishing activity during an experiment. Another example of the utility of two-way communication with the network from shore is that the sampling parameters of the sensor nodes can be modified in real-time, for example in response to an event noticed in the real-time data. This type of operation is known as adaptive, or 'smart', sampling.