Autonomous subsea platforms operating in the Arctic throughout the winter require a long-range acoustic navigation system to substitute for GPS while under ice and not able to surface. To allow the autonomous platforms to sample a large portion of the Beaufort, the navigation system must span several hundred kilometers, which in turn requires multiple sources with acoustic ranges of at least 100 km each. To enable vehicles such as gliders to navigate while performing their missions also requires an onboard acoustic receiver and accurate clock, plus processing to compute geolocations. Over the past four years an operational system that includes multiple fixed source moorings and compact, low-power receivers has been maintained in the deep area of the Beaufort Sea north of Alaska. The navigation system has been used to successfully allow gliders and profiling floats to operate throughout the winter, performing science missions and augmenting data collected on instrumented fixed moorings in the center of the Beaufort.

A bottom-to-bottom acoustic communications network is feasible in areas of the ocean where the combination of the depth and near-surface sound speed support direct-path refracted rays which connect multiple sensor nodes located on or near the seafloor. In this paper the initial results of modeling and testing the physical layer of a communications system that will support a bottom-based sensor network in the Mentawai Basin west of Padang, Sumatra in Indonesia are reported based on an experiment done in 2015. The acoustic communications system utilizes a carrier frequency between 3 and 4 kHz, and achieves data rates from 60-700 bps, depending on the range. The combination of a bottom depth of 1750 m and a deep and seasonally persistent thermocline provide the necessary conditions for direct-path propagation via surface refracting rays. The results that are presented include signal-to-noise ratio (SNR), adaptive equalizer mean-square-error and sample multipath measurements from real-time acoustic modems (the WHOI Micro-Modem) at ranges of 20 and 27 km. Raw acoustic data was also recorded in parallel for post-processing and additional analysis. Refracted rays provided direct-path ranges to 33 km, so the results shown here can be extended through use of lower frequencies to increase the range.

The Arctic Ocean is ringed by shallow areas along the continental shelves which are home to shore-fast ice throughout much of the winter. These areas are being used for both traditional activities such as fishing and hunting, and increasingly, at least in some locations, resource extraction and related exploration. Environmental monitoring under shore-fast ice in multi-use regions will increasingly be accomplished using autonomous underwater vehicles and non-cabled sensors, both of which will require means for acoustic data telemetry. In this paper the results of an experiment done to test the data rate, range and reliability of acoustic communications in 5-10 m deep water under shore-fast ice near Prudhoe Bay, Alaska are described. The experiment was carried out in late winter of 2014 using Micro-Modem communications equipment operating at 10 kHz at ranges of 1.5 to 5.6 km and data rates (burst) from 80 to 5000 bps. The achievable data rates were dependent on range and water depth, plus the presence of ice keels that blocked the acoustic path in some areas. The feasibility of long baseline (LBL) navigation at these ranges is also confirmed by the results.

A long-range acoustic navigation system with built-in acoustic communications capability has been developed for use by underwater gliders, drifters and vehicles under Arctic ice where surfacing to acquire GPS position may be risky or impossible. The system consists of multiple buoys placed on the ice with transducers suspended 100 m below, each of which is programmed to transmit in a specific time slot at regular intervals. The system operates at 900 Hz, and has programmable bandwidth, from 25 to 100 Hz. The communications data rate for the system is several bits per second, sufficient to transmit the GPS location of the buoys and several bytes of data to vehicles under the ice. The system was deployed in March of 2014 and operated through the fall of 2014, testing the performance of both the navigation and communications capabilities of the system in conjunction with Seagliders deployed by the University of Washington. Ranges of greater than 400 km were achieved with range accuracy of 40 m RMS for the case where the speed of sound is known. The long range and excellent accuracy were the result of ducted sound propagation in the Beaufort Sea.

Initial results of experiments performed under Arctic ice have shown that acoustic communications and navigation can be performed on scales of 10-100 km using relatively inexpensive and compact hardware. Measurements of the impulse response at ranges of 10 and 75 km reveal extensive scatter and both resolvable and unresolvable rays. Phase coherent communication using adaptive equalization was successful up to ranges of 70-90 km at data rates of 5-10 b/s. As the SNR drops to levels too low for phase coherent communication, short FM sweeps (5-10 s), are shown to provide sufficient gain to provide lower rate communications and also support navigation.

The recently developed wave glider has the potential to be an effective unmanned platform for acoustic applications. We present the results of a variety of experiments that quantify this potential. The radiated self‐noise of the autonomous platform is evaluated using an integrated passive acoustic recorder during a set of field trials off the coast of Hawaii. We present the radiated noise spectra from these trials to illustrate the dependence on hydrophone location and sea state. Using the same instrumentation, we demonstrate the ability of a modified wave glider to detect marine mammals using passive acoustic monitoring techniques. We also evaluate the performance of the wave glider operating as an active acoustic gateway, highlighting the potential of this platform to serve as a navigation reference and communications relay for scientific, industrial, and military subsea assets. To demonstrate the potential of the wave glider platform to support acoustic navigation, we assess the performance of time‐of‐flight range estimation and seafloor transponder localization. These tests were performed using commercial off‐the‐shelf acoustic positioning hardware integrated with the wave glider to illustrate that the low self‐noise of the wave glider makes it possible to achieve acoustic positioning performance similar to previously reported results. Finally, we show that the glider can operate as a station‐keeping surface communications gateway and provide recommendations for its use. © 2012 Wiley Periodicals, Inc.

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.

Different methods, such as ALG, STUN, TURN and FCP have been proposed for using SIP together with firewalls and NAT. The most suitable solution has to be determined in every situation. Some of these methods cause more network traffic and capacity requirements, while others require support from the clients being used in the network. In this paper a system which makes it possible to offer SIP service to customers behind NAT without making any changes to firewalls is presented. The system is based on an RTP proxy and a SIP server, which modifies SIP messages.

An acoustic communications system with the capability to operate at multiple data rates in two frequency bands has been designed and developed for use in 21-inch AUVs. The system is specifically designed around the 21-inch diameter Bluefin Robotics AUV, though it could be adapted to smaller vehicles (12-inch), or similar free-flooded vehicles. The system includes both high (25 kHz) and mid-frequency (3 kHz) modems and supports data rates from 80 bps to more than 5000 bps. Both of the modems utilize four-channel arrays to increase reliability. The high-frequency modem is also used to support multi-vehicle navigation via one-way travel time measurements using synchronized clocks on all of the vehicles in a work group

The micro-modem is a compact, low-power, underwater acoustic communications and navigation subsystem. It has the capability to perform low-rate frequency-hopping frequency-shift keying (FH-FSK), variable rate phase-coherent keying (PSK), and two different types of long base line navigation, narrow-band and broadband. The system can be configured to transmit in four different bands from 3 to 30 kHz, with a larger board required for the lowest frequency. The user interface is based on the NMEA standard, which is a serial port specification. The modem also includes a simple built-in networking capability which supports up to 16 units in a polled or random-access mode and has an acknowledgement capability which supports guaranteed delivery transactions. The paper contains a detailed system description and results from several tests are also presented

A moderate bandwidth observatory platform has been developed that integrates acoustic and satellite links to provide connectivity between sensors on the seafloor and investigators on shore. The system incorporates high-rate (5300 bps) acoustic modems, solar power generation, a low-power single board computer running Linux, and Iridium satellite modems to provide a lower cost, relocatable alternative to higher bandwidth systems. The prototype system, which supports an ocean bottom seismometer (OBS) and fluid-flow, temperature, and resistivity sensors, has been deployed near the Nootka Fault in 2326 m of water off the coast of Vancouver Island since May 2004. Since deployment, the system has transmitted more than 150 MB of data from the seafloor instruments

Work at Woods Hole in underwater acoustic networks in focused on several aspects of development with the goal of creating a general-purpose framework that facilitates research for multiple applications. These applications include fixed sensor networks for environmental or naval use, as well as mobile ad-hoc networks and mixed systems that include both stationary nodes and mobile underwater vehicles. Common to research in all of these areas is a need for a hardware and software test-bed that facilitates prototyping of approaches to networking. Our work in this area includes a complete approach: modems for the physical layer, general-purpose processor for networking and sensor interface, and a modular software environment.