Jim Partan

The WHOI micro-modem: an acoustic communications and navigation system for multiple platforms

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 survey of practical issues in underwater networks

Underwater sensor networks are attracting increasing interest from researchers in terrestrial radio-based sensor networks. There are important physical, technological, and economic differences between terrestrial and underwater sensor networks. In this survey, we highlight a number of important practical issues that have not been emphasized in recent surveys of underwater networks, with an intended audience of researchers who are moving from radio-based terrestrial networks into underwater networks.

The WHOI micromodem-2: A scalable system for acoustic communications and networking

A successor to the WHOI Micromodem-1 underwater acoustic modem has recently been developed. The Micromodem-2 has the same compact form-factor as the Micromodem-1 and will support all of the existing applications for the Micromodem-1, as well as interoperate with the Micromodem-1. Existing acoustic communications protocols using phase-shift keying (PSK) as well as frequency-hopping frequency-shift keying (FH-FSK) are supported, as are navigation features including narrow-band and broadband long-baseline (LBL) navigation. The Micromodem-2 is significantly more capable than the Micromodem-1 in computational ability and memory, bandwidth, non-volatile data storage, user expansion interfaces, and real-time clock precision. The expanded capabilities will allow new communications algorithms, modulation, errorcorrection methods, navigation features, and networking capabilities to be implemented. The improvements in processing capability and acoustic interfaces on the Micromodem-2 allow it to operate at acoustic frequencies from approximately 1kHz to 100kHz. The significant increases in available non-volatile storage enable the Micromodem-2 to capture data in-situ for diagnostic and research purposes. The Micro mo dem-2s firmware architecture is similar to the Micromodem-1s firmware architecture, using a real-time operating system based on modular signal processing blocks. It has been improved to increase modularity and facilitate future portability, and it offers significant improvements in timing for use with navigation and networking applications.

Multi-band acoustic modem for the communications and navigation aid AUV

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

A long term vision for long-range ship-free deep ocean operations: Persistent presence through coordination of Autonomous Surface Vehicles and Autonomous Underwater Vehicles

We outline a vision for persistent and/or long-range seafloor exploration and monitoring utilizing autonomous surface vessels (ASVs) and autonomous underwater vehicles (AUVs) to conduct coordinated autonomous surveys. Three types of surveys are envisioned: a) Autonomous tending of deep-diving AUVs: deployed from a research vessel, the ASV would act as a force-multiplier, watching over the AUV to provide operators and scientists with real-time data and re-tasking capabilities, while freeing the ship to conduct other over-the-side operations; b) Ridge-segment-scale (100 km) autonomous hydrothermal exploration: combined with conventional gliders or long-endurance AUVs, an ASV could tend a fleet of underwater assets equipped with low-power chemical sensors for mapping hydrothermal plumes and locating seafloor hydrothermal venting. Operators would control the system via satellite, such that a support ship would be needed only for initial deployment and final recovery 1-2 months later; and c) Basin-scale (10,000 km) autonomous surveys: a purpose-built autonomous surface vessel (mother-ship) with abilities up to and including autonomous deployment, recovery, and re-charge of subsea robots could explore or monitor the ocean and seafloor on the oceanic basin scale at a fraction of the cost of a global-class research vessel. In this paper we outline our long term conceptual vision, discuss some preliminary enabling technology developments that we have already achieved and set out a roadmap for progress anticipated over the next 2-3 years. We present an overview of the system architecture for autonomous tending along with some preliminary field work.

An acoustically-linked deep-ocean observatory

A buoy-based observatory that uses acoustic communication to retrieve data from water column and seafloor instruments has been developed and deployed in 2362 m of water offshore Vancouver Island. The system uses high-rate (5000 bps) acoustic modems that are power-efficient (on order 1000 bits per joule) to telemeter data from an ocean bottom seismometer and a sensor monitoring a cold seep site near the Nootka fault. The buoy includes a Linux-based embedded controller, the modem base station and meteorological sensors. Data is off-loaded from the buoy using ftp, and remote login capability allows the acoustic communication schedule to be modified when instruments are added or removed from the network. The system has been operational for one year, typically transferring more than 500 Kbytes of data per day from two seafloor instruments.

Tracking Large Marine Predators in Three Dimensions: The Real-Time Acoustic Tracking System

Large marine predators like sharks and whales can have a substantial influence on oceanic ecosystems, and characterizing their interactions with the physical and biological environment is an important goal in marine ecology. Studies of foraging ecology are of particular importance, but sampling prey aggregations encountered by these predators is extremely difficult because of the small spatial scales over which prey aggregations often occur (meters to hundreds of meters). We developed the real-time acoustic tracking system (RATS) to allow large marine predators to be accurately tracked over these small spatial scales to facilitate proximate environmental sampling. The system consists of an array of four free-floating buoys capable of detecting 36-kHz pings emitted by an animal-borne acoustic transmitter. Upon detection, the buoys transmit their position and the arrival time of the ping via a radio modem to a computer on board a nearby ship, and a software program uses differences in arrival times from all of the buoys to estimate the location of the tagged animal. The positions of the tagged animal, buoys, ship, and support boats can be monitored via a graphical user interface to allow proximate environmental sampling and maintenance of the array around the tagged animal. In situ tests indicate that average positional accuracies for a transmitter inside either a four- or three-buoy array (buoys spaced 1-1.75 km apart) are less than 10 m, and that accuracies remain near 10 m for transmitters located up to 500 m away from the edge of the array. The buoys can consistently detect the transmitter up to 1000 m away, but detection rates decrease between 1000 and 2000 m; no detections were obtained beyond 2300 m. Field deployments of the system have demonstrated an unprecedented ability to monitor the movements of baleen whales in real time, allowing a suite of prey and oceanographic observations to be collected within meters to tens of meters of a tagged animal.

Acoustic communications and navigation under Arctic ice

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.

Low complexity lossless compression of underwater sound recordings.

Autonomous listening devices are increasingly used to study vocal aquatic animals, and there is a constant need to record longer or with greater bandwidth, requiring efficient use of memory and battery power. Real-time compression of sound has the potential to extend recording durations and bandwidths at the expense of increased processing operations and therefore power consumption. Whereas lossy methods such as MP3 introduce undesirable artifacts, lossless compression algorithms (e.g., flac) guarantee exact data recovery. But these algorithms are relatively complex due to the wide variety of signals they are designed to compress. A simpler lossless algorithm is shown here to provide compression factors of three or more for underwater sound recordings over a range of noise environments. The compressor was evaluated using samples from drifting and animal-borne sound recorders with sampling rates of 16-240 kHz. It achieves >87% of the compression of more-complex methods but requires about 1/10 of the processing operations resulting in less than 1 mW power consumption at a sampling rate of 192 kHz on a low-power microprocessor. The potential to triple recording duration with a minor increase in power consumption and no loss in sound quality may be especially valuable for battery-limited tags and robotic vehicles.

Long range acoustic communications and navigation in the Arctic

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.