AUV Work and the Harbor Tracking and Observatory Project

PRIMARY CONTACT: Tom Weber

In 2006 we began our effort to explore the applicability of using a small Autonomous Underwater Vehicle (AUV) for collecting critical batyhymetric and other data. We teamed with Art Trembanis of the University of Delaware to obtain use of his FETCH 3 vehicle. We purchased, calibrated, and integrated a small multibeam sonar (Imagenix Delta-T) into this AUV and over the course of 2007 began to explore its applicability for collecting both hydrographic quality bathymetric data and seafloor characterization data. As described under the Visualization theme, in support of this effort, the Visualization group has developed mission planning and control software as well as an AUV simulator.

AUV Simulator with seafloor and physics-based fish

Unfortunately the DOERRI Fetch 3 vehicle suffered a catastrophic failure during a mission with Bob Ballard in the Black Sea. Fortunately, however, the system was fully insured and a replacement vehicle (this time a GAVIA AUV with a Submetrix Geoswath bathymetric sidescan and a Kerfot Inertial Nav system) has already been ordered (with delivery expected in April 2008). The new system is a much more mature AUV with imagery, bathymetry, and particularly positioning capabilities far beyond the original Fetch vehicle. We are thus very excited about the enhanced possibilities offered by this system and look forward to working with it in the coming year.

In support of our AUV effort as well as to provide a permanent ability to accurately position this (or any other) vehicle, samplers and other devices, we also began a project in 2006 designed to install a fixed acoustic navigation array in a portion of Portsmouth Harbor. When fully functional, this positioning system may also provide the ability to passively listen to ship-traffic in the harbor as well as to monitor changes in the physical oceanography of the harbor. We have called the project the "Harbor Tracking and Observatory Project."

During the past year, Tom Weber, Glenn McGillicuddy, Val Schmidt, Michelle Wierathmueller, Andy Mcleod, Lloyd Huff, and others have built and installed a semi-permanent installation of acoustic gear at Portsmouth Harbor Light. This is essentially a cable-to-shore acoustic tracking node that we are using for high frequency (30-40 kHz) acoustic research. This installation currently consists of a set of four hydrophones, signal conditioning electronics, and a small computer on a submerged mount located approximately 100 m from Portsmouth Harbor Light on New Castle Island. This hardware has been largely developed in-house. Power and data communications utilize a multi-conductor underwater cable connecting the submerged hydrophone system with a 'topside' computer housed inside the lighthouse. A wireless Ethernet system provides connectivity with the outside world, enabling the complete system to be controlled from any convenient location (e.g., on a research vessel or in project team member's offices). The analysis of data collected in late 2006 showed that for high signal to noise ratios (> 20 dB), the standard deviation of the acoustic phase detecting bearing sensor developed as part of this project was between 0.3-0.5°. However, there is some concern that there is an angle dependent bias in this measurement methodology that could be difficult to correct.

Further system testing in February 2007 focused on determining the useful range for this system by pinging at the hydrophones from various locations around the harbor entrance. The initial results look promising for ranges up to 1.5 km. We are currently conducting research on the various acoustic tracking methodologies in this challenging, shallow water (<50m water depths) environment. This research is being developed as a Master's degree thesis by Michelle Weirathmueller. At this time, the deployed system is being upgraded with new preamplifiers and the addition of a conductivity/temperature sensor. Acoustic pingers are also being developed in-house (with Val Schmidt, Andy Mcleod, Glenn McGillicuddy) as part of this research in order to help test time-of-flight ranging accuracies. The "UNH Pingers" are battery operated and are packaged in either a small Pelican case or buoy. Each pinger is GPS positioned and able to send both CW and stepped chirp signals from 1-100kHz at roughly 175dB. GPS 1PPS signals provide unified triggering to better than 200 nanoseconds. As described under the Visualization theme, the devices have already been used with great success for acoustically tracking tagged humpback whales in collaboration with the Stellwagen Bank National Marine Sanctuary. Preliminary results from this work were presented at the Acoustical Society of America conference in November. It is expected that these devices will be of great utility for many future studies.

In October 2007, Weirathmueller and the Harbor Tracking Team (under the guidance of Weber) conducted a series of experiments to measure the environmental effects on the acoustic path across Portsmouth Harbor. Both a static and a roving acoustic sources were deployed for the duration of a tidal cycle each day for several consecutive days. Environmental data including CTD casts, a CTD chain measurement, tide levels and weather measurements were collected as well. The analysis of the data set will be the focus of Michelle Weirathmueller's Master's thesis work and will be used to better understand the limitations of the time-of-arrival and phase-tracking underwater acoustic tracking systems. This data should help provide a fundamental understanding of the limitations of each of these approaches.