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Feature Article

Ocean Glider Payloads for Maritime Surveillance and Monitoring

By Dr. Alain Maguer

Spectrogram of raw data segment with cetacean activity automatic detection (green peaks in small figure).

Gliders are autonomous vehicles propelling themselves through the ocean for a long period of time, either by changing their buoyancy or by taking advantage of the waves from the ocean. The gliders offer capability for persistent surveillance and monitoring of the underwater environment. Gliders have been demonstrated to be a cost-efficient solution for the measurement of seawater properties, such as temperature, salinity and optical properties. To date, their potential as a discreet and cost-effective means to perform other missions is still under investigation, requiring much larger energy capability than the most common oceanographic glider applications, which are based on low-power sensors such as CTD and optic sensors.

This article describes the effort made at NATO's Centre for Maritime Research and Experimentation (CMRE) survey of seafloor to increase present capabilities of underwater/surface glider technology in order to address potential applications, including acoustic passive monitoring, survey of seafloor properties and communication across the sea-air interface between unmanned and/or manned systems, as well as environmental monitoring such as sea-state wave measurements.

Acoustic Payloads
Different acoustic monitoring systems were integrated into CMRE gliders since 2007 in order to address several fields of application, such as passive monitoring of the underwater environment, marine mammals risk mitigation purposes and seabed properties characterization. For the first purpose, the European community has a willingness to ensure that any human action that introduces energy, including underwater noise, is at levels that do not adversely affect the marine environment and therefore is looking for technical means to monitor noise.

A single hydrophone payload, a towed array payload and a volumetric array were built. They all included the development of a custom-made, low-power consumption acquisition system capable of acquiring, in real time, up to 32 channels. The low-power consumption is critical for ensuring the persistent aspect of the missions.

The single hydrophone payload integration into the Teledyne Webb (East Falmouth, Massachusetts) glider was performed in 2010 and was challenging due to the small amount of space available within the glider. The hydrophone was mounted on the stern of the glider, and a 10-day endurance was achieved within the glider while acquiring and processing data up to a 192-kilohertz sampling frequency.

The analysis of the at-sea data has shown very high quality acoustic data when the glider is in the water column. However, high noise was measured when the glider was on the surface due to splashing water or the hydrophone bouncing. Noise peaks were also measured during engine operation. The capability of the system to detect/classify marine mammals in real time within a glider was successfully demonstrated at sea.

The second acoustic payload is a towed array payload that was developed to assess new concepts of operation in anti-submarine warfare (ASW). The new concepts were based on the use of AUVs equipped with towed arrays. For this purpose, CMRE began in August 2007 to design and build a thin-diameter (31 millimeter), high-frequency nested towed array. Currently, the ASW program at CMRE is simultaneously using two AUVs equipped with towed arrays to perform multistatic operations. A demonstrated capability was observed in some specific operational scenarios.

Over the years, CMRE has undertaken several modifications to the original towed array design in order to enhance its performance by reducing its size and power consumption for use of smaller autonomous vehicles and/or gliders. In 2012, CMRE developed a thinner array diameter (18 millimeters), which was created to reduce the drag of the array to the vehicle and therefore able to be used by smaller AUVs and gliders currently on the market.

The purpose of this last acoustic payload was to transition a methodology of using underwater ambient noise (sea surface generated noise and distant shipping) for seabed characterization to an autonomous vehicle equipped with a tetrahedral and vertical array. This payload was also designed for performing small surface boats localization. The vehicle in subject was the newly developed hybrid glider/AUV named eFOLAGA, which has the advantage over ordinary gliders of more flexible maneuverability features, i.e., the vehicle can operate in either glider or ordinary propulsion mode.

The volumetric array consists of eight elements. Five are in a vertical configuration, and three offset from the center element to form a tetrahedron with the center element. The spacing between the vertical elements, as well as length of each edge of the tetrahedron, is 10 centimeters.

The GLASS experiment was conducted in July 2012 off the coast of northern Italy near La Spezia. Most of the acoustic data analyzed from the sea trial was acquired while the eFOLAGA was mounted on the bottom-moored frame. All electronic devices for navigation purposes were turned off, and only the data acquisition system was active. Some attempts were made to keep the vehicle at fixed depths by manually trimming the buoyancy and to perform a linear track at constant depth using the propeller for forward motion. However, the behavior of the vehicle was not as expected, and only the bottom-moored missions were considered.

A small rubber boat with an outboard engine outfitted with a portable GPS data recorder maneuvered in the vicinity of the array. It was detected through advanced processing of the volumetric array demonstrating that compact volumetric arrays are effective tools for the detection of small surface craft. To continue this article please click here.

Alain Maguer has a Ph.D. in acoustics and signal processing. He has worked for Thomson Marconi sonar in France and Thales Underwater Systems. In July 2007, he joined the NATO Undersea Research Centre in Italy, where he is currently the engineering division head. His research interests are sonar, autonomous vehicles, gliders and towed arrays.

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