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Feature Article

Towed, Moored and AUV Echo Repeaters Simulate Targets

By Alain Maguer,

Vittorio Grandi
Gerardo Parisi

The MERAS buoy during deployment. Hanging below the buoy frame is the lithium battery pack. On top of the mast are the GPS, FreeWave and wireless local area network antennas.
The NATO Centre for Maritime Research and Experimentation (CMRE) in Italy, known as NATO Undersea Research Centre (NURC) prior to July 2012, has developed its third-generation, towed echo repeater in accordance with new requirements from the center's anti-submarine warfare (ASW) program. CMRE regularly conducts at-sea experiments for ASW to develop and evaluate new concepts of operation in monostatics and multistatics. A key component of this work has been the echo repeater, which serves as an artificial target that returns a synthetic echo by receiving a signal and retransmitting it. This provides highly calibrated measurements that can be extrapolated to a real ASW scenario and allows for assessment of new operational concepts.

In 2009, NURC decided to design a new prototype, based mainly on commercial-off-the-shelf components and designed to be towed from small ships. The Towed Echo Repeater (TERAS) balances a flexible software structure, which allows scientists and users to customize system features, with the hardware implementation of modules to achieve accurate and deterministic behavior of core functionalities (e.g., response delay and time tagging). The system was fully tested and operated at sea from 2010 to 2011. Furthermore, in 2011, NURC started developing a moored echo repeater, as well as a version to be integrated into its 21-inch AUV.

Towed Echo Repeater
TERAS was designed to be towed by small ships with limited deck capability. It has its own slip-ring-equipped winch for flexibility, but the electromechanical cable can be spooled on any suitable winch with or without a slip ring. Dry-end electronics are powered by 110- and 240-volt mains and rack mounted for compactness and robustness. In this configuration, the echo repeater provides its maximum source level exceeding 180 decibels, referred to 1 micropascal at 1 meter.

The system's hardware consists of wet- and dry-end main units. The wet end comprises an electromechanical tow cable and tow body equipped with a broadband high-sensitivity Neptune Sonar Ltd. (Kelk, England) D/70 receiving hydrophone, an ITC (Santa Barbara, California) ITC-2010 transmitting transducer and a Kulite Semiconductor Products Inc. (Leonia, New Jersey) ETM-375 depth sensor. The dry end comprises a 19-inch rack with a signal-conditioning analog front end, industrial-grade computer running CMRE software under a Windows XP operating system, a tow-cable winch with slip-ring and a deck cable with extension.

The CMRE-designed operating software has a graphical user interface to configure and run the system, continuously monitoring the received signal. On a trigger event, it records the contents of the receive window, processes the received signal (i.e., adding gain) and retransmits it after a programmed delay. Some setup parameters are written in a configuration file (e.g., hydrophone sensitivity, amplifier gain) and are not modifiable from the front panel. All other parameters are adaptable to the specific requirements.

Once the configuration is set, the system can be started by pressing the start button on the system operation panel. The acquired signal is visualized in real time on a graph, together with all other related data, such as time, latitude, longitude and depth. The operator can decide to store or repeat the received signal or change configuration parameters to optimize performance.

TERAS was employed during the GLINT11 (Generic Littoral Interoperative Network Technology 2011) ASW experiment in the Gulf of Taranto in September 2011 and ExPOMA12 (Exercise Proud Manta 2012) multistatic ASW experiment in the Sicily Strait in February 2012, demonstrating reliable, calibrated target simulation capability from both the CRV Leonardo and NRV Alliance. GLINT11 was fourth in an annual series of experiments demonstrating real-time offboard autonomous AUV-based multistatic active sonar network technology. ExPOMA12 was the first opportunity to test the GLINT demonstrator as part of a NATO exercise.

Moored Echo Repeater
To replace outdated technologies and try to better understand the effect of seabed and water-column parameters on the performance and effectiveness of an underwater system, NURC started modeling activities and at-sea campaigns in 2011 to quantify the differences between the models and the actual measurements, while including techniques to reduce uncertainty in the input data.

The technology was to be moored on the bottom to limit signal fluctuations due to the echo repeater's motion. This new system also aimed to improve logistics and lower the cost of at-sea trials by eliminating a support ship for towing the echo repeater.

The Moored Echo Repeater (MERAS), now under final development and testing, comprises a moored surface buoy and a bottom-moored transducer assembly, which includes an Ultra Electronics Maritime Systems Inc. (Dartmouth, Canada) MPS2-100 Projector, Neptune D/70 hydrophone and Kulite ETM-375 pressure gauge connected together by electromechanical cable in a classical U-shaped mooring configuration. MERAS is particularly suited to situations wherein using a vessel is precluded by costs or weather conditions. To continue this article please click here.

Alain Maguer received a Ph.D. in acoustics and signal processing in 1986. He has worked previously for Thales' underwater systems and has been the engineering department head since 2007 of the NATO Centre for Maritime Research and Experimentation. His research interests are sonars and autonomous vehicles.

Vittorio Grandi is head of the electronics and data acquisition branch at the NATO Centre for Maritime Research and Experimentation. He has a master's degree in electronics engineering and more than 20 years of experience designing underwater acoustic and oceanographic equipment. He led the development of the CMRE family of echo repeaters and acoustic sources.

Gerardo Parisi joined the NATO Centre for Maritime Research and Experimentation in 2007 after more than 20 years of experience developing telemetry and data acquisition systems for military applications. His focus is now on underwater acoustic signal acquisition and generation. He designed the software for the CMRE family of echo repeaters and acoustic sources.

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