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Chelsea Technologies’ Acoustic Transducer Design Heritage

ERAS echo repeater acoustic source system: a low cost, compact and flexible sonar test system from Chelsea Technologies

A lesser-known aspect of Chelsea Technologies' maritime range and its acoustic design heritage is its proven capability in underwater acoustic transducer design and manufacturing. One such example is the echo repeater acoustic source system (ERAS), a low cost, compact and flexible sonar test system designed for hand-deployment from small vessels, containing both projector and hydrophone.

In fact, Chelsea Technologies has wide ranging experience of producing both sophisticated and simple echo repeater systems for the highly specialised defence market through its acoustic design heritage. This experience spans several decades from the first system “CUSTERD”, designed and built in 1995 and used for many years by the UK MOD on trials. More recently systems have been supplied to the USA with systems also deployed with non-NATO navies.

What is ERAS?

eras sonar calibration test and training system
ERAS sonar calibration test and training system

Chelsea Technologies’ Echo Repeater Acoustic Source (ERAS) is a self-contained sonar calibration, test and training system for use on small vessels. An included ruggedised laptop computer is used to provide all signal selection and control, via a bespoke graphical user interface that can be easily operated, without the need for specialist training.

“a self-contained sonar calibration, test and training system for use on small vessels”

The only other units are a deck unit containing the signal conversion and amplification and a hand-deployable transducer. The system is useful for testing active and passive sonar range, bearing accuracy and target identification. Two frequency bands are offered in the ERAS system:

  • Band 1 Low Frequency (LF) – 3 kHz to 23 kHz
  • Band 2 High Frequency (HF) – 23 kHz to 50 kHz

ERAS can transmit at up to 171 dB re 1 µPa@1m with a flat transmit response to within ±1.5 dB across these two frequencies bands. The LF source level can be increased to 180 dB re 1µPa@1m between 7 kHz and 19 kHz.

Dual Band Acoustic Source – part of Chelsea Technologies’ acoustic design heritage

Dual Band ERAS Transducer in Transit case
Dual band ERAS transducer in transit case

As a calibrated acoustic transmitter (source mode) ERAS can generate complex test signals. This mode is aimed at testing passive sonar. The test signals can also be used to simulate some active sonar pings and approximate acoustic signatures of known targets by combining noise and tones. An external input is also provided for replaying pre-recorded acoustic signatures through the system. In summary:

  • Up to 24 tones and 5 bands of band limited noise (white, pink or PRN)
    • Each with a selectable source level and frequency
    • Each tone selectable as pulsed or CW
  • AM or FM modulated Signals, (24 Tones and 5 bands of band limited
    noise)
  • Sweeps, LPM or LFM
  • Pulse trains up to 1000 pulses with stepped source level
  • Broadband noise (white, pink or PRN)

Dual Band Echo Repeater – part of Chelsea Technologies’ acoustic design heritage

The Echo Repeater Acoustic Source - Transducer Deck Cable
The echo repeater acoustic source – transducer deck cable

In “Echo Repeat” mode, ERAS can repeat signals received from a hydrophone contained in the hand-deployable, transducer head. It does this in response to a trigger event, based either on the incoming acoustic signal, an external input, or under operator control. The repeated signal can have a highlight structure imposed upon it to mimic the reflections from a target. A programmed sequence of echo repeated signals can be configured to simulate a moving target by appropriate addition of Doppler frequency shift and monotonically changing time delay. A transpond pulse can also be tagged onto the signal to indicate that the sonar ping was detected and repeated. This function is used to calibrate active sonar systems by providing a controlled acoustic target.

In summary:

  • Reproduce an incoming signal with variable range delay and target
    strength
  • Software defined bandpass filtering
  • Add up to 5 target highlights
  • Doppler shift and moving target simulation
  • Tag-on a transpond pulse.

Dual Band Transponder

The Echo Repeater Acoustic Source – Deck-Unit in a 19Inch flight case
The echo repeater acoustic source – deck unit in a 19 inch flight case

Transpond mode allows the system to produce any signal available in source mode in response to a trigger event, based either on the incoming acoustic signal, an external input, or under operator control. A transponder is a device that can transmit and receive signals but when it detects an interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transponder transmits an interrogation signal and measures the time between this transmission and the receipt of the other transponder’s reply.

The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two transponders. Transponders can be used as an active ‘pinger’ to test the range of the sonar system. This mode can also be used to test collision avoidance systems, search and location beacon testing. In summary:

  • Software defined bandpass filtering
  • Can produce any of the acoustic source types detailed above in reply to an acoustic trigger

A log file system is provided which stores all selections made. Also provided is the ability to save and recall complex settings to enable trials scenarios to be prepared in advance.
An integrated GPS receiver in the deck unit and a pressure sensor in the transducer allow the location and depth of the transducer to be determined.

“A transponder is a device that can transmit and receive signals but when it detects an interrogation signal it responds by transmitting a specific reply signal”

The ERAS underwater transducer can be used for reception and recording signals in the 3 kHz to 50 kHz range. In ‘Sonar receiver mode’ the user will be able to record from the hydrophone to a file on the laptop computer for later analysis. Recording is at 192 kSPs at 16 bits. The standard Deck-Unit has outputs for monitoring transmitted and received
signals.

“A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another”

Echo Repeater Acoustic Source – Deck-Unit Front Panel
Echo repeater acoustic source – deck-unit front panel

Long-Term Support From Chelsea Technologies

Chelsea Technologies was formed as a multi-aspect, underwater sensor supply company with a proven capability in underwater acoustic transducer design and manufacture, optical/magnetic/electric and biological sensors, electronics and software, signal processing and the packaging of sensors and electronics, including battery power packs, into small volume, underwater pressure housings. Chelsea Technologies was acquired by Sonardyne International Ltd. (Sonardyne) in October 2018.

Chelsea Technologies manufactures a wide range of products for the underwater marketplace and an important segment of ongoing business is the repair, maintenance, calibration and support of our sensor-based products. Chelsea Technologies therefore spans all of the required capability segments to supply ERAS systems and with the backing of Sonardyne as a parent company is well positioned to support such systems on a long-term basis. Further background information on Sonardyne can be found at www.sonardyne.com.