SONAR versus RADAR
SONAR is the underwater equivalent of radar, and like radar its functions are many and varied. These include the detection, identification, location and speed indication of 'targets of interest'.
There are basically two sonar modes of operation, 'Passive' (receiving or listening) and 'Active' (transmitting and receiving).
Radar operates on the principle of transmitting and/or receiving electromagnetic waves, travelling at approximately the speed of light (3x108 m/sec) through space or the atmosphere.
SONAR on the other hand, relies on sound or acoustic energy in seawater travelling at approximately 1,500m/sec, depending upon conditions such as temperature, depth, and salinity.
Why Sound Energy?
In the dull, dense and conductive environment of seawater, electromagnetic waves and light rays are severely attenuated. Acoustic energy however has proven to be more tolerant and controllable in this medium, but the sound is still influenced in many ways during its passage through the sea, consequently substantial acoustic energy is essential for many sonar applications.
The operational frequency of a specific sonar system is chosen to accommodate the required mode of application; this is critical to the system performance, and its receiving/transmitting transducers.
For example low frequency (say 1 kHz) systems may be used for long range 'listening' and detection, whereas higher frequency (>10 kHz) systems could be employed to resolve 'target signatures', and to pinpoint the target location.
The accepted formula for calculating wavelength (l) is -
l= c/f meters
Where c = 1,500m/sec (approx. speed of sound in seawater),
and f = the operational frequency, e.g.1kHz (1,000Hz),
The Acoustic Systems Trainer (AST) for SONAR
The study of sonar principles and techniques are equally as demanding as the study of radar principles and techniques.
With the appropriate scaling of the system operational parameters, such as an increase in frequency of operation (hence a shorter wavelength), and the subsequent reduction in the dimensions of the acoustic tank, lower power requirements in the active sonar mode, and a P.C. interface with software to perform the sonar signal analysis, a real-time bench top Acoustic Systems Trainer for SONAR has been designed and manufactured for underwater studies.
The system is comprised of a control Console, containing the CW and Pulse gating electronics, and the hardware/software-processing (Sonar Signal Analyzer - SSA) interface with the computer.
AST(Mk V) Console and Acoustic Tank with Target Transport System
The transparent Acoustic Tank is constructed from acrylic material, measuring 1.2 meters in length, 600mm deep, and 600mm wide, with a capacity for 400 litres of water. The built-in heater can be used to vary the water temperature which is monitored by a submersible probe thermometer.
A variety of active targets are mounted on a reversible, variable speed Target Transport System, which enables the Doppler principle to be demonstrated.
The passive targets consist of a set of scale model, brass propellers, with 2, 3, 4, and 5 blades, mounted on a variable speed motor unit with integral gearbox.
Acoustic damping material can be added to the tank for 'mill pond' quiet experiments to be conducted with the sensitive hydrophone, and spectrogram software.
Transient Recorder display
Both narrow beamwidth and wide beamwidth high frequency transducers can be used; allowing beam interaction to be clearly shown on the P.C. based virtual instruments.
Transducer Polar Diagram
Beam steering circuitry controls the phase drive to the transducers.
Digital Oscilloscope display of Active/Passive SONAR
Active and passive targets are studied, and the SSA - Spectrum Analyzer, displays the signatures of a selection of passive targets.
Digital Spectrum Analyzer
Sonar Techniques and Principles
The foundation of the sonar principle is based upon the physics of sound waves, and all of the influences affecting the sound waves as they travel through the medium of water.
Due to the wide range of variable electronic parameters, such as PRF, Pulse Width, Tx Frequency and Variable Phase outputs, theAcoustic Systems Trainer (AST) is ideally suited to a development environment where the 'what if....?' scenario, can be explored in complete safety.
A university MSc degree dissertation has already been written using the AST(Mk III) as a test bed at the University of Plymouth.
Some of the techniques and principles which can be demonstrated (and developed further) with the Acoustic Systems Trainer, include -
- Speed of sound in water
- Temperature effects on the speed of sound.
- The single element Transducer
- Transducer resonant frequency
- Two-element array
- Transducer beamwidth
- Passive SONAR
- Active SONAR
- CT-FM SONAR
- Parametric SONAR
- Depth finding
- The Doppler effect
- Underwater communications
- Active target profiles
- Passive target FFT signatures
- Shallow water backscatter
- Beam formation
- Beam steering
- Volume scattering & cavitation
- Dielectric phase shift
- Acoustic (ASW) Warfare (E.W. in radar)
The AST is currently installed in a number of universities, and naval training establishments in the following countries-
- United Kingdom
- United States of America
- Kingdom of Saudi Arabia
The largest installation of seven Acoustic Systems Trainers(Mk V) provides for fourteen students working in pairs.
Typical AST-SONAR laboratory
The AST has a comprehensive laboratory manual, data acquisition software, and a complete set of accessories, including-
- Active/Passive Targets
- Target Transport System
- Temperature Monitoring
- Acoustic Damping Material
- BNC Cables & 'T' Pieces
- SONAR Course on CD
Frequently Asked Questions!
FAQs and Answers about the Acoustic Systems Trainer (AST) for SONAR can be found at -www.itp101.com