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SONAR (Asdic)

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Janet H
1358076.  Wed Sep 09, 2020 4:34 pm Reply with quote


"The ASDIC, known to the Americans as Sonar, was basically a transmitter-receiver sending out a highly directional sound wave through the water. If the sound wave struck a submerged object it was reflected back and picked up by the receiver. The length of the time from transmission until the echo was received was used to measure the range, which was shown as a flickering light on the range scale. By mounting the transmitter head so that it could be directed almost like a searchlight, the bearing of the target could be read from the compass receiver."

1358081.  Wed Sep 09, 2020 6:20 pm Reply with quote

Janet H wrote:

"The ASDIC, known to the Americans as Sonar, was basically a transmitter-receiver sending out a highly directional sound wave through the water. If the sound wave struck a submerged object it was reflected back and picked up by the receiver.


Active Sonar actually uses omni-directional transmitters and highly directional *receivers* due to the near impossibility of sending out narrow-beam acoustic signals at the sort of frequencies they could use then*. The "ping" is sent out and then the directional receiver tells you which direction any returned echo comes from. In the original ASDIC sets a single receiving element would be mechanically moved for each ping until the operator heard an echo - this is the scene you see in old movies where a matlot moves a wheel like a truck steering wheel on the desk to scan specific directions and home-in on the target. This made it very slow and comparatively easy to evade.

Since the late 1940s the concept changed to using an array of multiple receiving elements pointing in different directions (usually wrapping around the full 360 degrees) to detect the echo from a single ping no matter which direction it came from. The amplifiers then fed electronics which compared the signals received from each one and indicated a direction and time to the operator. This concept was MUCH quicker and much harder to evade.

Actual sonar transducers (the bits that make the noise and hear the echo) tend to be "reciprocal transducers" in that the same mechanical device is driven with volts to make the noise and then connected to a very sensitive amplifier to detect the echo. The most common configuration for a general search sonar is called the "Barrel Stave Array" because groups of transducers are arranged in blocks like the staves of a barrel:

Each one of those square things is the front of a single transducer (only the covers are square - the transducers underneath are circular). They are all driven together to "ping" and then the electronics scan the staves individually to detect the echos. There are other configurations for specialist uses - planar arrays, bi-static arrays, parametric arrays, flextensional arrays and (for the military) towed tubular arrays, but the barrel-stave array is still the most common for general search sonars.

That's Active Sonar, which is almost never used because the target can always hear your ping from further away than you can hear the echo (by definition). Most military sonar use is using *passive* sonar. Passive sonar uses most of the same equipment, but it doesn't "ping" - it just listens very hard for the noise made by ships, submarines and whales and uses all the kit to work out which direction it's coming from. This is completely undetectable, but has the limitation that while it gives a direction it can't tell the range. Using it is a matter of moving between different positions and taking multiple readings. It takes a skilled captain and a well-tuned crew to do it well.

So that's sonar - all really simple and effective. Or at least it would be were it not for the minor detail that sound refuses to travel through water in straight lines for any significant distance. It bends with current, temperature, salinity, phase of the moon and ambient grandmother's age coefficient. It also gets refracted by pressure, which means that the ping progressively pitches down and heads for the sea-bed producing what are called "convergence zones" between which they can't detect anything at all. That's why even today sonar operator is a skilled artist playing the sonar like a musical instrument as much as technician.

I'm not going to give specific sources for any of this, although there's plenty out there if you want to look. My first job out of Uni was as a Sonar Trials Engineer, and then as an engineer in the investigation of the poor performance of the RN 2016 sonar during the Falklands war, and a lead engineer in the subsequent programme of modifications to address the issues found. That was 35 years ago, but it stays with you!


* Directional high frequency beams are now possible using parametric arrays with beam-formers, but the high frequencies used make them short-range and specialist use only


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