Sonar Basics

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	Sonar Basics
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What is it ?

SONAR stands for SOund Navigation And Ranging - a navigation (not used in 688) and targeting system based on sound. It comes in two flavors: active and passive.
The passive sonar has been used by subs for a long time. In W.W.II they had a simple hydrophone (microphone) that could be turned in different directions and a guy at the other end who listened to the sound. Today the passive sonar system of a sub consists of more than one hydrophone and they are usually fixed. The bearing of a sound source is determined by electronic and computers.

The active sonar is an add-on to the passive sonar. It's more or less a device that's capable of sending out loud noises (that famous PING) and then use the passive equipment to listen for reflected signals. It, too, saw combat action in W.W.II on surface ships to detect subs (it was called ASDIC at that time).

For more information on sonar as well as some better drawings take a look a the sonar sections in the manual.

How Sonar determines the bearing of a sound

Well it works pretty much like your ears. It consists of (at least) two hydrophones at a certain distance. The time lag between the moment hydrophone A receives the sound and the moment B does allows to calculate the bearing. Let's look at this.. ehm.. image:

At first my apologies for this poor drawing (if you can do better please contact me) but I think it gives you an idea how it works.
Obviously the sound from the red source has to travel the same distance to A as to B. (The colored lines represent the sound waves - actually those won't be straight lines but it's easier to draw this way)
The sound originating from the blue source reaches B first and then with some delay A. Finally the 'black' sound hits B first and with an even bigger delay A.
If you know the speed of sound in water and the delay you can calculate the bearing quite easily.

In fact this simple example shows you a problem that occurs if you put all you hydrophones in a row ( towed array). Take a look at the pink sound source. It's sound will reach A and B at the same time just like the sound of the red source.
Now if I sit in the sub and I hear a sound that has no delay between A and B I know that it is coming either from the North or from the South. Because one cannot decide where the signal comes from the display in 688 plays safe and shows a track at the two possible bearings.
Every direction has it's mirror (blue - cyan) except for the position directly in front or behind the array (black). The mirror's axis is the direction of the array.

On to the Towed Array section.