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Friday, April 11, 2014

The Malasian Airliner

I'm puzzled about the search for the Malaysian Airliner.  Once they had detected even a single ping on the towed array, I would have thought that they would have pretty well pinned down the location of the source of the pings.

Before I explain what I mean, I must admit that my "knowledge" of how these things work is based on novels by authors like Tom Clancey.  However, Tom was said to be a bit of an enthusiast on modern weapons and did his homework.  The basic premises were confirmed in other novels.

As I understand it, when an atomic submarine, either the missle carriers or the attack boats, are on patrol, they stream a long line behind them which is festooned with acoustic sensors (microphones).  I'll explain how they work with simple geometry but of course, the signals detected by these sensors are fed into a computer which gives a virtually instant firing solution.  And as far as I understand, a single ping from an enemy submarine or a noise such as propeller cavitation,will determine its position.

Lets look at the simplest situation in which the target is at right angles to the towed array and right opposite the middle of the array.  The pings arrive at the front microphone and the back microphone at the same time.  This tells you that the target is located somewhere on a disk that intersects the towed array in the middle and at right angles to the array.  In the case of a black box which is pinging, you know it is on the bottom of the ocean so the black box is on a line where the disk intersects the bottom.

However, you don't only have microphones at the front and back of the array.  Let's look at the microphone that is in the middle of the array.  It is closer to the target than either of the end microphones.  It therefore receives the ping before either of the end microphones.  The speed of sound in water is well known.  As the wave front of the ping moves outward, the circle gets larger and larger such that for an infinitely distant target, the wave front is essentially flat.  The difference in time becomes shorter and shorter the further away the source of the ping is located.  From the difference, with modern electronics you get an estimate of range.  In other words, our disk becomes the rim of a bicycle wheel and becomes a solution of the two points where the rim meets the bottom of the ocean.

Now it seems unlikely that all the microphones on the array are omni-directional.  I would assume that there are some directional microphones and the difference in the amplitude of the sound recorded by these various  directional microphones gives you a direction.  In other words, if the "bicycle wheel rim"  intersects the sea bed in two locations, this sorts out which of these is the target.

An added dimension is added when the black box pings for an extended time and the boat is towing an array of microphones.  You are getting a view from a whole bunch of different angles all pointing to a given location. 

The geometry is only slightly more complicated when the ping is in front or behind the array.  In one sense, it is simpler.  If, say, the ping is behind the boat and off to the side, the front and back microphone receive the ping at different times and this indicates in what direction the pinger is located.

Then you have the "passing train" effect.  The pitch changes if the train is coming toward you or has passed and is going away from you.  In this case, the listener is on the train and listening to a stationary sound.  Same effect.

A modern submarine is dependent on knowing exactly where an enemy is located and quickly before they can get off a torpedo.  I can't understand why the location of a stationary source of repeditive pinging is not pinned down almost instantly.   What is all this steaming back and forth over the location to narrow down its position.  They certainly can't be towing an array such as the subs use.

For that matter, why was one of the American or Russian atomic subs not diverted to the location.  Surly they would have pinned down the location within minutes of arriving on location.  Perhaps they have been.  For that matter, they could be pinging themselves and mapping the bottom.  Surly the echo from a bunch of scrap Aluminium would be different from the echo from a soft muddy bottom.

I don't get it.

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