ASW (anti submarine warfare)
is the discipline where, a surface ship, want to find enemy submarines
at as great a distance as possible, and preferably before the submarine
can fire torpedoes at your own ships. If the submarine's torpedo has an
effective range of e.g. 20 nmil, then you must be able to detect the
submarine at a greater distance. This can only be done if you use
Active Towed Array Sonar ) . An
effective active sonar is lowered into the water, where it emits some
very powerful sound pulses, which are dispersed in the water. If the
sound wave meets a submarine, some of these waves are reflected back. It
is in exactly the same way that a bat catches its prey. In order to be
able to receive the weak sound waves that are reflected from the
submarine, a long series of hydrophones are used which are lowered into
the water, preferably 400 to 1000 meters after the ship, to reduce the
propeller noise that the ship itself generates. (TAS) Towed Array Sonar.
A TAS typically consists of 60 to 120 hydrophones, which hang like beads
on a string. By calculate all the signals from all hydrophones, however,
it is easier to distinguish between the reflected signal from the
submarine and the background noise. (it's again the same way the bat
does, it only has two ears)
Of course, the submarine
designers try to make the submarine as invisible as possible. The entire
Submarine is therefore typically coated with a thick layer of
Polyurethane, to absorb the sonar sound instead of reflecting the sound.
How good the submarine is at absorbing the sound / reflecting is called
(TS = target strength) The lower the TS, the better the submarine is to
sneak past unhindered, in the same way as a "stealth plane" is very
difficult to detect with radar.
In the F341 model:
The model F341 has a Hull mounted sonar and a
TAS Towed Array Sonar system.
Design of the hull mounted
The Sonar is a 344kHz
piezoelectric (PZT) disc element with a diameter of 27mm (equal to 2m if
||The Hull mounted Sonar is placed below the keel, for optimal
visibility in all directions.
||The sonar is designed with an acoustic
front “lens” and a backing material made of a combination of
tungsten powder and epoxy. The backing layer closed to the PZT
is heavy density, to match the PZT acoustic impedance, and by
gradually reducing the amount of tungsten power, changing the
density. The result is a well damped transducer with a good
transient response. (It may sound easy to make at home on the
desk, it is not and it sucks, and a lot of epoxy and power is
wasted because I only produced a single piece)
Sonar can be powered by a 50W amplifier and controlled by the
1GHz raspberry pi. The main processor.
The TAS Towed
Array Sonar and ATAS Active Towed Array Sonar
The TAS is design to
reach a depth of a ship length (200cm) at a speed of 2 knots.
A real life TAS: A
hydrophone array is made up of a large number of hydrophones
placed in known locations and towed in a horizontal line behind
a boat or ship. Sound arriving at the array from a distant
source, such as a submarine, will reach each hydrophone at
slightly different times, depending on the direction from which
the sound is coming. Using this information from all the
hydrophones in the array, the direction from which the sound is
coming can be pinpointed. If the background noise is
uncorrelated, then noise can be reduced by the square root of
the number of hydrophones. An array with 100 hydrophones can
therefore listen / find noise sources that are (√100) 10 times
below the individual hydrophone noise floor.
In the model, TAS is implemented with a
sinking class 10 fly-line. If you use a "sink tip" line, it
places itself flat in the water at a nice depth a few meters
behind the ship, it is also so rigid that it can be wound in and
out completely automatically without any problems.
The TAS winch 3D drawings
The TAS winch is
designed with a modified servo and a multi-turn
potentiometer. The modified servo is mounted with a
wheel with teeth to be able to count how many meters the
array is pulled out. A 2nd motor ensure that
the winch coil rolls the array back onto the coil. This
motor is only active during retrieve.
The picture is showing the actual implementation,
everything is controlled via a single PWM signal.
At the bottom of the pictures you can see the shock
absorber which holds the whole "winch" in place, as well
as the motor which is used to pull the TAS in
||The ATAS is designed
with a 100mm linear servo motor that can open the rear
gate, and at the same time move the ATAS out into the
outer position. ATAS itself is designed as a two ring
transmitter with a resonant tube in the middle.
It does not work but if I had to design it, I would
probably design it this way :)
The transmitter can be designed in many ways, but it
is important to be able to send as much sound out into
the horizontal plane in the water as possible. Sound
sent towards the surface or towards the bottom must be
Towed Array Sonar in action (link to video)
|The model is prepared for defense against torpedoes
with a "hard kill" ATT
Linkl to the real ATT "Sea Spider" by Atlas in Germany