detection of submarines

Jemiba

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In a scientific magazine I found an article, explaining that a fish/submarine or everything
else moving under water, is displacing its own weight as a kind of pillar of water, which is
moving to the surface and so changing the water level, relatively to its depth.
(see the sketch as explanation ). This phenomenon is said to be used for the detection of
submarines. I know, that it is of course possible to detect the wake caused by a periscope or
even by a submarine at shallow depth, but I think that’s something different. Here it would
mean to find the “hump” on the water surface. Is this used and if, how is it done over the
constantly changing sea ?
 

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AeroFranz

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Good question. Also, what happens if the submarine is at rest? i am pretty sure the water around and especially above it is redistributed so that the sea surface is flat...
 

Jemiba

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Of course, a stationary object under water won't cause disturbance of the
surface, but moving objects only. I think, there will be just very small changes
and I don't know, how they could be recognised, even in a calm sea, not to
mention higher sea states. I was astonished to read this in the mentioned article,
publshed in a reliable magazine, as a translation from an issue of American Scientist.
 

Michel Van

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strange, i can recalled that US Navy look in 1980s for methods to scan the Sea level with laser or radar.
from space so exactly accurate as possible !

Wat came out was this map made with NOAA sat
800px-Ocean_gravity_map.gif


can it be they tested this method for find under water Submarine ?
 

r16

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ı remember reading such reports that there was no hiding for submariners anymore .I had somewhat felt sorry but I would hazard a guess but just a guess that IR detection of heated water around the submarine is far more easier than lasing wavetops .
 
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oky totally thinking aloud here..

Is an analogue in the air possible, say with a satellite tracking system?
 

Jemiba

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"Is an analogue in the air possible, say with a satellite tracking system?"

If I've understould your question correctly, I would say no as air is compressible,
but water isn't. that's the stated reason for a body moving under water, to constantly
press a certain volume of water upwards, as water flows around this body, so it cannot
be displaced to the fron and to the rear and not the bottom, too, but only upwards.
A good example as an explantion is someone, who is crawling under the carpet. ;D
Unfortunately, giving a number for the amount the surface would be raised in relation
to the depth of the object, was out of the scope of this article ( Spektrum Der Wissenschaft
.6.08, "Laufen=Fliegen=Schwimmen")
 

robunos

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what about the 'seasat conspiracy', see here,

http://www.astronautix.com/craft/seasat.htm

i remember reading, but don't remember where, sorry, that the 'results that alarmed the pentagon', were that the satellite could detect submerged submarines with total ease, so it was squelched before the soviets found out and built their own version.
don't know how true this all is, but seems plausible.

cheers,
Robin.
 
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If I've understould your question correctly, I would say no as air is compressible,
but water isn't. that's the stated reason for a body moving under water, to constantly
press a certain volume of water upwards, as water flows around this body, so it cannot
be displaced to the fron and to the rear and not the bottom, too, but only upwards.
A good example as an explantion is someone, who is crawling under the carpet. Grin
Unfortunately, giving a number for the amount the surface would be raised in relation
to the depth of the object, was out of the scope of this article ( Spektrum Der Wissenschaft
.6.08, "Laufen=Fliegen=Schwimmen")


Yes but a plane flying through air does displace some of it in all directions (motion in a fluid after all). again thinking really aloud and off the top of my head, isn't it possible to pick up the anomaly in air flow over a particular area?
 

Jemiba

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"isn't it possible to pick up the anomaly in air flow over a particular area?"

If it is possible to measure the density of air from a (large) distance, this
could be possible, maybe ...
 

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Low frequency sound etc.

The nice thing with the sea is that water is incompressible and also that it has a nice relatively clearly and flatly defined upper surface where to do measurements on displacement.
 
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what if you spray your airspace with certain "stuff" ( crude way to put it , but you get the idea) , I guess density differentiation would be possible..
 

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Of course, before radar, sound detection was very important in detecting enemy aircraft and was used extensively in WW2:
http://histru.bournemouth.ac.uk/Oral_History/Talking_About_Technology/radar_research/sound_detection_aforerunner.html

Nowadays with aircraft flying higher and faster it's of less use. If a craft traveling at Mach 0.9 can be detected when the sound (or air pressure) arrives from say 50 km distance, then the actual craft is already within 5 km. And this doesn't take into account the altitude.

Sonar can theoretically be used to detect low flying aircraft in the sea (one concern about the use of F-111 for anti-shipping roles in Australia btw) since the speed of sound is much faster in the water.
 

Jemiba

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As I had absolutely no idea, what numbers we are talking about, I made a calculation for the following example :

A submarine of 5000 ts , with a length of 100m and beam of 10m is travelling in a depth of 50 m. The displaced mass of water will be lifted to the height of the submarine, but of course, due its viscosity will be distributed more and more in relation to depth. As an estimation, I set the cone along which the mass of water is distributed at 90°. Then, 5000 ts of water would be distributed over an area of 110m x 200m (to ease calculation, I used a rectangle ...) , this could mean a rise of the sealevel of about 20 cm, at least in the center of this area. A very rough calculation, of course, but I can imagine, that such change should be recognisable with proper methods.
 

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20 cm ! that's a lot ! how did you arrive at that figure? the ocean that this submarine is operating in isn't really a confined space , and you are not accounting for currents..
 

AeroFranz

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Jemiba said:
... this could mean a rise of the sealevel of about 20 cm, at least in the center of this area. A very rough calculation, of course, but I can imagine, that such change should be recognisable with proper methods.

Jemiba, I appreciate the effort everytime somebody takes the bull by the horns and does a reality check using numbers and physics, even if you have to make rough assumptions, so thanks for doing that.
This being said, i think the next obvious question would be how the radar (or whatever sensor you are using) distinguishes between the displacement wave of a submarine and a "regular" wave. Unless the sub is in a lake, there will always be some sort of wave motion. Maybe you have to look at the overall pattern and find the wave that is still 20 cm taller than the rest? ???
 

Jemiba

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I was surprised, too, and honestly I have severe doubts. My very inaccurate
calculation is based on the statement in the mentioned article, that displaced
water will be forced mainly upwards, as, due to the incompressiblity of water, this
is the direction of the lowest resisting power. Due to the viscosity the amount of
water will be distributed over a wider area and I set the widening angle at 90°, which
I thought to be quite large. One factor I completely ignored, is the speed of the
submarine, as the effect will surely increase with higher speed. My submarine probably
is dashing really fast ! :D
To make this clear: I won't take a ruler and try to measure a 20 cm higher sealevel.
But to me it seems, that this effect could produce a really measurable increase in local
sealevel in the range of some centimeters. If and how this is detectable, is another
question, sealevel is a statistically affair. What's the attainable precision and, probably
as important, the attainable resolution ? If a radar beam measures the value of sqaremile,
a local increase of some centimers probably won't be detected.
 

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