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View Full Version : How does Sonar determine depth and distance?

The_Silent_O
04-18-2006, 07:56 AM
Actually I think I've got this figured out already, but for discussion purposes.

The destroyers always seem to know what depth and how far out in front of them you are. That means that they require the two dimensional factors of depth and distance.

So they ping you and get a rebound ping, that gives you a straight line distance to the object. (the Hypotenuse).

So how did they determine the (down) angle? where the receivers that sensitive?

scaseman
04-18-2006, 08:27 AM
Remember you also have the the doppler effect to take into account. That would have given more information.

Kaleun1961
04-18-2006, 09:59 AM
Calculating the depth of a submerged U-boat was an art, developed by experience. The sonars/asdics in use throughout most of the war only provided a bearing and range estimate. Clever escort commanders knew how to estimate the U-boat's depth by how far away they were when they lose contact on the run in to target.

The sound beam is shaped like a cone, or a flashlight beam in the dark. As it projected out from the pinging vessel, it reached forward and downward. Being cone shaped, there was an area above and below the cone that was a blind spot. This was why sonar was useless against surfaced U-boats. As the escort closed in on a submerged contact, the deeper a U-boat was, the sooner it fell below the sound cone. By calculating how far away they were when they lost contact, they could estimate the depth of the U-boat. If a U-boat was shallow, it was held in the sound cone longer. A very deep U-boat was lost further out from contact range.

One way around this limitation was to have one escort pinging and holding the sonar lock while one or more other escorts ran in, guided by the pinging vessel when to drop their depth charges. I have a book at home which illustrates this. I'll have to scan in the pictures which clearly illustrate how this worked.

To model this for yourself, take a flashlight and shine it downward at an angle and walk forward in a darkened room. As you move forward, you will see how the light shines forward in a cone shaped pattern. As you move forward, keep an object on the floor in sight. At a certain point, it will drop from view below the cone of light. This is how U-boats were able to break sonar lock by going very deep.

The_Silent_O
04-18-2006, 12:26 PM
Great explanation and analogy. I didn't think the sonars had an angle measurement device and you confirmed it.

Pretty much rule of thumb then for knowing when to drop charges and what depth to set them for.

BueJack
04-18-2006, 01:08 PM
K-61 says wrt sub hunting in WW2..

An interesting subject on this is Acoustics. Look it up on Google or Wikipedia. The basic idea of acoustics (or energy waves) in a medium (this case sea-water) is what is called the 'Refraction Index'. This has the effect of bending the energy wave. The amount of 'bending' is directly proportional to the density of the medium, ie: depth of water. So in effect the sound wave is eventually bent parallel (and upwards) to the surface of the water. If you're below this depth you're safe.

This is why Thermal layers (a band of preferably, cold water) are so effective in hiding a sub from the escort. If the sub was below the Thermal layer, the acoustic wave will be bent so much, held within the layer or reflected upwards. Thus the sub a few metres below remained undetected.

Closer to the surface, the acoustic wave has the effect of bouncing (inverted) against the surface of the water. Similar to how a tornado destroys each house either side of you, but yours remains intact. This has the effect of unreliably picking up a contact then losing it - driving the sonar operator around the 'bend'.

The only thing is that I don't think this is modelled in SH3 http://forums.ubi.com/images/smilies/bigtears.gif

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WilhelmSchulz.
04-18-2006, 04:13 PM
The amount of 'bending' is directly proportional to the density of the medium, ie: depth of water. So in effect the sound wave is eventually bent parallel (and upwards) to the surface of the water. If you're below this depth you're safe.

IE Covergence Zones. A subermeged sub can hear a ship/sub miles away because of this "bending" effect. The sound wave bounces down from the surface and then bends back up. This can be repeded several times.

This is why Thermal layers (a band of preferably, cold water) are so effective in hiding a sub from the escort. If the sub was below the Thermal layer, the acoustic wave will be bent so much, held within the layer or reflected upwards.

In my beliefs thermal layers is more of a insulator/wall. For example if your loungeing in a pool with your face/ears underwater you cant realy hear a person untill there a foot or two away, evean then they have to speak up to be heard. It works the same way with sub noises.

But this thermal layer can be a good thing(good for a DD bad for us). Say theres a sub above Kaleuns "sound cone". A escort could bounce it's ping off the thermal layer and ping the offending sub.

BueJack
04-18-2006, 06:06 PM
Originally posted by WilhelmSchulz.:
The amount of 'bending' is directly proportional to the density of the medium, ie: depth of water. So in effect the sound wave is eventually bent parallel (and upwards) to the surface of the water. If you're below this depth you're safe.

IE Covergence Zones. A subermeged sub can hear a ship/sub miles away because of this "bending" effect. The sound wave bounces down from the surface and then bends back up. This can be repeded several times.

This probably has a minor effect in contrast to the total noise made by a surface ship.

This is why Thermal layers (a band of preferably, cold water) are so effective in hiding a sub from the escort. If the sub was below the Thermal layer, the acoustic wave will be bent so much, held within the layer or reflected upwards.

In my beliefs thermal layers is more of a insulator/wall. For example if your loungeing in a pool with your face/ears underwater you cant realy hear a person untill there a foot or two away, evean then they have to speak up to be heard. It works the same way with sub noises.

But this thermal layer can be a good thing(good for a DD bad for us). Say theres a sub above Kaleuns "sound cone". A escort could bounce it's ping off the thermal layer and ping the offending sub.
It works both ways.. sometimes (this might be modelled) the sonar guy misses the depth charge firing...but generally I don't, I change course once the pinging has stopped or hear a ship above me.
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WilhelmSchulz.
04-18-2006, 07:53 PM
Heres a rundown on Thermal Layers (http://www.fleetsubmarine.com/bathythermograph.html)

Robards
04-19-2006, 09:39 AM
Im fairly certain that they triangulate it just like they do with earthquakes to determine the depth. In fact, you should notice that if there is only one or two escorts, they don't get the depth correct.

BueJack
04-19-2006, 12:48 PM
Clear straight forward descriptions http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Kaleun1961
04-19-2006, 05:36 PM
Here's the scan from my book I promised; sorry it's a day late. As a bonus, there's also a small section dealing with Hedgehogs.

http://i13.photobucket.com/albums/a299/K-61/Asdic/Asdic.jpg

Kaleun1961
04-20-2006, 07:55 AM
In the illustration above, the U-boat is broadside to the sonar beam, a bad situation. Anybody who plays this game knows you have to try to always present your stern or bow to the pinging vessel, in order to minimize your profile.

The size of the sound cone in this illustration is also exaggerated, which tends to happen with drawings not to scale. If the U-boat was really deep, it would be below the reaches of the sound beam and would not relect an echo to the sending vessel. Escorts also used hydrophones, just as the U-boats did. This meant that even if a U-boat was not being actively pinged, it could still be heard if it was being noisy. That's why we should try to run in silent mode when near escorts, even if we are not being pinged.

As an escort ran in to drop depth charges, at some point it would lose sonar contact with the submerged U-boat. The deeper it was, the sooner the escort lost contact. Hedgehog was developed to overcome this. If you read the article above, you will see that Hedgehogs could be fired ahead of the escort, while it still had the U-boat in sonar lock, out to 250 yards.

When the Allies geared up production and had enough escorts, they were able to form teams that trained and deployed together. Their team work improved and so did their tactics. Even without Hedgehog, there were means of overcoming the limitations of sonar. One tactic developed was known as the creeping assault. One escort would stand off a distance and hold the U-boat by sonar, while another slowly approached the submerged U-boat. The escort with the U-boat in sonar lock would guide the other vessel[s] over its position and tell it when to drop depth charges.

These support groups [British term] or hunter-killer groups [American term] became very skilled at hunting U-boats to their death. Earlier in the war there were simply not enough escorts available to allow them to train and deploy in numbers. Many convoys in the early years had only one or two escorts at most. Even if the escort found a U-boat, it could not leave the convoy long enough to hunt a U-boat. With the arrival of support groups, they could be called in to hunt a detected U-boat while the convoy escorts stayed with the convoy. The support group could spend the amount of time necessary to hunt a U-boat to its death. This often meant that they could pursue a contact until it was forced to rise because of depleted batteries or accumulated damage from repeated depth charge attacks.

BueJack
04-20-2006, 11:10 AM
Originally posted by Kaleun1961:
One tactic developed was known as the creeping assault. One escort would stand off a distance and hold the U-boat by sonar, while another slowly approached the submerged U-boat. The escort with the U-boat in sonar lock would guide the other vessel[s] over its position and tell it when to drop depth charges.

http://forums.ubi.com/images/smilies/25.gif In game I have the 'dirty' buggers doing this to me all the time. A lot of the time an Escort comes in fast, drops a few eggs, then races off trying to attract my attention.
But Kaleun Freddie is not fooled as, I counted the number of escorts prior to attacking the convoy, and monitor them at all times.

So when this escort makes maximum noise, I sweep on my hydrophone and find that there is one destoyer missing. Ole sneaky 'Cpt Smith' is trying to do a dirty on me. I usually find him directly on my 'six', so I turn in the direction of the noisy escort and monitor old Smithy.

Silent mode and at 1-2knots, and Kaleun gets away once again...yeeehahhh! http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif (Feb/March 1943)

The_Silent_O
04-20-2006, 11:36 AM
These support groups [British term] or hunter-killer groups [American term] became very skilled at hunting U-boats to their death.

Love the distinction in the terms...gotta love the killer instinct in the American Term. On the other hand, the british probably took tea time inbetween depth charge runs...but they got the job done eventually.

So who holds the record for most confirmed U-boot kills. (I know it's a British navy Captain...just can't remember his name now)

tisoy505
04-20-2006, 12:27 PM
Does anybody know what is and isn't modeled in the game as far as anti-uboat tactics, ie thermal layers, closeness to surface or sea floor, etc.??

It seems sometimes I can bang on the pipes sometimes and still get away. Also I never hear the sonar ping, is that my system, or was that real. I always thought it could be heard.

Celeon999
04-20-2006, 01:41 PM
I remember that someone said that thermal layers are indeed modeled in the game but there is of course no way to detect them.

scaseman
04-20-2006, 02:07 PM
You would need someway of reading the external water temperature. As different water layers vary in temperature. I did see this modelled in another submarine sim years ago.

BueJack
04-20-2006, 03:05 PM
You can use your hydrophone to pick up a thermal layer. It works both ways, and you can pick it up in the game.
You get a strong signal from a ship, which suddenly goes wobbly-goobly and much fainter. I picked this up a few times before I realised what it was...or at least I hope it's that http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

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Kaleun1961
04-20-2006, 03:36 PM
Originally posted by The_Silent_O:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">These support groups [British term] or hunter-killer groups [American term] became very skilled at hunting U-boats to their death.

Love the distinction in the terms...gotta love the killer instinct in the American Term. On the other hand, the british probably took tea time inbetween depth charge runs...but they got the job done eventually.

So who holds the record for most confirmed U-boot kills. (I know it's a British navy Captain...just can't remember his name now) </div></BLOCKQUOTE>

Captain Walker of the Royal Navy was the highest scoring U-boat killer. I can't recall his personal tally offhand, but I think it was something like 17. Then there the many that he helped, guiding them in for the kill. He may have been the one who killed Prien, or was involved in that action.

Kaleun1961
04-20-2006, 03:39 PM
Here's an interesting personal story I found while doing online research into Captain Walker:

http://www.war-experience.org/collections/sea/alliedbrit/wsmith/index.html

Kaleun1961
04-20-2006, 03:45 PM
This site credits Walker with 20 kills: http://ahoy.tk-jk.net/macslog/TheRoleoftheSubmarineinWo.html

Here is a quote from this site:

There is little doubt that the German U-Boat arm faced the most efficient anti-submarine force operating in WW2, namely the Royal Navy. eg. Captain F J Walker CB, DSO + 3 bars, DSC + 2 bars. Royal Navy, when Commander of the 2nd Escort Group comprising the sloops, Starling, Magpie, Wren, Wildgoose and Wood******, destroyed 6 U-Boats over the 31st of January and the 19th. of February, including the capture of U-264, the first German submarine to be fitted with a Schnorchel.

Captain (Johnnie) Walker had the amazing record of 20 U-Boat kills, but, at the age of only 48 he died from a stroke. The total Western Approaches Naval Command mourned his early death, and Admiral Sir Max Horton, at his funeral, held in the cathedral at Liverpool said: "No dust nor weight of stone, but all the waters of the Western Approaches shall be his tomb."

HMS Hesperus bore Walker's body out to sea, where it was consigned to the waters of the North Atlantic Ocean.