Noob question on sonar contacts

desertisland · 03-27-07, 07:23 AM

I have been wanting to ask this question for a while now:

When using the broadband sonar with towed arrays, is it true that, for every true contact, you will always receive a false contact opposite of the true one at exactly the same angle from your ships's direction?

I guess this is one of the properties of wave motion? Correct me if I'm wrong, I'm intrigued.

I just need to confirm if there will always be a mirrored false contact. Thanks! :hmm:

Captain Nemo · 03-27-07, 07:38 AM

Quote:

Originally Posted by desertisland

When using the broadband sonar with towed arrays, is it true that, for every true contact, you will always receive a false contact opposite of the true one at exactly the same angle from your ships's direction?

I guess this is one of the properties of wave motion? Correct me if I'm wrong, I'm intrigued.

I just need to confirm if there will always be a mirrored false contact. Thanks! :hmm:

That is correct. I can't quite remember the reason why this happens, but to ensure I always assign a tracker to the correct contact, I make a 10 degree turn and watch which one moves with the turn and which one doesn't. The one that doesn't is the true contact.

Nemo

Aegeus · 03-27-07, 07:45 AM

Way far from being a sonar expert but i think that is always the case with TA broadband or narrowband.

Dont mean to hijack the thread but here is another sonar question.
I found out that is possible to deploy both your TAs will that help in getting a better fire solution especially if you deploy them in a different length?

By the way first post here so glad to find you guys

Captain Nemo · 03-27-07, 07:59 AM

Welcome to the forum. Not sure that you can deploy both TA's at the same time, how would you select starboard and port individually at the sonar station? They also might interfere with each other. Never tried it though to confirm this.

Nemo

desertisland · 03-27-07, 08:21 AM

Quote:

Originally Posted by Aegeus

Dont mean to hijack the thread but here is another sonar question. I found out that is possible to deploy both your TAs will that help in getting a better fire solution especially if you deploy them in a different length?

[raise hand]
You can release both TAs in Seawolves but not 688s. Read somewhere (manual? yes I read it cover to cover] that one is only the backup for the other, releasing both will do you no good nor harm except the risk of wrecking both at once.

I also think it is better to keep you TA short. Reduces time for it to snap in line with the ship's direction so you can id the false contact faster.

Dr.Sid · 03-27-07, 09:00 AM

Towed array is string of individual hydrophones (many, more then 10).

Direction of incoming sound is derived from time delay of signal on individual hydrophones. If rear hydrophones receive the sound before front ones, the sound source is behind the array. If the time delay is distance between hydrophones divided by speed of sound it means the target is exactly behind the array. If the delay is shorter by sin(45)=0.707 it means the target is 45 degrees from the array axis, if the delay is 0 it means the target is 90 degrees and so on.

With just this you can't tell if the target is left or right. So there isn't in fact any mirror contact. You just don't have the idea if the contact is left or right, so both are displayed. Check FFG sonar, it only shows half the circle.

With sphere and hull arrays this problem does not exist because sounds does not reach all the hydrophones, especially not those on the opposite side of the boat.

Aegeus · 03-27-07, 09:29 AM

Quote:

Originally Posted by Captain Nemo

Welcome to the forum. Not sure that you can deploy both TA's at the same time, how would you select starboard and port individually at the sonar station? They also might interfere with each other. Never tried it though to confirm this.

Nemo

Thanks for the welcoming

If you go at the control station and select port or starboard on the TA selection switch you get info from the selected array in the sonar station.
I first found it on a 688 dont know if it works on a Seawolf.
My thought was that if you deploy them in a different length you would end up (?)with two different bearings at the target at the same time which is good for TMA i think.
I am noob so dont know much just learning at the moment

Edit
I ve just tried it on a Seawolf and you cant deploy both

sonar732 · 03-27-07, 10:39 AM

The main thing to think about with deployment of towed arrays is this....tactics!

The TB-29 is the more sensitive, while the TB-16 has a higher washout speed. In the book the TB-29 is listed as starboard and the 16 is port.

desertisland · 03-27-07, 10:53 AM

Quote:

Originally Posted by Dr.Sid

Direction of incoming sound is derived from time delay of signal on individual hydrophones. If rear hydrophones receive the sound before front ones, the sound source is behind the array. If the time delay is distance between hydrophones divided by speed of sound it means the target is exactly behind the array. If the delay is shorter by sin(45)=0.707 it means the target is 45 degrees from the array axis, if the delay is 0 it means the target is 90 degrees and so on.

Thank you, Dr. Sid, for your answer. It makes sense to me.

In other words, with 4 constants which are the distance between each receptors, the time-lag between each receptor's receiving the signal, the speed of sound, and the TA's aligment, we could determine 2 possible directions of the sound source?

And if all receptors receives the signal at the same time, we could tell that the source is either 90 degrees to the left or the right?

desertisland · 03-27-07, 11:07 AM

This is fun. Let me indulge myself a little more. A model for the towed array:

Ask 10 school boys to stand in a row, each 5 meters apart and carries a flag in hand.

Tell the boys to raise the flag if they feel the wind against their cheeks.

By the time lag between each raised flag, and the sequence in which they are raised, we could determined the direction of the wind. But we would get 2 possible solutions since the boys are not asked to tell which cheek is feeling the wind.

Dr.Sid · 03-27-07, 11:34 AM

Quote:

Originally Posted by desertisland

This is fun. Let me indulge myself a little more. A model for the towed array:

Ask 10 school boys to stand in a row, each 5 meters apart and carries a flag in hand.

Tell the boys to raise the flag if they feel the wind against their cheeks.

By the time lag between each raised flag, and the sequence in which they are raised, we could determined the direction of the wind. But we would get 2 possible solutions since the boys are not asked to tell which cheek is feeling the wind.

Well this could work only if you new speed of the wind

But generally that's it.
Well in fact it is done in slightly different way. Signals are summed with preset delay which amplifies signals from specific direction. It's called 'array steering' or 'beam-forging'. For BB display many such virtual beams are forged all the time, each for one bearing. It really takes a lot of math to simply see 'ah, there is something loud on bearing 030'

Edit: I should quit smoking ..

desertisland · 03-27-07, 12:24 PM

Do quit, I heard it's bad for health.

Another question:

Since the "pair" of contacts are not detected separately but deducted from a single group of data, why doesn't the computer specify them as "pairs" to simplify matters? Especially when you get multiple contact and it can be hard to tell which pairs with which? If they are group into pairs, the captian (or whoever is in charge of such matters) will remember to kill one of the twins. Wouldn't that be simpler?

Oh and I do believe the actual process is more complicated than this. Multiple sounds may overlap in their time of arrival, further complicating matters...:hmm:

Dr.Sid · 03-27-07, 03:20 PM

Check FFG sonar, it works more or less as you described. What screens and systems are used on real subs and ships is not known anyway (I mean not told).

For overlapping sounds there is narrow-band, which means even more math with Fourier transformation. With that you can follow one frequency .. and it should be possible to choose some which does not conflict with anything else.

Again .. how much this is used IRL is hard to tell. From what I read you can assign auto-markers even for BB contacts, especially because they can measure actual rate of bearing, even if the bearing changes too slowly to see it on the screen.

But to auto-follow contacts without being fooled by crossing lines you have to follow concrete NB line I guess.

Wim Libaers · 03-27-07, 04:42 PM

Quote:

Originally Posted by desertisland

When using the broadband sonar with towed arrays, is it true that, for every true contact, you will always receive a false contact opposite of the true one at exactly the same angle from your ships's direction?

Almost. The ship's direction is not important, the array direction is. This matters in turns, where ship and array direction are different.

desertisland · 03-27-07, 09:36 PM

Quote:

Originally Posted by Wim Libaers

Almost. The ship's direction is not important, the array direction is. This matters in turns, where ship and array direction are different.

Thanks for pointing that out, Wim. A more accurate description would be "the alignment of the towed arrays".

03-27-07, 07:23 AM	#1
desertisland Gunner Join Date: Feb 2007 Location: Desert Island Posts: 96 Downloads: 34 Uploads: 0	Noob question on sonar contacts I have been wanting to ask this question for a while now: When using the broadband sonar with towed arrays, is it true that, for every true contact, you will always receive a false contact opposite of the true one at exactly the same angle from your ships's direction? I guess this is one of the properties of wave motion? Correct me if I'm wrong, I'm intrigued. I just need to confirm if there will always be a mirrored false contact. Thanks! :hmm:

03-27-07, 07:59 AM	#4
Captain Nemo Ace of the Deep Join Date: Jan 2002 Location: London, England Posts: 1,144 Downloads: 54 Uploads: 0	Welcome to the forum. Not sure that you can deploy both TA's at the same time, how would you select starboard and port individually at the sonar station? They also might interfere with each other. Never tried it though to confirm this. Nemo __________________ "I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic". Sir John Slessor GCB, DSO, MC, DL AOC-in-C Coastal Command RAF ___________________________________________

03-27-07, 07:45 AM	#3
Aegeus Nub Join Date: Mar 2007 Location: Athens Greece Posts: 3 Downloads: 0 Uploads: 0	Way far from being a sonar expert but i think that is always the case with TA broadband or narrowband. Dont mean to hijack the thread but here is another sonar question. I found out that is possible to deploy both your TAs will that help in getting a better fire solution especially if you deploy them in a different length? By the way first post here so glad to find you guys

03-27-07, 09:00 AM	#6
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	Towed array is string of individual hydrophones (many, more then 10). Direction of incoming sound is derived from time delay of signal on individual hydrophones. If rear hydrophones receive the sound before front ones, the sound source is behind the array. If the time delay is distance between hydrophones divided by speed of sound it means the target is exactly behind the array. If the delay is shorter by sin(45)=0.707 it means the target is 45 degrees from the array axis, if the delay is 0 it means the target is 90 degrees and so on. With just this you can't tell if the target is left or right. So there isn't in fact any mirror contact. You just don't have the idea if the contact is left or right, so both are displayed. Check FFG sonar, it only shows half the circle. With sphere and hull arrays this problem does not exist because sounds does not reach all the hydrophones, especially not those on the opposite side of the boat.

03-27-07, 10:39 AM	#8
sonar732 The Old Man Join Date: Jul 2003 Location: Central MO Posts: 1,562 Downloads: 6 Uploads: 0	The main thing to think about with deployment of towed arrays is this....tactics! The TB-29 is the more sensitive, while the TB-16 has a higher washout speed. In the book the TB-29 is listed as starboard and the 16 is port.

03-27-07, 11:07 AM	#10
desertisland Gunner Join Date: Feb 2007 Location: Desert Island Posts: 96 Downloads: 34 Uploads: 0	This is fun. Let me indulge myself a little more. A model for the towed array: Ask 10 school boys to stand in a row, each 5 meters apart and carries a flag in hand. Tell the boys to raise the flag if they feel the wind against their cheeks. By the time lag between each raised flag, and the sequence in which they are raised, we could determined the direction of the wind. But we would get 2 possible solutions since the boys are not asked to tell which cheek is feeling the wind.

03-27-07, 12:24 PM	#12
desertisland Gunner Join Date: Feb 2007 Location: Desert Island Posts: 96 Downloads: 34 Uploads: 0	Do quit, I heard it's bad for health. Another question: Since the "pair" of contacts are not detected separately but deducted from a single group of data, why doesn't the computer specify them as "pairs" to simplify matters? Especially when you get multiple contact and it can be hard to tell which pairs with which? If they are group into pairs, the captian (or whoever is in charge of such matters) will remember to kill one of the twins. Wouldn't that be simpler? Oh and I do believe the actual process is more complicated than this. Multiple sounds may overlap in their time of arrival, further complicating matters...:hmm:

03-27-07, 03:20 PM	#13
Dr.Sid The Old Man Join Date: May 2005 Location: Czech Republic Posts: 1,458 Downloads: 6 Uploads: 0	Check FFG sonar, it works more or less as you described. What screens and systems are used on real subs and ships is not known anyway (I mean not told). For overlapping sounds there is narrow-band, which means even more math with Fourier transformation. With that you can follow one frequency .. and it should be possible to choose some which does not conflict with anything else. Again .. how much this is used IRL is hard to tell. From what I read you can assign auto-markers even for BB contacts, especially because they can measure actual rate of bearing, even if the bearing changes too slowly to see it on the screen. But to auto-follow contacts without being fooled by crossing lines you have to follow concrete NB line I guess.