False contacts in TA

[Pishky]

Hi there gurus.
I would be really grateful if someone could explain to me please why there are always two contacts, one true and one false on the TA and why you have to turn to resolve the difference.

Thanking in advance

Peter

[LuftWolf]

All sonar sensors work as a combination of the physical sensor surface interacting with the environment and a block of computers processing that raw sensory data from the physical transducers into the sonar signal that we as humans read in the sonar display.

The particular acoustic geometry of the TA (how it works) means that an ambiguous contact is always generated when there is a real signal, it is a by-product of the way the array functions when it takes the physical sensor signal from the source and turns it into processed sonar data that we can read.

[Pishky]

Thanks for your answer LUftWolf

My 'picture' of a TA is a series of cylindrical microphones arranged like beads on a string. The first and last one are separated by a large distance and should therefore be able to triangulate the contact. The computer should be able to take out the false contact. I guess this just goes to show my ignorance on the subject.

Once again many thanks for your reply, it is much appreciated.

[LuftWolf]

Yes, but the individual transducers and thus the transducer network as a whole can't extract information as to whether the signal is coming from x or x+180 mod 360.

[Wim Libaers]

Originally Posted by Pishky

Thanks for your answer LUftWolf

My 'picture' of a TA is a series of cylindrical microphones arranged like beads on a string. The first and last one are separated by a large distance and should therefore be able to triangulate the contact. The computer should be able to take out the false contact. I guess this just goes to show my ignorance on the subject.

Once again many thanks for your reply, it is much appreciated.

Well, yes, but that sensor geometry doesn't allow you to determine an exact direction, it only gives you an angle between the array direction and the direction the sound comes from. This means the target could be anywhere on a cone with the apex on the array. If you assume the target is not above or below you, but to the sides, intersecting that cone with the horizontal plane gives you two bearings.

[moose1am]

Here is what I have problems understanding. Why does the Nose Sonar. Spherical Sonar Array only show one contact when the Towed Array shows a true bearing and a false bearing?

Must be a difference in the sizes of the two types of arrays?

Originally Posted by LuftWolf

All sonar sensors work as a combination of the physical sensor surface interacting with the environment and a block of computers processing that raw sensory data from the physical transducers into the sonar signal that we as humans read in the sonar display.

The particular acoustic geometry of the TA (how it works) means that an ambiguous contact is always generated when there is a real signal, it is a by-product of the way the array functions when it takes the physical sensor signal from the source and turns it into processed sonar data that we can read.

[LuftWolf]

The arrays work in a completely different manner in terms of how the individual transducers are networked... I think that's about as far I can go without being wildly out of my depth.

[compressioncut]

Originally Posted by moose1am

Here is what I have problems understanding. Why does the Nose Sonar. Spherical Sonar Array only show one contact when the Towed Array shows a true bearing and a false bearing?

Must be a difference in the sizes of the two types of arrays?

Bow-style sonar is directional. Imagine a dome comprised of many hundreds of small transducers (pressure-sensitive thingamajigs) and the sound hitting it at one point, only registering on a few transducers. The sensor knows where the sound is coming from because it's only hitting at one point on the dome, and the computer end of the system can figure out the exact bearing from that - normally calculated from a time delay of the sound energy hitting a series of the transducers. That process is called beamforming.

A towed array is quite different. The sound travelling through the water hits one side of the array, but it also hits the other side of the array (it's OK to imagine it as a two dimensional object here) simultaneously. The array processor is unable to tell which side the sound is actually on, which is why you get an ambiguous bearing.

[Pishky]

AHA!

It is as if a lamp got turned on in a dark room. Your answer actually makes a lot of sense to me.

Thank you for your time.

Regards

Peter

[Henson]

Try not to think of the towed array as hearing sounds in a 360 degree environment like the spherical array does. The FFG's TA broadband display actually does a much better job of modeling how the TA really hears things, that is Front to Back.

Let's say you are heading north and have a spherical array contact bearing 030. On the FFG towed array (which most closely models a real display), that contact would be about halfway up the front section of the array on relative bearing of 030...OR 330. The array can only hear front to back (180 degrees vice 360), so it has no way of knowing which side the contact is actually on until it is turned. The procedure in the manual for resolving bearing ambiguity is much closer to the way it is done on a real TA platform, including submarines. If the contact draws up the array (or forward, whichever you prefer), that means the TA turned towards the contact. Draw a birds-eye perspective picture of that and it will make sense to you.

The reason it shows two contacts instead of one on the submarine platforms has always been a mystery to me. I suspect that it was a decision made when writing 688I and the programmers just decided it would be too much of a hassle to fix in subsequent new releases. Basically, the current DW sub display is just two TA displays connected to each other, one going from front to back, and then returning back to front. If you think in those terms it will help. If you want the real in-depth theory of it, look to the FFGs TACTASS.

[MaHuJa]

Originally Posted by Henson

The FFG's TA broadband display actually does a much better job of modeling

Ugh, this forum has gotten obsessed with that word.

Strictly speaking, the word you are looking for is probably showing, not modeling.

The reason it shows two contacts instead of one on the submarine platforms has always been a mystery to me. I suspect that it was a decision made when writing 688I and the programmers just decided it would be too much of a hassle to fix in subsequent new releases.

I believe it may be a matter of the real subs actually using that "dual contact display" which may be because they wanted similarity with the spherical display - to avoid making that much of a distinction between "sonarman sphere" and "sonarman towed".

However, it would appear that IRL there is a separate seat and display for the towed array, which doesn't look quite like the spherical watchstations, so there may indeed be differences.

Either way, what we have now works.

[Henson]

Originally Posted by MaHuJa

I believe it may be a matter of the real subs actually using that "dual contact display" which may be because they wanted similarity with the spherical display - to avoid making that much of a distinction between "sonarman sphere" and "sonarman towed".

However, it would appear that IRL there is a separate seat and display for the towed array, which doesn't look quite like the spherical watchstations, so there may indeed be differences.

Either way, what we have now works.

The second part is true in US boats. There are separate SA and TA operators, and their displays are very different. The SA broadband display shows 4 separate waterfall pictures in three different time averages. The TA display is one single waterfall display that runs from 000rel to 180rel, or forward to aft if you prefer, and generally displays two time averages (this last depends on the exact conar system). That single waterfall is all we need, because we already know that the contact shows up on both sides, and we resolve ambiguity by watching to see if the contact draws up or down the array in a turn. If we had to track dual path on a two-sided TA display it would be a visual nightmare.

If you take the FFG TACTASS broadband display and turn it on its side you have a typical US TABB display.

And you're right, what we have now works. I only brought all of this up to answer the original question.

[Dr.Sid]

Well .. in DW where you usualy handle both sonarman and captain, I find russion displays the best. They give best idea about situation. It is also great you can see all arrays at once. US waterfalls come next, the history and time-average function are great and better in some situations, like weak contacts.
On FFG it is just mess .. you must mark the contact and look at the map what direction it actually is. Luckily that does not matter much in FFG, since you don't evade torpedoes that much and you don't use CM's for it.