Using TA+hull sonar for triangulation

[NFunky]

Hey again,

I've been playing with the 688(i) again and just had a very strange experience vis-a-vis triangulation using two arrays. I picked up a Han class sub pretty strongly on my towed array (3 frequency lines) and so I thought I might look at my other two arrays to see if I could get him on either. Sphere was blank in NB, but I had a line in the lower window in BB. Unfortunately, DW wouldn't let me mark that line no matter how many times I clicked the designate button on and around its bearing. I then looked on hull NB and bingo, I had the second frequency line coming in feintly. I designated it and merged the contacts in TMA.

Here's where it started to get weird. After a few LOBs appeared, I tried to use the bearing cross from the two arrays to triangulate the target, but the solution I got using the crosspoints was vastly different from what my solution using DEMON said it sould be. I made a new TMA leg, waited for more bearings and again, the triangulation was way off from the solution I got by using just the TA and DEMON.

I got to what I thought was in range and fired two fish, but they didn't seem to lock onto anything and eventually ran out of gas. I was frustrated, so I quit the scenario and viewed the replay and you know what I found? The target was actually way further out than my triangulation suggested. He was at around the exact position my solution using just the TA and DEMON had given me, but I'd thought that the triangulated solution would be more accurate.

Any ideas why this happened? I seem to remember having luck with triangulation before, but I think that was most commonly using the sphere and TA, not the hull. Does the hull array give somewhat inaccurate bearings? Also, why was I able to see an intermediate timescale contact on sphere BB, but was unable to mark it for the life of me?

[Castout]

I'm an idiot in making TMA solution but as far as I know the precise triangulation you mentioned is no longer applicable in DW and it's more realistic that way.

In the previous title like Sub command you will get a perfect precise triangulation when you have a contact tracked with two or more sonar arrays.

I'm not sure as to the exact why but my guess is that for one the different positions of the sonar arrays it self would render perfect triangulation almost impossible. And another that sound doesn't travel in a straight line underwater so you are bound to have bearing differences that would make perfect triangulation almost impossible.

TMA is not a science to me but a kind of an art . That's another way of saying a thoughtful guessing.

[Molon Labe]

Triangulation with the TA isn't nearly as good as triangulation you get with a UUV because the lines are almost parallel. A very small bearing error can lead to a significant change in range on the solution. Does that mean it's worthless? No, far from it. At times it can be quite accurate. But if you have something better to go on (like a solid unambiguous solution with a known contact speed), go with that.

Something else to keep in mind is that the first bearing lines for new contacts/sensors will be off a little because they aren't on the 2-minute interval so that is going to throw a triangulation off, and obviously there is also the issue where the TA bearing gets thrown off during turns.

[Pisces]

I do not know how big the bearing inaccuracies are for each sensor. But you can make best use of it if you beam the contact, and stream the towed array as far as possible.Then the bearings of the 2 sensors are as far apart as possible. If you happend to have your bow and tail line pointed towards or away from the contact then triangulation is useless on it. Then there is no seperation from the contacts point of view.

[suBB]

But you can make best use of it if you beam the contact, and stream the towed array as far as possible

the problem with use of maximum scope, is, in shallow waters, you'll be dragging your TA, running deaf, and forced to operate at higher speeds to compensate for the drag. Under those conditions, you'll be compromising yourself if you want maximum scope. It's best to optimize maximum scope vs speed based on your environment..

[goldorak]

Originally Posted by suBB

the problem with use of maximum scope, is, in shallow waters, you'll be dragging your TA, running deaf, and forced to operate at higher speeds to compensate for the drag. Under those conditions, you'll be compromising yourself if you want maximum scope. It's best to optimize maximum scope vs speed based on your environment..

No, even if you extend the towed array to its maximum length, a speed of 7 knots is sufficient to have the towed array in line with ownship.
No problem of having it drag over the ocean floor.
Go less than 7 knots and yes in shallow waters you'll find the towed array being dragged all over the place.
And frankly the tactical speeds of the 688/seawolf/virginia are quite high that going at 7 knots is no problem at all to search for enemy subs.

[goldorak]

Originally Posted by Pisces

I do not know how big the bearing inaccuracies are for each sensor. But you can make best use of it if you beam the contact, and stream the towed array as far as possible.Then the bearings of the 2 sensors are as far apart as possible. If you happend to have your bow and tail line pointed towards or away from the contact then triangulation is useless on it. Then there is no seperation from the contacts point of view.

Yes, you need maximum separation between sensors. Next you must proceed with a minimum speed of 7 knots so as to have the towed array in line with ownship. The bigger the depth difference is between ownship and the towed array the bigger the error in the tma solution.
Last but not least, to get a precise triangulation mark the contact on the towed array and hull or sphere sonar AT THE SAME TIME whenever possibile.
This single rule will simply your solution as you can't even imagine.

Edit : when using triangulation 6 minutes (4 minutes to wait for the first couple of bearing lines + 2 minutes for the second couple of bearings line) is all you need to fix a solution. No need to go into several legs, one is more than sufficient if the contact isn't maneouvering.