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Old 08-05-2021, 06:22 PM   #1
Efshapo
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Default Four bearings while moving, with variable time intervals

Hello everyone!

I developed a graphical method to get target data using four bearings while moving. The new thing is that it does not require time intervals to be constant.

You can find a tuto here: 4 Bearings, Moving, Variable Time Intervals – v1.2

Do you know if something similar exist? I mean, beside usual TMA that requires a lot of maths, something graphical and simple.

Last edited by Efshapo; 10-14-2021 at 09:37 AM.
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Old 08-05-2021, 07:02 PM   #2
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I can’t say I’ve seen any versions put out there with unequal time intervals, this is very novel. I’m not sure it’s simpler, but I think you’ve laid it out well. Putting my personal feelings about the 4-bearing method aside, I think you’ve done a good job here.
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Old 08-07-2021, 01:03 AM   #3
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Ok, thanks!

And I think we agree on the lack of practicality of the 4-bearing method. :]
I just find the exercise kinda fun. Also, it can be useful with the hydrophone, when the convoy is not in sight. That rarely happens in WP though.

By the way, I'm not saying it is simpler to do the 4-bearing method this way, I'm saying it's the simplest way I found to do it with variable time intervals.

Variable time intervals is a cool feature though, because it's more flexible. Here are some of the advantages from the top of my head:
- You can take your time to do an accurate bearing measurement (E.g. at a round angle value) and then note the exact time it was done, without the need to do it at a specific moment.
- You can also re-do any measurement if you find out it has been done at a bad position (E.g. close to a singularity) without having to ditch all the previous work.
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Old 08-08-2021, 12:31 AM   #4
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Elegant!

So I was actually using the fixed interval version of this method (the first half) for a long time? (https://www.subsim.com/radioroom/sho...d.php?t=240883)

I guess the philosophy is to find out the segments that are of the same ratio as time intervals.


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Old 08-08-2021, 06:32 AM   #5
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Yes, this method is quite elegant. You are doing 2 times 2 the same procedure. Although it may be a lengthy procedure, it's routine procedure all around. No need for choosing a particular bearing-line to do further geometrical constructions that looks more usefull than others.

First starting with a near and far assumption on position on the first bearing. Then choose a point on the second bearing to assume a target course (for that particular case). Then these are projected forward to the 3rd bearing time and target course corrected (for that particular case) to find the point on the 3rd bearing (using the parallel of the 1st bearing) that satisfies the constant target speed across the 3 bearings at that distance. From those 2 near and far lines you predict the point of the locusline for the time of the 4th bearing. The target has to be somewhere on that line at the 4th bearing time depending on the initial unknown distance. Then the position of the target at the time of the 4th bearing is fixed by the actual 4th bearing intersecting the locusline.

To find the position of the target at the time of the 1st bearing Efshapo does the same thing in reverse, starting from the 4th back to the first. Then he fixes the line through the 2 points which is the target course. And the overall distance between them, or the intermediate bearing intersections, indicate the target speed.
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Old 08-08-2021, 07:53 AM   #6
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You might be interested in this:





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Old 08-08-2021, 04:36 PM   #7
Efshapo
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Thanks everybody!

@ljqcn101
What you described was actually the starting point that made me wonder what would happen if I used a ratio different than 1:1 for different time intervals! This standard 4-bearing method seems to be a particular case of the general case that I propose. (And, as you, I also found out that the "segment from mid point" was a really effective tool.)

@Pisces
You got that right! It's indeed 4 times the same procedure, and you also understood perfectly why it's done this way!

@Sean C
I know well the Maneuvering Board, I've actually recently been asked to correct a step-by-step tutorial about it.
The cons here are that it requires 6 bearings instead of 4, that it requires that you keep your course and speed during each series of 3 measurements, and time intervals have to be equal. In my opinion, this technique is less elegant than the standard 4-bearing method.
EDIT: Sorry, I read too fast, the last line gives the same formula as me indeed! I still prefer to use them with 4 bearings instead of 6, but thanks for sharing!
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Old 08-09-2021, 04:50 AM   #8
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Interesting! Thanks!

Actually a lot more complicated than I thought...

Do you have any estimates of the accuracy of this method depending on the angles involved? For example if you happen to end up directly behind the convoy and the sub is going towards it you will have extremely low accuracy. Would it be better to make a 90 degree turn at some point, to improve accuracy?

Is the method the same if your sub is moving in the opposite direction (but same path) as in the guide?
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Old 08-09-2021, 11:35 AM   #9
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This method doesn't work if the lines are all overlapping or narrowly parallel to each other. They need at least some separation. So trailing that target won't do you much good. You need to make a course change anyway at some point. (When you are exactly behind it then you already know where it is going. Yuo follow. You just don't know the distance.)

To get best accuracy you want large changes in bearing directions. This is best done by moving to an angle of the bearing with significant speed. And if you are moving opposite to the target in question then those bearing changes are the quickest. But you'll also get behind it fast, and might loose sight of it. Though hydrophone is longer distance than visual bearings, those hydrophone bearings are also far less accurate. The method does not change.
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Last edited by Pisces; 08-09-2021 at 11:52 AM.
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Old 10-06-2021, 10:22 AM   #10
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For anyone interested, I simplified the steps after getting G (by starting from G). If you want the latest version of the step-by-step procedure, I updated the link in my first post.
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Old 10-06-2021, 05:31 PM   #11
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And I just made a video to illustrate the guide (using the periscope):

You can skip to 20:30 to see the result.

Last edited by Efshapo; 10-08-2021 at 08:34 AM.
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Old 10-08-2021, 08:35 AM   #12
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And just now another one, actually using the hydrophone:

You can skip to 38:45 to see the result.
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