How do you Get Accurate Range with No Map Contacts On

[Dignan]

So I'm in a "no map contact updates" phase and was wondering what people do to plot courses for far off targets? Does everyone just use the ctrl-v to get "nearest visual contact" and then plot the bearing and range on the map?

This usually works pretty well until you have a convoy. I've found that your WO will choose different "nearest targets" to estimate the range for which will throw off a course plot. What techniques do people use when trying to plot the course of a far off convoy (without map contacts)?

Up close it is easier because you can use the stadimeter and start plugging data into the TDC but by this time, it's hard to have a good speed estimation if your course estimation is all out of whack.

Discuss.....

[Luken]

Four bearings method can be pretty accurate.

[jason210]

Given that the convoy is ahead of you, ie within a 180 degree arc around the bow of the sub...

1. There's usually a warship in front of and behind the convoy so ignore those and turn the dial to where the "big sound" of the convoy either starts or ends and use that point. Track it for a while, taking bearings periodically - for example every 10 minutes. (Ideally the Uboat should be still for this, but so long as it is moving very slowly and not turning it will be ok).

2. Note which way round the dial the sound appears to be moving, this will tell you whether the point where the big sound starts is the front or back of the convoy.

3. If the convoy is moving away, the angle detected on the Hydrophone will be successively decreasing with each periodic measurement; if it's approaching, the angle will be successively increasing with each peridoic measurement.

These simple steps tells us whether the convoy is getting closer and gives us an idea of its bearing.

Range
The range is related to the size of the angles between the periodic measurements. If the angles are very small, then the target is a long way off. Another thing you can use is the size of the angle occupied by the "big noise". Again, if this is small angle, the convoy is far way; a large angle, the convoy is close. This gives you a basic idea of the situation.

For more detail, you need to know the speed of the convoy. This is vital, but unfortunately without intel we can only guess that. Most convoys move pretty slowly, so you could perhaps go for 12 kts. But the success of any calculation using passive sonar depends on this.

Now for math:

a) How many metres will a ship in that convoy travel in 10 minutes, moving at 12 kts?

Answer:

First convert to metres per minute using 0.032:
12/0.032 = 375 metres

In ten minutes:
375 x 10 = 3750 metres


b) Now to work out range using using distance as represented by the angle on the hydrophone. You need to use trigonmetry for that, and it gets messy if the target is not perpendicular to your course. But if you use 3750/sin (5) you'll get a reasonable estimate, ie 43 KM. The more the ship is turned away from a perpeniducular course to yours, the less accurate this result will be.

This is as much as I am prepared to do manually, but in order to get a good estimate on the convoy's bearing, more work is needed. And it get's messy just using pure forumulae. The best way to approach this is to draw geometric diagrams on paper. Also, if you have access to any of the Sonalyst Sub Simulators like Sub Command, Dangerous waters etc, then there's a good geometric representation within that simulator that shows you periodic plots of sound contacts and proposed solutions that you can move about.

[Dignan]

Thanks for the ideas. I guess it just takes some practice.

I've tried the 4 bearings method but I felt like I was back in highschool geometry and subsims are about having fun, right

[jason210]

I know what you mean. I'm making some slide rule type instruments for this, the main goal being ease of use.

[Maki4444]

Originally Posted by jason210

I know what you mean. I'm making some slide rule type instruments for this, the main goal being ease of use.

Great job Jason, can't wait for it I generally have the feeling that all of these methods while in fact correct weren't used in real life and that subs relied mainly on radio contact reports. Just look at Das Boot or read Iron Coffins, there is no mention of a sub laying low listening to the hydro and then you know just triangulating the course :P

[jason210]

Originally Posted by Maki4444

Great job Jason, can't wait for it I generally have the feeling that all of these methods while in fact correct weren't used in real life and that subs relied mainly on radio contact reports. Just look at Das Boot or read Iron Coffins, there is no mention of a sub laying low listening to the hydro and then you know just triangulating the course :P

I honestly don't know how they did it back then, but I do know that during the cold war and today computers do this kind of analysis, and sims like sub commander and Dangerous waters let you do this passive sonar plotting graphically. However, passive sonar is very good for giving you an idea of range and heading of a convoy, which must have been useful.

Btw, here's a link of my torpedo calculator:



This version works fine, but I have since tidied it up a bit. For example that text has been removed, and replaced with other information. The part (b) is wrong also because this device won't work for gyro angles - it can give you the pseudo torpedo track but that isn't useful, so torpedos must be fired at 0 gyro angle when the target aappears at a specific periscope angle. I might add a slide rule down there also, for estimating speeds.

I will release it on my website as a free downloadable kit, and also as a ready-assembled product that you can buy. It will only be cheap, but I think with the free kit there may be a problem attaching the rotating parts because this requires the use of a riveter - not something we all have access to.

[Michael13]

Originally Posted by Maki4444

Great job Jason, can't wait for it I generally have the feeling that all of these methods while in fact correct weren't used in real life and that subs relied mainly on radio contact reports. Just look at Das Boot or read Iron Coffins, there is no mention of a sub laying low listening to the hydro and then you know just triangulating the course :P

Well, they was, but they made it by different way. Pursuing ships/convoys whole day (sometimes few days) at safe distance, watching their behavior, calculating average speed and course and etc.