Calculating position, real course and speed only by hydrophones!

[Mittelwaechter]

Interested in plotting a contact-course, position and speed only using hydrophones? Read on!


U 42 is hunting for single merchants in the Western Approaches. It's 2300 hours, the sea is calm. The sky is cloudy and two hours ago a light fog came up.
The Kaleun has ordered all engines stop at periscope-depth, Heinz is listening at the hydrophone for contacts.

"Contact! Freighter, inbound. Bearing 60°."

Konrad - our WO - activates the stopwatch and orders Heinz to report the bearings exactly all 8 minutes. Konrad notes the contact on the map. He draws a line of 20 km length from our position - bearing 60° to our bow.
Additionally he grabs a green pencil and draws a perfect circle (he's a genius!) on a free area of the map and marks the center with M. He draws the horizontal diameter and marks the ends with P1 and P2. The contact is reported on the starboard side, therefore he marks the right point P1. The line P1-M shall represent the distance the contact will travel in 8 minutes, the line M-P2 shall represent the distance for the following 8 minutes. Logically the length of both lines is equal as long as we assume the contact is moving with constant speed on a straight course.

8 minutes have passed, Heinz reports: "Contact now at 50°."

Again Konrad starts the stopwatch and notes the bearing on the map - a line of 20 km length - 50° to our bow. He notes the angle of the bearinglines - 10° - and names it A1.
He calculates: (180° - 2A1) : 2 = 80°. He grabs a blue pencil and the protractor to construct an isosceles triangle under the P1-M baseline - both angles read 80° - the sides meet in Point C1. The angle at C1 reads 20° (2A1).
Konrad - a golden colour pencil in his hand - draws a circle using C1 as center and the line C1-M as radius.

"Contact at 34°."

Our WO orders "Full speed, Heading 270" and reactivates the stopwatch. He notes the new bearingline carefully on the map. The new angle reads 16° - marked as A2.
To complete his auxiliary drawing he calculates again: (180° - 2A2) : 2 = 74°.
Using the blue pencil he constructs the second triangle similar to the first procedure, but now under the M-P2 baseline. Both baseangles read 74° - the sides meet under an angle of 32° (2A2) - the point is marked as C2 and used as center for a circleline - golden coloured - which meets M.

The two golden circles show two points of intersection - one is already marked M - Konrad names the other one U using the black pencil.
"U" represents the position of an U-Boot that locates a moving contact under the bearingangles A1 (10°) and A2 (16°).

Konrad draws the angle U-P1-P2 in red - it reads exact 33°.

He transfers this angle to the first bearingline representing the first contactreport at 60°. The new arm reads the quite exact courseDIRECTION of the freighter.
Fat chance the freighter doesn't move along this line but on a parallel courseline.

The intersectionpoints of the transfered courseline with the three bearinglines cut the courseline into two equal parts. They represent the distance our freighter moved in 8 minutes.
Konrad extends the courseline by 50% ("s") to mark the theoretical fourth bearing Heinz would have reported after 8 minutes without changing the position of the U-Boot.
He draws the bearingline to meet the new end of the courseline.

Seven minutes have passed since Heinz's last bearingreport. Our WO orders all engines stop to enable Heinz a proper report after 8 minutes.

"Contact! Freighter at ..."

Konrad notes the new bearing on the map. The line meets the theoretical fourth bearing in the freighters current position.

All necessary information to start an optimal attack is collected: position, course and speed (well you have to do some calculations on your own!) of the contact.

"Kessler! Wake up our Kaleun! We've got a job to do."


To confuse you even more - this shows Konrads auxiliary drawing. The colours are used in the correct chonological order. Well therefore it's in german.


And here Konrads NavMap notes.


Good Hunting, Gentlemen.

[GreyOctober]

Wow Mittelwaechter very nice narrative and very informative. But i somehow find your method too complicated. What i usually do is once i locate a contact by hydro i allow myself a minute or so to figure out the general direction of the contact my moving the hydro angle 2-3 deg left and right of the target. Once i figure this out, i make a mental note on the amplitude of the sound and start the stopwatch. I then move the hydro angle ahead of the target by several degrees and wait for the contact to intersect it. Once it does so, i stop the stopwatch, listen for the sound amplitude and note the bearing change. This will give me a rough estimate of the contacts speed and bearing and i can move to intercept.

Cheers!
Grey

[raduz]

you guys can find all this and even much more here:

http://www.subsim.com/radioroom/show...ophone+hunting

[Mittelwaechter]

Complicated but capable to sink contacts without even viewing them.
You may position your U-Boot at the right place and do some manual TDC settings to hit the contact in heavy fog or with periscopes destroyed.

You get correct speed, AOB and distance whenever you want.

[P_Funk]

I felt good when I figured out AoB way back.

This makes me want to cry.

[Sailor Steve]

Originally Posted by Mittelwaechter

Complicated but capable to sink contacts without even viewing them.
You may position your U-Boot at the right place and do some manual TDC settings to hit the contact in heavy fog or with periscopes destroyed.

You get correct speed, AOB and distance whenever you want.

Pretty good, since no one ever did that successfully in real life. Once again the game cheats.

[Mittelwaechter]

Well "the game" allways cheats as long as you know your exact own position on the NavMap.

Have fun with it - or simply don't do it.

[Nuoz]

Originally Posted by Sailor Steve

Pretty good, since no one ever did that successfully in real life. Once again the game cheats.

That's probably because they had to identify the ship they were hitting to save torpedos, hitting them in the most adequate spot amd avoid a friendly hit.

[Kubryk]

Hi,

this tutorial is what I was looking for. It's simple and well explained. Thanks!


But...
I tried this method 3 times and... I must be doing something incorrectly.

First try:
When my sonarman shouted contact I ordered engine stop. Only when in dead stop (I checked on the map) I took 1st bearing, started stopwatch, after 8 minutes another, after 8 minutes 3rd. Then I started engines again, after 8 minutes since last bearing I took bearing nr 4. Here I paused. I did all calculations required. Angles on triangles p1-m-c1 and m-p2-c2 were close to 90 degrees, so my circles were big.

As you can see on screenshots, I was able to calculate contact's position. I ploted an intercept course. I was really amazed that my guy is shouting 'long range', from my map it looked like the ship is less than 3km from me. After checking with periscope and hydro I surfaced and chased contact with all my speed. He was additional 7km away. It's course was more or less the same as I calculated by hydro-method.

screenshot nr 1



screenshot nr 2


Now I see that I could make mistake - I misplaced P1 with P2. Contact was on my left side, so P1 should be on left side of the helping circle, and I can see I used right end of a diameter as P1. My bad. I'm pretty sure that in other cases I was right though and this mistake is an exception

Second try:
After some time I heared another contact. I sailed towards it, so eventually my crew reported bearing. Again, only when I was in dead stop I started the Mittelwaechter's procedure.

Again, I heard 'long range', so I instantly knew the ship wasn't where it was suppose to be

This time however I tried to estimate. I assumed contact's course-direction is valid, so in order to find him I have to calculate distance between me and contact. I knew it was a merchant. I calculated distance it would travel in 8 and 24 minutes, and using angle calculated with Mittelwaechter's method (29 degrees in this example) I put circles on my map.

This may be confusing, so here's a screenshot:


screenshot nr 3

I assumed - if the contact's speed is constant he would travel x km. I calculated distance for (AFAIR) 5, 6, 7 and 8 knots. I was drawing circles over the protracted angle, aligning circle arrowhead with protractor, so later all I had to do was to put circle over the bearings.

Than I took another bearing, it was 313, 47 degrees to my port.


screenshot nr 4

This bearing line crossed my 4.4 cirlce. Accidentally, it the same spot it crossed the 4th (virtual) bearing to contact I would made if I wouldn't leave my first position.

Anywho, I was already going on perpendicular course. Now funny thing happened. For 14 minutes (game time) bearing to contact was 313 degrees. I was going 8 knots. When my speed changed to 7 (I don't why, I still had juice in batteries) bearing started to change. By this accident I knew two things: contact was doing 8 knots and I really was perpen... our courses would form 90 degrees angle

(maybe if my speed wouldn't drop I would be part of a sea collission? )


screenshot nr 5

When I saw contact, I plotted it course etc. Turned out it was different than I calculated with hudro and I (my course hasn't change) was no longer perpendicular towards it. The difference between hydro and real course of the contact was ca. 10 degrees.

I sunk the merchant, but was starting to think I cannot use hydro to locate contact with precision. Or - I have badluck and in stock SH3 my merchants are zig-zaging.


screenshot nr 6


3. This gave me a lot to think about. BdU gave me spotted contact on my map. I knew its course (+- 2 degrees). I tried to use hydro-method for the third time.


Again, from my calculations I learned that contact should be very close to me
. I didn't bother to mark that place on my map.

Those two small circles you can see on screenshot represents contact position marked by periscope observation. Note that real course of the contact was true to BdU report and was ca. 10 degrees off than my hydro calculations. BdU was right, not me.


screenshot nr 7



screenshot nr 8

Now to questions: what can be wrong? I'm 100% positive there's nothing wrong with my bearing. I can put things like that on my map. There's nothing wrong with my helping circle (except for example 1). I often pause the game to draw everything as perfect as possible. So?

I noticed one thing - the biggest challenge for me is to draw a 'virtual' bearing, one I would have made if I didn't start engines after initial 3 bearings. My problem is I'm having hard time judging distance. When I use angle calculated from helping circle (giving me course-direction) distance between 1-2 and 2-3 bearing are different. Eg. it's 2km in first 8 minutes, but 1,8km in the second. So how can I put that 'virual' bearing correctly? In your example Mittelwaechter you seem to use circle with radius = distance between 2nd and 3rd bearing. But in your picture this is the same as between 1st and 2nd.



Can anyone help me?

Sorry for suprisingly long post
Kubryk

PS
I don't know how to put clickable thumbnails hence the link under every picture.

[Mittelwaechter]

Hi Kubryk,

nice to see you are working on this, you are doing a good job there!

Whenever the angle between first and second bearing is small, add a few minutes and ask for the second bearing again to reduce error probability.

The bearing of the hydrophone operator is a cone. In 20 km distance the bearing covers a width of ~ 350 meters (2r pi /360)
The SH3 map tools are not designed to do these 'hydrohunt' calculations.
Try to center the bearingline in the 350 meter cone.

The best way to calculate the virtual bearing is by dividing the complete distance travelled on coursedirection and add the average.

After third bearing estimate the virtual bearing and try to achieve a big angle for the fourth bearing. You may want to surface and go flank for some minutes. Start immediately and do the calculations while sailing.
Try to come to a full stop before asking for the fourth bearing.
Make sure to follow your U-boats coursepath and try to work the fourth bearing exactly.

I have no idea what's going on, but checking your screenshots the initial contact would have been in ~ 5 km distance - according to your fourth bearing and the resulting real courseline. Your hydro operator is not that bad.

There is allways a difference between the real courseline and the calculated courseline, due to the map tools limitations - but it is small enough to still get a proper target solution. Even the distance is irrelevant as long as you head perpendicular to the courseline.

Try to 'correct' the weapons officers perfect settings and change the AOB for +/-10° - you'll still score a hit. Change the distance - Pythagoras will give you a smile.
The real source of error would be the contact speed. But if you want, you can calculate the speed from the complete travelled distance!
(timestamp the initial contact report... - or keep on restarting the stopwatch...)

The more you close in to your contact, the more error tolerant your setup.
Well, this all requires an exact 'cross bearing' to get your real contact position.

Have fun!


Edit: your conclusion about same speed is correct (your 8 knots story), but the assumption of a perpendicular course is not consequently correct.

[Kubryk]

Thanks for your reply! I noticed that small angle difference between bearings is somewhat inconvenient, still I'm worried that target would escape. It happened once, I took two bearings but third one I had to do manually, sonarman no longer could hear him. I will try to make that distance bigger though.

I try to accomplish two things thanks to your tutorial - it would be nice to know target's course just by hearing, and I would really like to sink at least one ship while submerged and without periscope. Must be a great feeling

I didn't know that bearing is a cone 350m wide, thanks for that info. I always try to center things, must be something wrong with my brain (I don't use this rule in photography, thank God )

I did all my calculations sumberged. I will surface after 3rd bearing and go flank, we'll see if it's going to help. Thanks for the tip!

Originally Posted by Mittelwaechter

There is allways a difference between the real courseline and the calculated courseline, due to the map tools limitations - but it is small enough to still get a proper target solution. Even the distance is irrelevant as long as you head perpendicular to the courseline.

I've been meaning to ask about this, I read that sort of statement a lot on subsim. How is it possible that distance doesn't really matter as long as you have speed and AOB correct? I don't get it, it's against my intuition

Originally Posted by Mittelwaechter

Try to 'correct' the weapons officers perfect settings and change the AOB for +/-10° - you'll still score a hit. Change the distance - Pythagoras will give you a smile.

I only use weapons officer to see how long till loading a tube, I'm my own weapons officer for calculating firing solutions

Originally Posted by Mittelwaechter

your conclusion about same speed is correct (your 8 knots story), but the assumption of a perpendicular course is not consequently correct.

I don't have a ruler on me so I will ask - same speed means same bearing ALWAYS? No matter how courses are related to each other?

It's true with parallel and perpendicular for sure, hm... maybe it's true always... no. It can't be.

[Pisces]

Originally Posted by Kubryk

Thanks for your reply! I noticed that small angle difference between bearings is somewhat inconvenient, still I'm worried that target would escape. It happened once, I took two bearings but third one I had to do manually, sonarman no longer could hear him. I will try to make that distance bigger though.

The hydro-man only starts calling out within 20km or so (or was it 25km?). But you can hear upto 34km. That range is 70% bigger, and almost 3 times the area of 20km range. At the bearing where the contact has an AOB of 90 degrees it only halve-way through your hydrophone range. So there is usually plenty of time before you really loose it. Unless it happends to be very near the extreme end of the range. Oh well, can't have everything.

Listening yourselve for the soundbeam-sides where the sounds die out is tricky. You do loose a bit of bearing accuracy that way. But listening for maximum volume at the middle bearing is VERY tricky. Then bigger bearing differences are a must.

Use the smaller bearing difference drawing to estimate how much time there is before it is near 90 degree AOB, or it leaves hydrophone range (if you don't mind catching up to it later). Remember, speed of contact is constant, so any distance it has moved along the course is proportional to time between bearings. Then use halve of the time available as the new interval (but be conservative as it is very crude). Or use the old 3rd bearing as the new 2nd bearing if that 1st new interval has allready past, and double the interval before taking the new 3rd bearing. The 3rd bearing doesn't have to end up at 90 degree AOB. It just a way to not fall behind.

But you must be suffering from information overload now. I'll let you digest it first. And alcohol doesn't help.

[Mittelwaechter]

Well, Herr Kaleun, your Navigator Konrad Schlumberger has been asigned to command his own U-Boot.
But during the last shoreleave Heinz Voigt - your hydrophone operator - visited the new hydrophone operator seminar at the U-Boot school in Kiel.

He came back with some great skills to support you in hunting contacts only with hydrophne.
He is able to calculate most of the tricky stuff to get the real courseline of an inbound contact under those conditions where the initial angles between the first three bearings are small. It indicates the contact is sailing along a course coming close to your position.

Heinz will give you constant reports of the bearings, including the information whether the contact is closing in or whether it sails in constant distance.
At the bearing this information changes, the AOB of the incoming contact is 60° (!).

Additionally Heinz reports proper distance statements and tells you if the contact is far away or at medium distance.

Whatever occurs first - note its bearing on the map. Draw a circle to represent the medium distance around your U-Boot.

(please someone provides the information what distance is medium in GWX - I changed my settings in Contact.cfg and didn't back up! I guess it was 8000 meters, but I'm not sure.)

At the time you have both informations, copy the courseline of the 60° AOB to the "medium distance" bearing at the point it intersects the medium circle.

You may want to change your position if the 60° AOB information comes first, of course heading perpendicular to the "courseline", to ensure the contact will cross your medium distance circle.

Well, the rest should be easy...