Real WWII Nav Accuracy

Lanzfeld · 12-23-08, 05:36 PM

I am playing around with doing real navigation and, for me, the best realism comes from not having my sub displayed on the map and use dead reckoning to navigate. When I have a clear day at sunrise or sunset I allow my nav to get a sextant shot to update my position. I used to do this by zooming in and "control-left-click" (CLC) to get a fix but this was too accurate for me. So what I am toying with now is not using the CLC meathod but I zoom wayyyyyy out to the 1000 km or the 2500 km map (the scale in the lower left corner) and have enabled my sub icon to be visible there. I click on my icon as best I can and then zoom in so the sub icon goes away again. Obviously I lose some accuracy but that is the intent!!! The farther out I zoom to mark my sub icon the less accurate my fix is. I have been testing this and immediatly zooming close in to CLC and see how accurate I was. Sometimes I am 6km off, other times I get it down to 1km. It really depends on my zoom when I do the marking plus some slop. Sounds real to me!

What I wonder is how close to real position could an average german navigator be with his sextant? This will give me a good idea what zoom to stick with so my error will be close to what real errors were. With a good sunrise, were they accurate to 1km or were they lucky to be within 20km ??? Anyone know this??? Anyone with sextant experience?

Lanzfeld · 12-23-08, 05:39 PM

I googled it and it said that a good navigator can get it down to about 1/4 mile (460 meters) but that is with a modern sextant. What about a WWII model???

GoldenRivet · 12-23-08, 07:11 PM

i remember reading in "Torpedo Junction" that the margin of error was something on the order of 5 to 15 miles give or take depending on conditions and skill of the navigator

Lanzfeld · 12-23-08, 07:29 PM

Quote:

Originally Posted by GoldenRivet

i remember reading in "Torpedo Junction" that the margin of error was something on the order of 5 to 15 miles give or take depending on conditions and skill of the navigator

That is quite a bit different then 500 to 2000 meters. Hmmmmmmmm

Anyone use one of these things on your boat?

geosub1978 · 12-24-08, 02:55 AM

Quote:

Originally Posted by Lanzfeld

I googled it and it said that a good navigator can get it down to about 1/4 mile (460 meters) but that is with a modern sextant. What about a WWII model???

Modern sextants are the same. Only some few fixed mounted-stabilized eixst, which are very rare, as a common GPS reciever is much cheaper, faster, more accurate!

Pisces · 12-24-08, 07:47 AM

It's time for Don1reed to show up again. He usually does when this subject rears its head.

For the time being, ... and for what it's worth, as I never used a real sextant:

I think that 5 to 15 miles would be the deadreckoning accuracy after long periods without star fixes. Not the sextant/celestial navigation accuracy. That would depend in a large part on the measurement resolution of the sextant. As 1 arcminute is about 1 nautical mile on the surface it depends on how fine the readout is. Ofcourse it also depends on how steady the sextant is held by the navigator.

6SJ7GT · 12-29-08, 05:28 PM

Interesting thread,

from my reading, 1.5 miles is the practical accuracy from a moving platform. A good sextant is capable of 1/10 arcminute of accuracy, and as the design hasn't changed much for a long time (it has been that way since the 18th century). Here is site on the history of the sextant.

here's a couple of screenshots using celnav in the game map.

This uses the method of transferring the boat's position to Stellarium.

the last sight on Dubhe, I used the USNO almanac so I only had to calculate the diff of Hc & Ho. Use a convenient close intersection for the AP (32N 139 30'E):
at 03:04:30Z
Hc 17 48.9' Zn 26.7 deg both from the USNO Almanac.
Ho 17 33.6
Dist +15.3

I used the compass to find the 15.3 mi offset, then just had to use the protractor (with the tool helper to get the proper Zn) to draw the sight line and perpendicular. Here is the fix.

And the boat revealed. It was approximately 1000m from the fix but has travelled some.

I hope you get this working as using a sextant in game would enhance the immersion.
Mike

6SJ7GT · 12-29-08, 05:41 PM

ps

date on the above is 8 May 1942.

Mike

12-23-08, 05:36 PM	#1
Lanzfeld Grey Wolf Join Date: Jan 2002 Location: New Tripoli, PA Posts: 994 Downloads: 64 Uploads: 0	Real WWII Nav Accuracy I am playing around with doing real navigation and, for me, the best realism comes from not having my sub displayed on the map and use dead reckoning to navigate. When I have a clear day at sunrise or sunset I allow my nav to get a sextant shot to update my position. I used to do this by zooming in and "control-left-click" (CLC) to get a fix but this was too accurate for me. So what I am toying with now is not using the CLC meathod but I zoom wayyyyyy out to the 1000 km or the 2500 km map (the scale in the lower left corner) and have enabled my sub icon to be visible there. I click on my icon as best I can and then zoom in so the sub icon goes away again. Obviously I lose some accuracy but that is the intent!!! The farther out I zoom to mark my sub icon the less accurate my fix is. I have been testing this and immediatly zooming close in to CLC and see how accurate I was. Sometimes I am 6km off, other times I get it down to 1km. It really depends on my zoom when I do the marking plus some slop. Sounds real to me! What I wonder is how close to real position could an average german navigator be with his sextant? This will give me a good idea what zoom to stick with so my error will be close to what real errors were. With a good sunrise, were they accurate to 1km or were they lucky to be within 20km ??? Anyone know this??? Anyone with sextant experience?

12-23-08, 05:39 PM	#2
Lanzfeld Grey Wolf Join Date: Jan 2002 Location: New Tripoli, PA Posts: 994 Downloads: 64 Uploads: 0	I googled it and it said that a good navigator can get it down to about 1/4 mile (460 meters) but that is with a modern sextant. What about a WWII model??? Last edited by Lanzfeld; 12-23-08 at 05:39 PM.

12-23-08, 07:11 PM	#3
GoldenRivet Subsim Aviator Join Date: Mar 2007 Location: Texas Posts: 8,726 Downloads: 146 Uploads: 0	i remember reading in "Torpedo Junction" that the margin of error was something on the order of 5 to 15 miles give or take depending on conditions and skill of the navigator __________________

12-29-08, 05:28 PM	#7
6SJ7GT A-ganger Join Date: Mar 2008 Location: Wi. USA Posts: 71 Downloads: 10 Uploads: 1	Interesting thread, from my reading, 1.5 miles is the practical accuracy from a moving platform. A good sextant is capable of 1/10 arcminute of accuracy, and as the design hasn't changed much for a long time (it has been that way since the 18th century). Here is site on the history of the sextant. here's a couple of screenshots using celnav in the game map. This uses the method of transferring the boat's position to Stellarium. the last sight on Dubhe, I used the USNO almanac so I only had to calculate the diff of Hc & Ho. Use a convenient close intersection for the AP (32N 139 30'E): at 03:04:30Z Hc 17 48.9' Zn 26.7 deg both from the USNO Almanac. Ho 17 33.6 Dist +15.3 I used the compass to find the 15.3 mi offset, then just had to use the protractor (with the tool helper to get the proper Zn) to draw the sight line and perpendicular. Here is the fix. And the boat revealed. It was approximately 1000m from the fix but has travelled some. I hope you get this working as using a sextant in game would enhance the immersion. Mike __________________ If you like realism, navigate the way they did in ww2. Celestial Navigation for SH3 / SH4

12-29-08, 05:41 PM	#8
6SJ7GT A-ganger Join Date: Mar 2008 Location: Wi. USA Posts: 71 Downloads: 10 Uploads: 1	ps date on the above is 8 May 1942. Mike __________________ If you like realism, navigate the way they did in ww2. Celestial Navigation for SH3 / SH4

12-24-08, 07:47 AM	#6
Pisces Silent Hunter Join Date: Dec 2004 Location: AN9771 Posts: 4,904 Downloads: 304 Uploads: 0	It's time for Don1reed to show up again. He usually does when this subject rears its head. For the time being, ... and for what it's worth, as I never used a real sextant: I think that 5 to 15 miles would be the deadreckoning accuracy after long periods without star fixes. Not the sextant/celestial navigation accuracy. That would depend in a large part on the measurement resolution of the sextant. As 1 arcminute is about 1 nautical mile on the surface it depends on how fine the readout is. Ofcourse it also depends on how steady the sextant is held by the navigator.