Real Navigation Mod

Originally Posted by ichso

A new attempt 2 days later:
Lat: 47.5°N
So referring to the Van's Sun Almanac and interpolating between 40°N and 50°N again I should have seen the sun rise at 7:49 ( 8:49 loacl time at 15°W)
I tried to use the 'red light off' moment as my local SR time. this was at 9:10.

So I am 21 minutes behind the SR time for 15°W which would mean that I am ~5° farther west, so at ~20°W, which would meet my real longitude well (still being ~20.5°W).
I am not sure what to take as my referring point for the SR yet but the moment when red light turns off (= sun is to be seen by half of it's amount) fits well in this example. I will test this a little further tomorrow.

When you write 47.5*N is that 47*5'N or 47*50'N ? I assume the later

[ichso]

Originally Posted by Mav87th

When you write 47.5*N is that 47*5'N or 47*50'N ? I assume the later

Yes, sry, I meant 47°50'N

Originally Posted by don1reed

1)Those SR/SS tables, both van's and IRL are in "LOCAL TIME". Its up to the mariner to compute UT (GMT).

Am I missing something again ? Van's SR/SS Almanac gives no Information about the longitude for any specific SR time. So it's the local time of what local point ? The only thing these SR/SS times can refer to is the GMT or am I wrong there ?
As I wrote in my previous post, the last time I tried to calculate my longitude I just used the time difference between the SR time from the almanac and the time my red light got turned off. At that moment I read the time from the upper clock, the one that pops up when you point with the cursor at the clock.
This difference was about ~1.3 hours (it was 9:10, the SR from the almanac read 7:49). And that calculated into my longitude ~20°W which was correct.

So I have the theory:

Originally Posted by don1reed

3) I found out yesterday, while sailing NW in U.47 (AP 55° N, 7° 31' E), that the lower (white background) clock in SH3 dropped back one hour when the boat crossed meridian 7° 30' E, the boundary line between Prime Meridian 000° and 015° east.

That this doesn't matter for the SR/SS times you get from the almanac if you use the points of "redlight on/off". This seems to be exactly the time when the sun sticks half out of the water and maybe just that little time difference to local time to even out the effect that the local time switches not at 15°W/E, 30°W/E, ... but at 7°50'W/E, 22,5°50'W/E, ...

I will test that for some different longitudes this evening to see if it fits then too as it did yesterday.

[don1reed]

Am I missing something again ? Van's SR/SS Almanac gives no Information about the longitude for any specific SR time. So it's the local time of what local point ? The only thing these SR/SS times can refer to is the GMT or am I wrong there ?

Since the globe is, for navigational purposes, considered round, then its been established that the sun rises and sets at the same time in each of the 24 time zones.

When the sun is over my RL meridian, its RL noon in local time. When that occurs, it happens to be around 1800 GMT. There is a 6 hour difference between my RL local time and GMT for my RL location. Based on seasonal earth moving events (earth 23.5° tilt), SR/SS changes in timing perspective depending on your Latitude. Thats why the times are different day to day, month to month, etc.

If you take another look at the sun almanac you will notice that every SR and SS occurs at around the same time, i.e., SR in the morning and SS in the evening. If the table were based on GMT then the SR/SS times would all be different.

example:

Say you and I are stationary; I'm at 55-N 8-E and you're at 55-N 90W...

Based on the Almanac, the sun will rise and set at the exact same times...except your SR/SS will occur 6.53 hrs. later




As the earth rotates, the sun rises the same time locally as seen from the same latitude.

[ichso]

The method I mentioned, using the moment when red light turns off as your SR time, seems to work well.
Just know your latitude as precise as possible. From Van's SR/SS almanac get the current local SR time by interpolating between the next bigger and next smaller given lat.value there.
Then just wait for the exact red-light-off-moment (minute wise) and get the difference from the above calculated SR time. This time difference is needed in hours, for example are 75 minutes of time difference to be used as 1.25 hours.
Multiply that number by 15 and that are the degrees your off from Greenwich.

I worked myself from the coast of Northern Africa up to Scotland and from there back to St.Nazaire with very good results of position calculations.

Had no ship encounter on the hole patrol though, nearly ran out of fuel before returning to base

We are basicaly working with 3 "times" here

BASE TIME: Gameclock (in SH-4 thats easy as there is only one! In SH-3 Don's findings shows that its NOT the one with white background)

GMT: Well that says it self. Van's SR/SS almanac is GMT times for SR/SS at 10 different lattitudes for each date all year in 1939 (Valid for all the war years)

LOCAL TIME: The "missing link". The time we need to convert the two others into in order to make our calculations. (Or if you want to skip that then convert all the SR/SS's into BASE TIME or vicaversa GMT's to BASE TIME)

Those convertions are what i ****ed up the most in the start, now im starting to get the hang of it. Just crossed the I-dateline in SH-4 after a great start from PEARL. First Sunset at around Lng 177E sat me 30nm off my real Lng. Im now waiting for the stars to shine through so i can take a fix on a couple of low stars. (I have found when using stars hanging low on the horizon to be most precise).

The fast and easy thing would be to shoot Polaris and take that reading for granted for Lat., witch is the way i do it when crossing the pond on the long jurneys. Then ocationally i take a 6 star fix and calculate the position from the star GP's etc etc. with Navigator. That takes me within 15nm most of the times (when im controling it)


Redlight time in SH-4 is DEFINETLY the correct SS/SR time - only thing missing is seconds in van's almanac to make it more precise, but i can get that from Navigator.

It's correct that you need to know your Lat. pretty precise, but if you sail a heading for an amount of time you are not going to end up to far from where you plan, unless you fastforward x1024 or more. In that case you have to sit "watch" at the helm and click it every time it strays from the course.

[ichso]

One thing that is still missing (I think) is a good speed conversion chart for the various weather conditions.
I found that looking at the speed gauge and trying to estimate the average speed from that differs too much from the average speed unter 512x TC so this would still result in wrong position markings by tracking course.

A really complete chart would be to huge I think. Then one would have to write everything down for each uboat available and with each combination of speed effecting upgrades available for it.
Maybe there are just some factors by which the sub's maximum speed is reduced under certain wind strenghts, which wouldn't be all surprising.

For example: In early 1941 I travelled many days in my VIIC under wind speeds of 9. Some time I calculated a speed of 9,8kn from my course, which would be 18,47km/h. This was good enough to use to travel in any direction as long as the wind stayed at 9.

So it seems as this would really only depend on the wind and a little helpful table would be nice. So that one could just read the values he needs.

[don1reed]

Van's SR/SS almanac is GMT times for SR/SS at 10 different lattitudes for each date all year in 1939 (Valid for all the war years)

Van's sun almanac and SR/SS MR/MS tables on the right hand pages of the Nautical Almanac are in Local Mean Time, not GMT.

See:
http://www.irbs.com/bowditch/pdf/chapt18.pdf

section: 1809 Local Mean Time, paragraph 2

Today I'm testing my patience as not much to do at work, probably everybody's waiting for US Interest Rate Statement...

Stormy weather but managed to pick a brake in overcast. I'm halfway between Ponto Delgada and Lisbon CF65.

Trying to nav without that silly helmsman and correcting my course constantly at max x128... Calculated speed deviation and checking my positions every 9 hours by sending a report to BdU (it gives your grid) so it fits very nicely when I should enter next grid. Also did some math to calculate my lat at SR time (didn't catch SS though). As checking with the maps lat looks fine - 38°N. I like that part very much because lat fits very well, I'm on 215° course so my adjustment for lat was also not that simpe. Also I don't align my bottom of the screen but give it some space to precisely adjust sextant.

Now the hard part. SR was at 5:34 (4:34 local as I should be between 7,5°W and 22,5°W). Which value should I take, GMT? With GMT that gives 1h 6 minutes difference so 1,1x15= 16,5°W and it also looks fine! Impressive Ichso. Very simple method and probably you proved it to be right.