The basic is TDW's real navigation. For additional tools look here under "equipment" => http://www.subsim.com/radioroom/show...&postcount=110 Also you need stellarium v11 and higher, which emulates the real refraction of stars and planets.

Is the difficulty with using a sextant on a submarine simulated? Using a sextant on a moving boat is another world from using it on dry land. How about atmospheric refraction and scintillation? Scintillation alone can make a 1º position difference in a star, forget about measuring minutes and seconds!

Thankfully, over the open water, the atmosphere tends to be much steadier than over land but scintillation and refraction are variable effects having different importance for each individual observation.

But that ability to actually do the mechanics of reducing observation to position is an incredibly valuable experience.

Now, in SH5 are the navigational map and tactical plot combined as in SH4 or do they have completely separate navigation and tactical plots? Because in SH4, introducing manual positioning destroys your tactical ability. In reality they were completely separate functions.

Edit: Yup! The refraction correction is there! Well done. Is that built into the game or did you introduce it with your mod?

Speaking of Stellarium a realistic way of simulating a sextant sight in there would be getting the apparent alt. instead of geometric when you click on any celestial body correct? If I'd like to have more errors as in real life would it be more realistic by using the angle tool and measuring myself?

That's what ETsd4 means when he says "which emulates the real refraction of stars and planets." It is the lens-like refraction of the light from astronomical bodies that shifts their positions, varying mostly by altitude above the horizon, but also in a lesser way by temperature and atmospheric pressure. In practice, you only need to consider the effect of altitude because positional shifts caused by the other two are smaller than the variations in your ability to measure.

BTW, regarding finding your position with the sun other than using the "LAN" or noon shot, I found this:

http://www.oceansail.co.uk/Downloads...Reduction.html

The thing is that when getting the geometric alt which in my understanding the the most accurate and using the apparent I get a huge difference from my assumed position (DR). In real life what is a good margin of error acceptable between actual position and Assumed position? I kind of find that using the geometric (once I was only 700 yards apart!!) is cheating.

No matter when you shoot the sun its position is beastly difficult to determine due to its position. The thing is big, half a degree in diameter and it heats the air up something fierce, making it swim all over the place like a blob of jello.

All that uncertainty can be boiled down into an error deviation in your position that would be quite large. That's not to say that it isn't better than having absolutely no idea!:D Navigators sometimes grab anything available in their quest to avoid COMPLETE ignorance....

Even in a three star computation done on dry land you end up with a triangle drawn, and your position is "somewhere in this here triangle." And from repeated use, you get an expectation of how large the triangle should be. If it's way too large, you start looking for computational or observational errors. If you find none, you do the whole blasted thing over again. "Hey Cap, can we surface again?"

From I've seen from a video is that you get the sun's declination, than compare it using your LHA or GHA then with GMT you can kind of guess which latitude you are near. Is this correct? I just don't want to be limited in noon shots during the day...

Yes it is true that you can get some idea of your longitude by measuring the altitude of the sun at a given time. Since the sun moves 15º per hour through the sky (360º / 24 hours), by knowing GMT and knowing the sun's altitude you can determine how many degrees you are from the zero meridian, within limits.

The reason noon sights are useful is that by repeated sightings before and after noon you can interpolate the sun's position exactly at noon local time. It just gives a more accurate number than using a randomly chosen time of observation and being a prisoner of one shot.

Could you give me a step by step as how you would perform a "sun shot" other than noon ? I am going nuts about this. I know noon is better but I gotta do something during the patrol in daylight other than noon. Keep busy. me real time. me no like fast forward time :salute:

Let's do one for sunset. Atmospheric refraction is about 34' at the horizon, raising the position of the sun by almost exactly one diameter. In addition, when the limb of the sun touches the horizon, the position of the sun (measured at the midpoint of the disc) is 15' above the horizon. (Yeah I know the radius of the sun changes a bit with the time of year and isn't exactly 15' and........go away, my head hurts already!:timeout:)

So the moment the sun's lower limb touches the horizon, the true position of the sun must be corrected 15' higher (correcting for the diameter) plus another 34' lower (because refraction raises the apparent position). So your true altitude is 0º -34' + 15' or -11 minutes.

Now let's have fun! There are two kinds of minutes and two kinds of seconds, those of arc and those of time. And we're going to translate minutes of arc to seconds of time. Why? We know that at the exact instant of sunset our local time is 6:00 pm.

Because we are measuring something we can more easily detect, first contact of the sun with the horizon, instead of something quite difficult to observe, the time the center of the sun is coincident with the exact position of the true position of the horizon in the sky (uhhhhh......that's 34' above the horizon--no point of reference there) we have to determine our exact local time when we see that limb touch the water.

Basic math: the sun moves 15º through the sky in an hour, 360º divided by 24 hours. That means it moves one degree in 4 minutes of time. (60 minutes divided by 15 degrees gives you 4 minutes per degree).

Now it gets messy. That means 4 minutes time for the sun moving 60 minutes of arc. Let's change that to seconds of time: 240 seconds of time for 60 minutes of arc. We can further reduce that to 4 seconds of time per one minute of arc. Do you follow okay?

So we can apply our correction. The time we measured was not 6:00 pm, but the time when the sun was 11' of arc lower, later in time. We know that 1 minute of arc takes the sun 4 seconds to move, so 11 is 44 seconds later. Our local time was 18:00:44. Hope you recorded exact GMT at the instant of observation because you need it!

I would subtract GMT from local time to calculate the time difference. Express that in decimal hours instead of hours:minutes:seconds and multiply by 15º. That is your number of degrees east of the prime meridian. Convert to deg:min:sec. Translate for E and W longitude by subtracting 360 if the number is greater than 180º (it then becomes west longitude, expressed as a negative number) and there's your longitude!

I'm doing this off the top of my head with no reference materials so if I've made a mistake somebody whack me and get the right info out there!:salute:

Wow Robbins, that was too much of a crash course on celnav. Too fast too much info for me :dead::dead::dead::dead: :D

Anyway thanks for the info I'll save it and will try to do it live when I get my sextant in a couple of weeks (Davis Mark 15 or 25? doubts doubts...)

OK. Going back coming out from Warp speed.

From your last post I recognized that you were doing the Longitude by noon sight/GMT but instead of noon it was during sunset. I'm still trying to sort out LHA/GHA and d' (I started to read celnav only 10 days ago). Well, my question is that in what sort of way does making this "sun shot" during sunset and sunrise helps? i.e. Having the Sun at zero degrees from the sextant...

Other one: Can I still find my latitude with the so called "reduction sight tables"? Match the Sun's declination with my GMT at the nearest Latitude on the table? This being done with a noon/sunrise/sunset shot.

Last one (I promise :DL)

The SH4 nav pack file comes with info such as GHA and Sun's declination from 1939-45 plus meridian passage and sunrise/sunset. Is that all it takes or I need more info from the almanacs?

Sorry to ask these I do not intend to have you present a full lecture on celnav...

I am doing some experiments with Stellarium 11.0 and SH4 and will post a separate topic regarding this once I am done.

I was just curious with the instructions located here: http://www.oceansail.co.uk/Downloads...Reduction.html

at the bottom of the page.

ETsd4, I think I understand now. The apparent altitude takes account the refraction and other "real" factors from the real declination. Using the geometric alt. in Stellarium would be cheating or having a super sextant with a bionic eye attached taking accurate readings...So using the apparent altitude is much more realistic.


In one of the forums at stellarium's page they state that the altitude that you set is to simulate the refraction and atmosphere. Not the actual altitude of the observer. Since in SH you are always at sea setting zero or 2000m won't make a difference in the sextant reading.

http://www.cse.yorku.ca/~amana/personal/navigation/

That's roughly 1200 feet, or 400 yards, or 365 metres. Very accurate for the open sea, not so good in a harbor, which is why harbor navigation is done by eyeball. :sunny:

In practice, at sea, in a small craft or a submarine, you're doing incredibly well to get within 20 miles of your true position. Larger errors than that are very common, leading to methods of navigation that take into account a presumed statistical error.

For example, you know you're likely within (don't you love that word likely? If you're a worrywart, it sure doesn't help the sleep process!) 30 miles, that's plus or minus 30 miles! That means there is a 60 mile range north/south and east/west that you could be in. You're approaching a coast and want to make sure you go pretty directly to a port.

Do you navigate to the position of the harbor? NO!!!!! That would be stupid.:arrgh!:

Why is it stupid. Well, your position is not a point but a probability distribution 60 miles long. When you achieve landfall there are as many possible positions north of the harbor as there are south. You ARE Shoedinger's Cat, and you've just been let out of the box. Quick, which direction do you turn to get to the harbor? Choose the choice of your choice, but either way there is a 50% chance that you have actually turned away from your destination! How cool is that?:rotfl2:

Now let's do it taking into account the error range of +-30 miles. If instead of aiming at the port, we aim for a point 55 miles to the north, we know two things. If we end up on the south end of the error probability range, we'll be in sight of the port and can easily turn whichever direction we need to. When we get there we'll brag about how accurate our navigation is.

If we come to landfall and can't see the port, we KNOW we're north of it. We turn south decisively, like we know what we're doing ('cause we do!) and sail directly into port, where we brag about how accurate our navigation is. In practice it is normal to aim 100 miles from your destination to ensure that you turn in the correct direction. If you wanted to end up at Daytona Beach you would aim for Jacksonville! Of course I'm ignoring that when you get within 50 miles of Daytona you could pick up a radio station and vector in on that. I'm assuming you are using only celestial navigation.

The gradations on the sextant are irrelevant in determining how accurately you can navigate. On a boat getting to within half a degree is darned good shooting. Like I said, atmospheric scintillation can make Sirius dance around like it hasn't been to the toilet for 24 hours. I've seen it move more than half a degree and there is no way you can fix its exact position. The accuracy of the instrument far exceeds the accuracy of the man.

These videos are not extreme examples. Note that Sirius also flashes all the colors of the rainbow while it does its "I gotta go" dance.
https://www.youtube.com/watch?v=jwn6..._order&list=UL
https://www.youtube.com/watch?NR=1&v=4f35gIWdCz0
https://www.youtube.com/watch?v=umvgCr32ssk