Real Navigation with SH4

[don1reed]

Howdy, Mike.

Like Father, like Son

DLing now.

...I'm back in the Pacific (Sulu Sea), mainly due to the SH4's F5 screen being in Lat-Long and Ruler tool measuring in nautical miles.

SH3 really gets to be a hassle with all the conversions, both metric to nm and Kriegsmarine grid to Lat/Long.

Now that's where I believe you & Python could do your miracles. Make a script that had the ability to convert KM grids into Lat/Long and visa versa.

[jmr]

Originally Posted by don1reed

The use of sights by sextant was completed 5 time per day, religiously like call to prayer--
Dawn star fix,
before noon sun sight,
rfix with noon sight + before noon sight,
rfix with noon sight + afternoon sight,
dusk star fix.

Don, what is rfix?

[don1reed]

Howdy jmr,

rfix = running fix

The formula: Long cos Lat is actually

Minutes of longitude x cos Degrees of latitude

I only abbreviated the formula ( sorry)

Caveat:

Just be aware when using this little gem of a formula on an 8.5" x 11" sheet of paper that it will only be accurate for about 2 degrees (120 nm). As a result, around here I go through a lot of paper.

Cheers,

[jmr]

Ahhh so that's what it was with regards to Long cos Lat. I asked how you chose your numbers originally but then edited out think I need to spend more time going over your explanation and accompanying chart. But I still didn't get it and now I know why :}


So do you still do your plotting with paper or is the F5 map in game suitable for that purpose?

With regards to accuracy on paper - could you just use a mammoth sheet instead of 8.5 x 11 ?

[don1reed]

Honestly jmr,

I use paper with SH3 due to the authentic feel of what "they" had to use back during the war years and the tediousness of F5 screen conversions. It slows down the game-play a bit but at my age I like to savor the experience...and...unlike the F5 I now have a visual record of my cruises.

I switch between paper & the F5 screen with SH4 as the F5 is in Lat/Long and a lot more convenient.

If you use those downloadable plotting sheets offered by 6SJ7GT (sprinkled throughout his posts) you can save some time and avoid the extra math...(that's the part I like, however )

Cheers,

[jmr]

What's the rule of thumb when picking stars to shoot? The tutorial says he chose Gacrux, Regulus, and Kaus Australis as they're about 40 - 50 degrees apart on the horizon. So, the farther apart the better the accuracy in determining your position?

Another thing I've been wondering about is whether or not you can use this method of finding your location if you don't have an assumed point to start from. Is it possible to just get your long/lat from the heavens alone or do you always need some sort of assumed starting point?

[don1reed]

3 stars are usually all you need & ideally 120 degrees apart.
Mariners try to make it easy on themselves and most often choose the brightest stars, no need to select the faintest, obscure ones...unless you're bored and have nothing better to do.

2 stars will work if they are at 90 degrees to each other. (sometimes thats all the clouds will allow).

The more bodies you use in your sight reduction, the more accurate your fix during foul weather.

[don1reed]

Oh man o man, Mikhayl, that is sweet !

[don1reed]

Neat. I'm really partial to Sugar boats in SH4 & Canoes (type II A thru D) in SH3.

All the best,

Don

[don1reed]

Holy moly, it blows me away just how much the smallest or unexpected details adds to the immersion.

Hats off to yo...

never mind, you guys keep getting better 'n better, might as well keep my hat off.



cheers,

[6SJ7GT]

Nice job, Mikhayl with your blessing I'll add those to the rest of the files.

To All.
I noticed the Almanac tables had a few problems and am updating them, I'll post when they are ready to DL.

Don,
If I put sunrise/set for lat 0 long 0 is there an easy calculation for the latitude difference? Also, have you taken a noon sight in Stellarium? how did that work.
I think I can add sunrise, transit and set to the daily tables.

Mike

[don1reed]

Hi ya, Mike.

Since ships at sea are usually moving, targeting the sun for a noon-sight is always an "educated guess". This is why the navigator(s) begins taking sights 1/4 of an hour before and after. This process develops a curve that is mathematically discernible, wherein Noon can be determined.

Ashore, one would pretty much know their latitude/longitude and can easily and precisely predict the exact moment of noon, i.e., extract GHA of hour before your own longitude from the daily pages:
example:

Lets say for sake of argument that my long is 90˚16!1W.

Daily almanac pages (today) shows the GHA for the hour before my meridian as 17 hours, GHA 75˚54!6.

I subtract the two and get: 14˚21!5--goto increment & corrections pages of the almanac and thumb through the pages until I find that number at 57min 26sec. I now have the precise time of my noon @ 17:57:26 UTC.

Finding the times for Rise/Set at sea is, again, a thing of educated guesswork; however, that is what the Captain's Standing Orders are for. They're instructions for the pertinent watches to send the duty messenger to fetch the navigator to the bridge at the appropriate times for taking sights. (Of course the Navigator would have already figured out the approximate time beforehand and would already be dressed by the time the messenger got to him.)

The right-hand pages of the almanac is where Twilight (Naut. & Civil) Sunrise/Sunset...Moonrise/Moonset is determined.

Longitude is interpolated from the data presented on those pages and is based on the ships dead reckoning position, so, again, it behooves the Navigator to be on deck during the right time “window”...all guesswork.

...and yes, I have used Stellarium to determine my noon fix. I begin as in RL, I start taking sights every couple of min or so 15min before calculated noon, and I continue taking sights until 15min past. I draw my time vs. alt. curve and determine time & alt of noon to do my calcs.

edit: I might add that sometimes I save and exit SH3 or SH4 20 or more times taking these noon sights. It gets to be time consuming but no more so than in RL.

I'm not always precise, but pretty dang close.

Cheers,

[6SJ7GT]

Thanks Don,

After looking at the noon sight I realized meridian transit is just dependent on longitude so that is easy to include. Sunrise/set changes with lat. I can put in the equatorial sunrise / set, is there a calc to determine the change for your lat? (the official almanac has sunrise/set for different latitudes).

edit: I might add that sometimes I save and exit SH3 or SH4 20 or more times taking these noon sights. It gets to be time consuming but no more so than in RL.

I usually leave a browser open, then I can just alt+tab and start stellarium with the game minimized. The game pauses when I do this, so I cant do ssomething else while the game is running. The game needs an XO popup that could summon the capt to the bridge or control if something came up.

Cheers
Mike

[don1reed]

I usually leave a browser open, then I can just alt+tab and start stellarium with the game minimized. The game pauses when I do this, so I cant do ssomething else while the game is running. The game needs an XO popup that could summon the capt to the bridge or control if something came up.

Sorry, my peanut whistle PC (WindowsXP) doesn't have the guts and Stellarium locks up on me when I try this...oh well...my real cpu is an iMac.

Formula:

Arcsin [(sin L sin d)+(cos L cos d cos LHA)] used for determining Hc is reworked...

Arccos [(sin d - sin L sin Hc) / (cos L cos Hc)] = Z.

background: @ sunrise the center of the sun is actually 50' (or 0.83 deg) below the horizon. Therefore, we assign an altitude Hc = -0.83 deg to our fundamental formula:

Legend: HA = Hour Angle.
L = Latitude.
d = Declination.

Arccos HA = [(sin -0.83) - (sin L sin d) / (cos L cos d)] <----This Formula covers both Sunrise & Sunset.

Then: LMT sunrise(mean sun) = (180 deg - HA) / 15

Finally: LMT sunrise +- Equation of time = sunrise (true sun)

Same formulae for Sunset except (180 deg + HA) / 15

-...-

Nautical Twilight center of sun is 12 deg below the horizon, then:

Arccos HA = [(sin -12) - (sin L sin d) / (cos L cos d)]

then:

LMT, nautical twilight = (180 - HA) / 15 +- eq. of time.

-...-

Civil twilight = center of sun 6 deg below horizon...etc.

[jmr]

So I finally have some time to do some real navigation with Stellarium but I've noticed that you need to have a pretty accurate assumed position for this to work. So what do you do if your AP is way off or if Bernard puked all over your charts?