Real Navigation with SH4

[6SJ7GT]

I got time for a closer look at your plot sheet, and it is a piece of artwork. I see you have a running fix and sun sights, as well as star sights and dead reckoning. I am relatively new to this, and your plot brings together some things I have read about but haven't yet tried.

I have been working a plot, then transferring the position onto a larger scale area map, but I see you keep working up new plots on the same sheet. Post some of your interesting ones when you get them.

Thanks,
Mike

[don1reed]

Don't know if you have one of these, but it's indispensable for chart work and can be purchased from any good seafaring store. (Celestaire, Starpath, etc.)


page 2

Here's Page 2 from the post above. Mariners always (in real life) denote Dead Reckoning (DR every hour on the chart, symbolized by a dot surrounded by a half circle, a FIX is symbolized by a dot surrounded by a small circle on the course line). All course changes are shown as DR + time. BTW, I didn't place alot of DR's on the plot chart due to its small scale and it would really start to clutter up; but, on a real chart, there is plently of room to draw them in.

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.

In the next pic, you'll notice that I let the helm run free and over time, the boat is beginning to drift to starboard, all picked up by celestial navigation.

Hope these helped.

Edit: btw--you'll notice that I use each junction of Lat/Long nearest my course as Assumed Positions (AP). In real life when using HO pub 229 AP is based on GHA and Real Longitude. If you're trying to reinact and be authentic, its useful to know that 229 and 249 weren't around during the war years; however, 211 (Ageton) and 214 was if you have a copy of those. Ageton is very compact compared to 214 which was later replaced by 229.

Forgive me, edit2:

Don't know why I didn't think of this sooner--BUT, a mariner doesn't really need any of the tables to navigate by--All that is really needed is two fomulae and a Nautical Almanac for the time period.

1) Hc = asin [( sin Lat sin Dec ) + ( cos Lat cos Dec cos LHA )] and,
2) Z = acos [( sin Dec - sin Lat sin Hc) / (cos Lat cos Hc )].

These two formulae can be easily solved with electronic calc or the old way with sliderule.

Cheers,

page 3

[castorp345]

these are great to see and very inspiring for the budding virtual navigator.
many thanks for your fine posts, Don!



cheers
hc

[don1reed]

Tnx castorp345; but the guy that needs to take a bow is, 6SJ7GT. His script plus "Python" & "Stellarium" have knocked the ball out of the park!

...lol...you can't imagine how much paper trash I've got lying about my feet!!!

[castorp345]

Originally Posted by don1reed

the guy that needs to take a bow is, 6SJ7GT. His script plus "Python" & "Stellarium" have knocked the ball out of the park!

absolutely!!
i've installed the "Python-Stellarium" setup and it works brilliantly! my only "issue" as such is that in stellarium the deck is too steady (not to mention anchored)...


seriously though, this is a great and game-practical way of practicing navigational trig.

i'm not familiar with python, but would there be a way of introducing a randomized 'fuzziness' to the location given to stellarium based on in-game prevailing weather conditions? also (just throwing ideas around here), would it be possible via the python script to increase the fog-factor in stellarium to simulate in-game visibility/obscurity?

in any event, many thanks 6SJ7GT for your wonderful work!


cheers
hc

[6SJ7GT]

I'm glad to see this getting some use.

HC, you could introduce some randomization to the sub location but this would still give you an "anchored" position in stellarium. You could speed up the time in Stellarium, this would increase the rate of change of Altitude and make your reading a little less accurate. There are settings for fog, minimum brightness of displayed stars, and a few other settings you could play with. You could put these in manually via the script, but I would play with those settings in stellarium first to see if that would accomplish what you want.

I like Don's plot sheets, they would be a nice touch on the navigator's table in the control room

Mike

[don1reed]

I realize that all of this is not "Bill or Rights" or of constitutional proportions, or TMO/GWX quality yet; however, let me say that my first excursion using this method has proven to me to be the best yet.

The Wx I experienced from Manila to Formosa has been:
Clear skies; no rain or fog; Wind: 5 kn/360; Sea: Beaufort 2 the entire journey, so far.

In my estimation, the thing to do while serving as Navigator:

1) Be on the bridge in the sim and select the heavenly bodies to be used for navigation purposes.
2) Save game, then resume--and while sim is running so as to keep the clock running,
3) Enter Python with UTC date/time, enter Stellarium
4) Only use the bodies previously selected, gather the necessary data. Exit Stellarium and return to the sim.
5) With sim running all along, reduce your sights and plot them accordingly.

The fact that the clock has been kept running adds another real life adventure. As all know, RL doesn't have a pause button.

(Some single-handed Captains I've known on certain voyages didn't get a chance to plot their positions until a day later due to conditions--interesting--old data is sometimes better than nothing)
edit: sometimes the overcast is so thick all that is visible of the sun is a fuzzy cotton ball without clear definition of outline...so, center of mass is used. It's not as accurate, but it works.

More to follow, I'm hoping...

Don

[castorp345]

@ 6SJ7GT: can you recommend some good python source documentation (online) that would outline the sort of code that would be necessary to achieve these sorts of randomization effects? thanks in advance.


@don1reed: these are great tips, esp about leaving sh4 running... many thanks and keep 'em coming!


cheers
hc

Is the concept of using a sextant inside SH4 given up ??

I have redone the draggable sextant so it works in SH4 ver. 1.4 and with any patch on request (i just need some files from your install and plug new values for the cameras.dat, scene.dat and menuxxx.dat unless you want to hassle with that your self)

If we can live with using the 30 most (or lowest) degrees of sky then we can make a pretty precise overlay for the sextant covering 30 degres of sky over your total screenheight. Thus making it much more precise then before(twice the precision pr. pixel)

So is the problem with the stars that they just do not show the right place or is it that the timing on the watch is not good enough(second hand missing)??

I have found that using the draggable sextant (moves with the stopwatch) at the deckgun position works pretty fine as it is stabilized and enables good reads. Problem is the sextant has very thick lines for the readings and no intermedient ticks to cover more precise readings. Correct me if im wrong in my memorys here please - i just "surfaced" in the SH4 world again after a longer brake....

[don1reed]

First off, here are a couple of items that might be worthwhile sharing:
Items needed to navigate with SH4-
1) Small protractor
2) Scale with cm/mm on one edge and 10th of inch on the other.
3) You don't have to D/L special plot sheets all you need is a piece of blank paper. See above, I show my current position just SW of Lambay Island on the SW of Formosa (Now Taiwan).
Use the formula Long cos Lat to convert your scale mm, 10th of inch, or cm into lat and long. Here's how:

example: in above pic my last position is Lat 22° 18'N, 120° 19.9'E. I happen to be using 10th of inch as nautical mile on my plot, therefore 19.9' x cos 22° = 18.5 tenths of inch (1.85") east of 120° meridian indicates my longitude. Since I'm using 10th of inch as my nautical mile, Latitude will be read directly off the 10th of inch scale.

4) Nautical Almanac for war years 1941 to 1945
5) Formulae for determining the calculated height of the body and it's ground position azimuth:
1) Hc = asin [( sin Lat sin Dec ) + ( cos Lat cos Dec cos LHA )] and,
2) Z = acos [( sin Dec - sin Lat sin Hc) / (cos Lat cos Hc )].

In the above pic I've shown the math envolved in determining the Hc and Z using the above formulae. If you'll note once the Greenwich Hour Angle (GHA) of the sun was obtained, since I'm in East Longitudes, I must add my Assumed longitude to the GHA to obtain my Local Hour Angle (LHA) as a whole degree (317° in my example above). if I were in West Longitude, I would subtract my longitude from the GHA.

The increments of 120° 20.6'E = 20.6 x cos 22 = 19.1 (1.91") east of 120° on my plot indicates my assumed position. From this spot I construct a line 134° toward the sun.

Since the calculated sextant altitude (Hc) was larger than what I observed (Ho) the line of position (LOP) will be away from my assumed position. The rule of thumb = HO MO TO ( if Ho is More then the LOP is Toward the body).

On the plot sheet above, you'll also notice that I did a 45°-90° running-fix on Lambay Island. It's a timed event and it put me about 1.5 nm off and as you can see that it put me about 1 nm SE of my Sun sight LOP...so, what are you going to believe--your lying eyes through the TBT or the sun sight?

Pic below show my position on the F5.


One final thought about making your own plot sheets. Accuracy starts to fall off beyond 2° (120 nm) so you'll have to make more of them.

Cheers,

[6SJ7GT]

Looks like this thread is picking up some interest again.

castorp345 - you can find all of the python documentation including a tutorial at docs.python.org. Also, look at Dive Into Python, another excellent learning source. Don't let the title "for experienced programmers" scare you away as it has detailed explanations about python programming. also found this while looking around - Pyephem - With this you could make an almanac.

Mav87th - I looked at the game sextant before coming up with this method. The game sextant only reads to the degree, which is 60nm in distance (approx 1 deg. latitude). if someone could come up with a vernier or dial for reading to the minute of degree this might turn into a useful instrument depending on the accuracy of the game sky.

Don - I continue to be impressed with your posts. Reading about celnav and muddling through sight reductions gave me some basic knowledge of the subject, but seeing your plots really helps my understanding of the whole picture. You show how the plots fit with dead reckoning and coastal nav techniques. A few posts back you showed a pic of your TSD calculator, I use my old flight calculator as it works the same way.

On my SH4 nav page is a link to an Excel spreadsheet I put together. It has most of the intermediate tables, several worksheets you can print out, and links to online almanacs. The worksheets are set up to print out for manual sight reduction (no formulas). After working through the tables for sight reduction I am beginning to see how you would find using the formulas easier.

Regards,
Mike

[don1reed]

Tip 'o the hat to your genius, Mike. Well done. Have you tried it with SH3 yet?

[6SJ7GT]

I don't have SH3.

I'm Working on a .5 minute version of Ageton's for the spreadsheet, but have to check it some more. Building them from the formula's is fairly fast and gives me some understanding of the tables relation to the formula's
My son and I took some sights with the aircraft sextant, I'll let you know how they came out later.

Mike

[don1reed]

Have you been to Henning Umland's site at www.celnav.de/ ?

I believe he already has a compact Ageton tables avail as freeware.

[6SJ7GT]

I found Dr Umland's site and did a quick read through his online book. I also have his Ageton's tables. reading through the explanation gave me the formula's to build my own into the spreadsheet, mainly because I wanted all the tools in one place for my own convenience. Using the spreadsheet to build the tables from formulas also gave me a better understanding of the table relationship to the formulas you listed. I may eventually build a spreadsheet with your formulas, but right now I am having fun learning to use the tables. This project makes math interesting again.

Mike