Learning celestal navigation

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Re: Learning celestal navigation

Postby laukejas » Mon Nov 07, 2011 11:00 pm

Yes, I understand we are testing, I just hope to find some temporary solution, like these 41min 18sec correction.
I just made few more sightings - sometimes this 41:18 correction works, sometimes not... Especially it doesn't work with sun.

Here I reupload: http://www.sendspace.com/file/0p2ki2
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Re: Learning celestal navigation

Postby laukejas » Tue Nov 08, 2011 3:45 pm

Or take a look at this http://www.sendspace.com/file/2yk56j scenario. Sight sun (pause as soon as scenario loads). With default GMT time, LOP is off by 41.3nm. To get correct LOP (0nm), I have to subtract 4 minutes and 53 seconds from GMT.

Can it be that sun sighting requires 4 min 53 sec (or some other) correction, while stars - 41min 18 sec, as you suggested? See if you get same results.

Edit: And this http://www.sendspace.com/file/1dfmrr scenario requires minus 2 min 40 sec correction for correct LOP. You can check, if you want.

I can't see any pattern here. Looks like that celestial sky is unstable or something, since I can't find constant correction for the sun sighting.
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Re: Learning celestal navigation

Postby ron » Wed Nov 09, 2011 7:34 pm

Hi, yup we are on a good track here.
I think stars move on 1 sphere in VS, the sun moves on a second sphere, the planets move on a third one and the moon on a fourth one.
Now, for each of these 4 you will have to find the offset? Don't look at Ilan to update his VS code. VSF is the new platform.

Saying that, I have sailed (commercially) for some 15 years and have made I don't know how many sextant fixes, there must be 10,000's.
Sun I understand, I would shoot twice when on watch (2 hours apart) and "guess sail" the first lop. The second taking would be the true 12:00 one giving you your longtitude (sun at highest point) with your clock on GMT (so you can measure the "difference").
When you take your first sighting of the sun, do you shoot the bottom with sufficient accuracy? Do you visit the correction tables (to get to "center") depends on date (size of sun changes as the year progresses), I'm almost positive that those tables are not in VS (sun = sun no matter which date).

Since both our findings are about 2 spheres of rotation in VS (stars/sun), it could very well be that we are looking at 2 offsets.
Wouldn't surprise me one little bit....?

The sun offset should be the easiest one to figure out since we are only talking about one object in one sphere, still we have to allow for how this object moves (24H) on which "tilt" (ecliptic in our words) in VS.

Your Polaris fix depends a bit on date as well (if I'm not turning senile with age, I remember there is about 1 degree of rotation on the axle), not sure if VS has that variation incorporated in sims code?

Please note that VS is not a sextant simulator of the skies, we are way above the average user here....
(if you posted a poll on how many users use the sextant? 0)
Cheers,

Ron
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Re: Learning celestal navigation

Postby laukejas » Wed Nov 09, 2011 9:15 pm

Yes, your idea of different celestial spheres is very good! I think that is the case here.

Yes, Ilan won't do anything about VS, but if we can determine offset for each sphere, and post a tutorial (as you already did with stars, finding that 41:18 offset).

Problem with sun is, that I can't determine offset for it, it is not constant, as you may see from my earlier tests.

I can't compete with your sailing experience - I just have finished my studies with celestial navigation (with various tutorials on net), and I have made less than 50 sightings, all in VS (at least I now know they were so inaccurate not because of me!), and my sailing experience is less than 1nm in real life. But I guess I understand how system works with sextant.

Yeah, when I sight in VS, I aim for the best accuracy I can get, so I know for certain that at least one element (elevation) is entirely correct in calculations.
In that program, NAV32, corrections are automatic, so I can't change, for example, semidiameter, which is dependent on date, although VS probably does not simulate it. So error up to 0.3' can occur because of this. To avoid this, I would have to do all calculation by hand, keeping semidiameter 16' at all times (which probably is constant in VS).

So I have no idea how to track down offset for the sun, since every time I get different numbers.

About that poll - well, at least there would be 2 people who use sextant - you and me :D
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Re: Learning celestal navigation

Postby Sailorjohn » Wed Nov 09, 2011 11:40 pm

I've stayed pretty much out of this discussion, but wish to make an observation (pun). Like Ron, I'm a long-time professional mariner, and still active in recreational boating (trawler and sail). I learned celestial navigation as a USCG cadet aboard the barque EAGLE ('59-'62)...hundreds of obs. Subsequently I served as Navigator aboard 3 ships, later commanding 3 more. This was in the days before GPS, SATNAV, and even LORAN-C. I did celestial 'til I could do it in my sleep...and we also used sextants for horizontal angles in buoy positioning. I have my own personal sextant (WWII vintage USN, made by Lionel!) That said, I'm one of those long-time VS'ers (now exclusively VSF) who never attempted to use the VS 'sextant', because there was no novelty in it for me...and I had heard from others, like Ron, that there were serious problems with VS' 'Great Sidereal Angle, by which time is generally reckoned'. So I wish you guys well in your venture, and for your sakes, hope Ilan eventually implements measureably accurate sidereal time, and sextant, in VSF.
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Re: Learning celestal navigation

Postby Victor » Thu Nov 10, 2011 9:33 am

Concur SJ. I am hoping sufficient information can be gathered in order that they can be sent to Ilan so that he can incorporate in a sextant for VSF. I know he is working on one - not prioroty one, though.

(O/T - I used HSA for station keeping as well)

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Re: Learning celestal navigation

Postby ron » Thu Nov 10, 2011 8:23 pm

well, we are all in agreement so, which is good to know.
Findings only come about with testing and noting down our observations/discrepancies, which is what we do in this topic, it is a good one.
Since Ilan incorporated the sextant instrument, I can only assume he intended usage? Since he has an objective to improve the instrument, he needs input?
Which brings me back to an earlier statement (this topic is a good one).
The more the merrier..... , I will consolidate findings into one summary for Ilan, the subject is advanced so I'm not 100% on how Ilan can sell the feature (the classical sextant), with everybody on GPS nowadays.

I'm in here for nostalgia, it is a fun thing for me... John, I envy you keeping your instrument, I didn't keep mine (and it was a pretty good one as well), please love it and take it out of it's box now and again and pretend you know what you are doing, your audiance will be chuffed to bits, make no mistake, I remember our passengers always marvelled at the procedures, it's like magic.

Today it is not a big thing because there is hardly anybody here (this topic), still a classical sailor will get excited with a working sextant, that's my take (I know my 2 kids got very excited when I produced a sextant fix in VS, they were flabbergasted).
Cheers,

Ron
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Re: Learning celestal navigation

Postby Sailorjohn » Thu Nov 10, 2011 10:50 pm

Yeah, Ron...I still take it out of the box every now and then. Thought about leaving it out on our 'dinghy' coffee table...but it deserves to stay in it's box...which is in itself a beautiful piece of tongue-and-groove, brass, varnished hardwood, and felt padded workmanship. Incidently, my sextant has polarized filters, whereas most of the CG sextants used color discs.

Victor, almost all my early seagoing was on buoy tenders...navigator on 2, CO on the third. We used HSA almost exclusively for buoy positioning (watch out for the 'slider', ie right, center and left objects at same distance from viewer...big no no!!!!). We used a Stadimeter for UNREP and stationkeeping.
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Re: Learning celestal navigation

Postby Victor » Fri Nov 11, 2011 9:01 am

I think we'll leave this thread to Ron and laukejas who are doing a good job of troubleshooting celestial navigation à la VS ,hopefully before the VSF version comes out. I am not sure if we could use HSA in VS/VSF though. Should be quite interesting. I don't have VS any more , so can't do much on that score.A thread for the use of sextants, other than the subject of this topic could be started when the VSF version come out.

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Re: Learning celestal navigation

Postby ron » Fri Nov 11, 2011 7:37 pm

Yeah, we'll keep experimenting on the sextant feature.
I have this notion that with thousands of students, a working simulator (celestial navigation) for the "institutions" might be a line of revenue for Ilan if he gets this feature accurate to an extend (in VSF). It only takes one e-mail to all the Maritime Schools" which can be found Googling away. For 50 bucks they can beam up a working practice?

I also think that us "old hands" have produced too few learning scenarios/scripts to progress VS/VSF within the student community (our young ones).
This is something on my radar but..... (for time). It would only need one good scenery with some nav-aids (like the Florida by JT3D) and loads of situations/scripts can be created.

I have to talk to Ilan anyway about this sextant (but there is more). Where we (users) fail is in the publishing of realistic situations and how to deal with those (in published scenarios/scripts).
Cheers,

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