Learning celestal navigation

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Learning celestal navigation

Postby laukejas » Thu Nov 03, 2011 7:59 pm

Hello,

I'm trying to learn using sextant and finding position by stars with Virtual Sailor 7.

My first calculations attempt - sighting sun - failed horribly, off by 785 nm :D

Of course, it was some calculation error, but still, I'm wondering a few things.

What sight corrections do I need to make? Dip, refraction, paralax, semidiameter? Are all of those simulated in VS to get correct Ho?

Also, since I red in one tutorial in this forum that VS GMT clock is off by +44 minutes, I always minus 44 minutes from my sighting GMT time.
And I'm using Nautical Almanac 2011 (of course, date is set correctly in VS).

Apart from these questions, is there anything else to know? Maybe there is some kind of tutorial with examples?

Thanks in advance!
laukejas
 

Re: Learning celestal navigation

Postby Victor » Thu Nov 03, 2011 10:28 pm

There is tutorial section.

viewtopic.php?f=46&t=863

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Re: Learning celestal navigation

Postby laukejas » Thu Nov 03, 2011 10:44 pm

Well, as I said, I already red that tutorial, and it did not contain the answers I'm asking now...
laukejas
 

Re: Learning celestal navigation

Postby Sailorjohn » Thu Nov 03, 2011 11:16 pm

I think you would be better served by using VSF, as everything therein, including the celestial model, is vastly superior to VS...which is obsolete and unsupported.
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Re: Learning celestal navigation

Postby laukejas » Fri Nov 04, 2011 12:03 am

I'm sorry, for I have heard that sextant isn't included in VSF. And what concerns me - navigation and sailing - hasn't improved a lot (as much as I tried demo). There are no dynamic sails and ropes, sailing engine pretty much the same, some instruments are lacking (such as sextant), there are too little sails control (take a Sail Simulator 5 for example - there you can control pretty much every line that you can find in the sailboat). VSF went on different path, not concentrating on improving sailing complexity. So why buy VSF?
laukejas
 

Re: Learning celestal navigation

Postby laukejas » Sat Nov 05, 2011 1:47 am

Since no one is answering, I made few tests, and here is my results:

Since there is no way to tell which of sextant corrections needed to be included in calculations when navigating in VS, not real life, I made all four corrections, and chose my true position as my assumed position, so I would get zero intercept (so my true position would be perfectly on the line of position).
My observed altitude (Ho, after all four corrections) was 2.61792 degrees higher than computed altitude (Hc).
I do not know what this difference includes (probably celestial sky out of sync with time), but I just written down that every time I take a sight in VS, I shall make "VS correction" - subtract 2.61792 from my observed altitude (Ho) to get "VS Ho", which would give me accurate position.

By the way, I used GMT time shown on sextant. No -44 minutes correction as before.

So, all procedures like in real life, just subtract 2.61792 from observed altitude (Ho).

Or something around that number. There might be observation errors, although I tried the best I could.

Hope my experiment helps someone.
I will attempt a sail trip from Kriti to Italy, and on route, see if my method works. If I find any error, I'll write about it, but I'm quite sure this should work (at least with sun, although it shouldn't make any difference with different celestial bodies).
laukejas
 

Re: Learning celestal navigation

Postby Victor » Sat Nov 05, 2011 10:22 am

Keep up the good work. perhaps when you have finished your experiments you could add to the tutorial.
Ilan left out the sextant from VSF because, as you have found out, it was not accurate. He does intend to include a sextant in a future version but he is more concerned which physics etc of vehicles, sceneries and a few other things operational at the moment. As for individual sail control, he left it to Sail Simulator as it is their speciality just like Flight Simulator is better at flight than VSF but as the saying goes, you pays your money and you takes your choice.

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Re: Learning celestal navigation

Postby laukejas » Sat Nov 05, 2011 12:14 pm

Yeah, I understand that,
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Re: Learning celestal navigation

Postby laukejas » Sun Nov 06, 2011 1:29 am

My previous experiment involved some calculation errors.

Corrected "VS correction" - minus 41.118 minutes, or 0.6883 degrees. This needs to be subtracted from Ho.

I'll have to make two more experiments - with sun in zenith (I'll go for 90 degrees) and at sunset (less than 10 degrees to horizon) to confirm the accuracy of this correction. This will tell me if refraction is simulated in VS, since dip must be simulated (I think), parallax - maybe, but anyways, it is a small correction, not too important, and semidiameter should also be simulated (unless celestial sky is very off sync). So, only refraction is in question.

I'll post results as soon as possible.
laukejas
 

Re: Learning celestal navigation

Postby laukejas » Sun Nov 06, 2011 2:29 am

Well, experiments are complete... And, well, I'm really stuck. Really. Here is report:

sight 1:

Date: 2011 01 01

13:19:17 GMT

Hc= 84.4561 = 84:27:22
Ho= 84:41.4'
refraction correction (applied) -0.4

LatAP 23:00:10S
LonAP 14:08:30W


intercept off by +52.4' (52.4nm) (need to add this amount to get accurate fix)




sight 2:

Date: 2011 01 01

19:25:21 GMT

Hc= 2.7157 = 2:42:56
Ho= 3:12.6'
refraction correction (applied) +15.2

LatAP 23:00:10S
LonAP 14:08:30W


intercept off by +12.6' (12.6nm) (need to add this amount to get accurate fix)




As you see, things are irregular here. Even if I don't take refraction in account, at zenith and sunset sightings I get DIFFERENT intercept error, and it is large. Strange enough, that sunset sighting is more accurate, if I don't take refraction in account, intercept error is only 2.6nm, which is close enough. But for zenith sighting, this error is 52.8nm (when not taking refraction in account). This is really weird. I hoped that intercept errors will be the same at both experiments WITH taking refraction in account, or WITHOUT. In either of these cases, intercept error should be the same, but for whatever reason, it is not.

I'm confused, really. Can anyone help me here? If only I could ask these questions Ilan... Things would get clear. Results of experiments are paradoxal.

I would very, very appreciate any help or opinion on this.
laukejas
 

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