How to:celestial navigation

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How to:celestial navigation

Postby ron » Wed Nov 12, 2008 8:51 pm

SJ, this sextant function was tested at length, the VS clock for celestial navigation is off by about 45 minutes, here the experience......


I'm realy trying to get a fix using the sextant. I have no problem getting a star on the horizon and thus obtaining it's height and azimut (azimut I note down so I can check later I have the correct star).

The position I'm getting is way off and I have attached a screenshot that highlights where my problem occurs. The problem is a simple one: Time
Here the shot, below it my question.

In the top left corner you see the local VS data, position, time, height of the eye etc.

At the bottom center of the image you see the data the sextant is producing, I believe the elevation data I obtained by using the bottom one of the 3 wheels.
I'm not sure about the value for time as in GMT, for one my timezone is GMT +2 hours, being anchored in the Oslofjord. In any rate the GMT time should be able to be related to the local time in the left top corner?

It doesn't, hence my fix being way way off?

Question, which time do I use to fix the objects measured height above the horizon to the GMT time of the measurement?

I've just had a thought! I should be able to tell how much the clock is off by having a look at the calculated position compared to the anchored one (15 miles EW = 1 minute in time).
I'll do it and show you folks how I've calculated the position, with which tools. As the sight reduction tables (I used 249, Pilot Version in my previous live) are not available on the web, I have found a bit of free software that will do the calculations. It has a lot of extra's like telling and showing you which star is where at which hight, given your postion and time. It does all the calculations for you and it shows how (graphically) how the position is arrived at (it shows you the LOP's). It's free as in shareware, no limit that is important, just a little nag that is irrelevant.

Here is the link to the program that I'll use to get you going on the sextant:
Get it and please ask questions on it's use, I'll answer them as best as I can.

I have an example below that'll explain some stuff, let's ignore the time issue for now.
I apologize for the slightly over size limit, there's no way I can explain it on 800x600. Charlotte, Kevin, bear with me please?

Here is the picture, I'll talk about it below

So the program is called Navigator Light 32, you see it above set as my VS session for GMT and position. Best thing to do is press F10 to freeze your VS session so you have all the time in the world.
Here I have the GMT froozen at 18:17:59 see that in both?

I have clicked the TAB visible stars, I have selected a nice bright object low in the western sky as I know I can get an excellent horizon and bright object. This comes with experience. My selected object is the star Arcturus.
You see that you can expect that star to be visible at a bearing (azimut) of 278 degrees very low in the western skies, about 18 degrees above horizon? Now, VS will show you many many stars! Not to get confused by all of these and to more or less reflect reality at dusk? Set your amount of visible stars to 70 (Change/Weather/Date, the bottom slider).

Now on to the sextant in VS. When you have selected View sextant, use the third knob (E) and put it on 0 (zero). This will allign the sextant with the actual sky. Keep the right mouse button pressed down and move the sextant sideways till the arc is 278 (your expected azimut). We are lucky here because Arcturus should be the brightest object a bit above the horizon. You see that even brighter object below the horizon? That's the planet Jupiter, you see it here but you would'n't in real life, it's dipped below the horzon about an hour

So here we are, we have a star in mind and know where to find it. Let's get it's height above the horizon.

Use the bottom knob to move your star toward the horizon, in real live you'd move your sextant from side to side so the star would sort of arc on the horizon, here in VS, just get the light smack center on the horizon.

You can read off the sextant now it's Hs being 23:43:38.

Enter this value in Instrumental Altitude (Navigator Light 32) and press the calculate button. The calculator will take your time/your position/your star/your measured height and produce a LOP, you can save that LOP and move on to the next star. You can get a fix using 2 LOP's. That is not normal practice though, you'd want at least three and we'd do 5 in real live. There's only one reason for that and that is certainty, a mistake is easily made.

I'll show you in a next post how to do that and how to look at the LOP's. But there is enough above to get you going.

Here continues the second and final part of my sextant session.

We have seen above that I selected and shot one star, Arcturus. I did select 3 more and obtained their height above the horizon.
So in short, you select a star from the Visible Stars tab, put your sextant to zero, hold rightmouse button down and swing sideways to the azimut of your star, move the E knob until your star is smack on the horizon.

Back to the Navigator Light:

Go to the Line of Position Tab and select your star in the drop down box Celestial Object, enter the value for height in the box instrumental altitude.
Press calculate, your sighting will be calculated and shown below the button. Press Save LOP. Your sighting will be copied into the Astronomical Position Tab, you can see it there.
So, I have done 4 sightings, saved 4 LOPs. If everything is okay, the four LOPs should more or less cross in one small area (your position), depending on the quality of your reading and time and somewhat your estimated position.

So let's visit the Astronomical Position Tab.

You'll see the four sightings, press the Calculate button and the LOP's will be drawn and a position calculated. Above you see that the LOP's are more or less in one area (although I need to practice a bit to get them closer). At least that tells you that you've shot the right stars.

The astronomical position = 58 20.6N and 001 50.2W

Which brings me back to the time bug on the sextant. I believe the GMT is off by:
001 50.2W
010 04.5E
011.54.7 degrees or (15 degrees = 1 hour) or some 48 minutes.

Given that the LOPs quality decreases as your estimated position differs with your true position, above fix is not realy a good one (which could change the 48 minutes up or down a

Now I remember this, seen in the screen shots above:
The clock in the top left of my VS screen reads: 18:58:17
The GMT clock reads: 18:17:59

I have taken the measurements again, sim was frozen (F10) , I needed the practice anyway.
Now I used the Navigator software on the GMT which I think is around the correct one: 17:33:35 (or less some 44 minutes).

Loh and behold!!! I get an excellent fix very close to the spot I'm anchored in?
Anchored position:
58:27.3N and 010:04.5E

Celestial Position:
58:34.9N and 010:02.2E

I guess Ilan needs to slightly rotate his celestial map, depending which time it is rotating on? (the sextant clock or the clock in the top left)

So, yes dleedvm, you are right.

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