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» Online Classroom   »   » Public Discussion of Cel Nav   » Going Negative

Author Topic: Going Negative

 - posted June 11, 2007 07:38 AM      Profile for drlmk           Edit/Delete Post 
When I calculate my sextant shots with my known GPS position as the assumed position, the intercept is frequently slightly negative. Does that mean I am taking the celestial body slightly below the horizon, thus giving me an angle a bit larger than it should be? Your diagnosis?
From: Spokane

 - posted June 11, 2007 10:24 AM      Profile for drlmk           Edit/Delete Post 
I was just sitting in boring meeting, thinking this question over and drawing some diagrams and now I think I was leaving the sun slightly above the horizon to get the negative intercept.

Well, I guess the object is to put the sun right on the horizon, anyway...

From: Spokane
David Burch

 - posted June 13, 2007 09:46 AM      Profile for David Burch           Edit/Delete Post 
Most cel nav computer programs as well, a negative intercept means the label is Away.

Thus we have a = difference between Ho and Hc and when Ho is greater the label is T (positive) and when Hc is greater the label isAway or negative. So you are getting Hc greater than Ho routinely.

That means your Ho is too small as a rule. If you can be sure you are doing everything else right, meaning IC is correct (maybe a more likely candidate?) and you have HE correct then you could conclude that Hs is too small, which would mean the body is actually a bit below the horizon at the time you are calling it aligned with the horizon.

We could do a whole lot better with this analysis if you can tell me how much the sights are too negative... list at least 5 values if you have them. thanks.

Also, are all the sights from the same place? what was the HE, on a beach or overlooking a beach? is so what was the tide? was the IC the same for every sight? what type of sextant? what bodies were shot for each error, etc. the beauty of cel nav is we can figure out what is wrong, but we need all the data. thanks.

From: Starpath, Seattle, WA

 - posted June 13, 2007 11:47 AM      Profile for drlmk           Edit/Delete Post 
I don't have the sight data with me, but I took 4 or 5 sights and (as I recall) the negative intercepts were from 0.3 to about 1.l from the known GPS position as the assumed position in the calculations.

All sights were taken from the same place on a dock, within the span of 10 minutes. HE was 6 feet. IC was the same (zero) for each sight. The sextant is an Astra IIIB. Lower limb of the sun for all sights.

I may be able to find the raw sight data, but I'm not sure I saved it.

From: Spokane
David Burch

 - posted June 13, 2007 01:07 PM      Profile for David Burch           Edit/Delete Post 
Maybe too early to conclude there is a systematic error in alignment. I would just keep an eye on this.

You are right to take note when all errors are in the same direction. That means something is wrong, but you need more data on different bodies and different locations, etc to conclude that you have what is called a "personal error" on alignment. Just keep good records and save all your data.

If all the error is from just one session, it could be such exotic things as just a big patch of light air (warm air) in your way at the time... ie sort of a weak mirage effect.

Also keep in mind that if these were all from the same sight session we cannot rule out the Hc was wrong. Note that a time error would yield a systematic error of this type. Could not be date, that would be too large an error.

From: Starpath, Seattle, WA

 - posted June 14, 2007 07:46 AM      Profile for drlmk           Edit/Delete Post 
Thanks for your insights on this matter. I'll keep on keeping on...
From: Spokane

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