# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: USCG Student Example for Low Altitude
From: Marcel Tschudin
Date: 2013 Apr 26, 11:04 +0300

```Kermit, you wrote: "... the refraction differences between US N.A. and
French N.A. Tables reach 1 full arc-minute by 0� observed height."
Does the French N.A. provide somewhere the information to which
empirical formula the table values correspond to?

You asked further: "... should the DIP correction be performed first,
and then the Refraction correction, or the other way around ?"
I presume that you have on one side your observed Hs which is related
to your apparent horizon and on the other side your table values
(inclusive the astronomical refraction values) which are related to
the astronomical horizon (=horizontal line at observer). In order to
use in this situation the table values you have to relate your
measured height (Hs) first to the astronomical horizon; for this you
have to detuct also first the dip from Hs.

Marcel

On Fri, Apr 26, 2013 at 9:31 AM, Antoine Cou�tte
wrote:
> ________________________________
>
> RE :
> http://fer3.com/arc/m2.aspx/USCG-Student-Example-for-Low-Altitude-Rudzinski-apr-2013-g23682
>
> Hello Greg,
>
>
> Thanks for this example.
>
> From your chart, I can guess that for your published Azimuth of 077.5� you
> obtained an intercept (between GPS position itself and LOP) close to 0.8 NM
> (A).
>
> I find i = - 1.4' (A) / AZ = 077�7
>
> Our intercept difference (close to 0.6') comes from a difference in the
> Value of Refraction (the SUN is at quite a low height). As an example, the
> refraction differences between US N.A. and French N.A. Tables reach 1 full
> arc-minute by 0� observed height.
>
> Best Regards
>
> Kermit
>
> AN INTERESTING QUESTION (Any reply anybody ?)
>
> As the differences between both end results can be appreciable at such low
> altitudes, should the DIP correction be performed first, and then the
> Refraction correction, or the other way around ?
>
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```
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