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Re: USCG Student Example for Low Altitude
From: Marcel Tschudin
Date: 2013 Apr 27, 23:34 +0300

```Re: http://fer3.com/arc/m2.aspx/USCG-Student-Example-for-Low-Altitude-Cou%C3%ABtte-apr-2013-g23710

Yes, you could help. Thank you, Kermit, for looking it up and
providing the details. I will see whether this value at zero degree
altitude agrees with an other formula. Just to be sure: Does the
temperature indeed relate to 10 deg C? I ask this because the standard
conditions in metric units are normally 15 deg C and 1013.25 hPa.

Cordialement,
Marcel

On Sat, Apr 27, 2013 at 10:55 PM, Antoine Cou�tte
wrote:
> ________________________________
>
> RE :
> http://fer3.com/arc/m2.aspx/USCG-Student-Example-for-Low-Altitude-Tschudin-apr-2013-g23708
>
>
> Hello Marcel,
>
>
> Bureau des Longitudes sub-chapter 7.3 deals with refraction from pages 190
> to 207.
>
> I own the (first) 1997 Edition ISBN 2-86883-298-9 , Editions de Physique. I
> would think that a second edition recently came out of the press.
>
> A significant part of sub-chapter 7.3 is devoted to the Laplace's formula
> (Obvious : Marquis de Laplace was French ...).
>
> Laplace's formula is not directly applicable to very low altitudes. The book
> gives a table for standard refraction a low altitudes. For apparent altitude
> 0� it gives -32'58"
>
> For apparent altitude 0� the French Eph�m�rides Nautiques indicate -33'80
> for 760 mm Hg (i.e. 29.92'' Hg, or 1013.25 mb/hPa) and 10�C (50�F) . The US
> NA value is -34.5' for slightly different "standard" conditions (same 10�C
> and 1010 mb as I can deduct from Table A4 additional corrections).
>
> If we reduce the US NA 0� refraction value to the same conditions as the
> French EN (1013.25 mb), then we would get -34'6 for US NA value. Therefore
> the difference between both Almanacs is actually 0'8 (and not 1' as I
> earlier stated from memory).
>
> As earlier discussed in depth in NavList, Refraction at low altitudes
> (certainly below 10� for our CelNav applications) can be extremely variable
> if not unpredictable ...
>
> refraction also definitely depends on the light color (wavelength) and some
> time ago I was given this as an explanation for the differences between both
> Almanachs : apparently they would not be using the same wavelengths at very
> low altitudes.
>
>
> Best Friendly Salutations
>
>
> Kermit
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```
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