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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,
    >
    >
    > In the book "Introduction aux Eph�m�rides Astronomiques" published by the
    > 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.
    >
    > Hope it can help you ...
    >
    > Best Friendly Salutations
    >
    >
    > Kermit
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