NavList:
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Re: USCG Student Example for Low Altitude
From: Marcel Tschudin
Date: 2013 Apr 27, 23:34 +0300
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�ttewrote: > ________________________________ > > 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 > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- > > : http://fer3.com/arc/m2.aspx?i=123710