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    Low Alt Refraction Re: USCG Student Example for Low Altitude
    From: Antoine Couëtte
    Date: 2013 Apr 28, 06:21 -0700


    Hello Marcel,

    Please find here-after various Values for Refraction published for 0° Apparent Height. The applicable environmental conditions are also given whenever available.

    * US/UK NA for the YEAR 1983 (1010 mb, T = +10°C) -34.5'

    * French Ephémérides Nautiques for the Year 1981 (carefully rechecked 760 mm Hg, T = +10°C) -33.80'

    * French Introduction aux Ephémérides Astronomiques 1997 (+15°C, 1013.25 hPa, Wavelength 0.590 micro meter, Water pressure = 0 Pa, Latitude 45°, Altitude 0 m) 32'57.971"

    * Ephémerides Astronomiques 1990 (Radau Theory for 1000 mb and 0°C) -36'36"

    * Explanatory Supplement Sep. 1977 (unspecified conditions) -34'

    * Explanatory Supplement 1992 : same as E.S. 1997

    * André Danjon's Astronomie Générale 1980 (0°C, 76 cm Hg, Wavelength 0.575 micrometer) -37'52"

    * Jean Meeus' Astronomical Formulae for Calculators 1988 (unspecified conditions) -0.34'

    * Jean Meeus' Astronomical Algorithms 1991 (1010 mb and +10°C) gives Bennett's formula which is believed to be accurate to +/- 0.07' and yields -34.48' (and with an additional correction bringing its overall believed accuracy to +/- 0.9" it yields -34.50'). It is therefore quite likely that when deriving his (quite "magic") Formula M. Bennett was targeting the US/UK NA values indicated here-above.

    When appropriate I leave upon you the task of reworking these different results into the very same environmental reference (e.g. 1010 mb and +10°C) so that you can get a good feeling of the Refraction Values dispersion at 0° apparent altitude. My guess is that they all should match the US NA -34.5' value to within +/- 1'. Let me know your results.

    Hope it may help you better this time.

    Best Friendly regards


    pS : As you can observe the so called "European" Standard conditions (+15°C, 1013.25 hPa) were used only in the French Introduction aux Ephémérides Astronomiques.

    Such "standard" conditions (+15°C, 1013.25 hPa) are also used into the ICAO Standard Atmosphere model used in Aviation. And ...

    ... if you do not them by heart day AND night, .. don't ever EVER expect to be granted cockpit access ! :-)
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