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    Re: Refraction
    From: Nicol�s de Hilster
    Date: 2007 Nov 09, 11:27 +0100

    In NavList 3849 Gary LaPook wrote:
    
    > Back in March there was a discussion of various formulas to use to
    > predict refraction. I have found some other formulas that may be more
    > precise. The U.S. Naval Observatory used to publish its "Almanac For
    > computers" but stopped it after 1991. That publication has several
    > formulas for refraction:
    >
    > (1)     R' = cot(a + (7.31/(a + 4.4)))
    >
    > (2)    R = R' - 0.06sin(14.7R' + 13)
    >
    >
    > R is in minutes and a is the observed altitude. "At sea level, with
    > air temperature of 10�C and pressure of 1010 mb, R' of eq. (1) is
    > accurate to 0.07' and R of eq. (2) is accurate to 0.015' for an
    > altitude range of 0� - 90�."
    >
    >
    > "For nonstandard conditions the following formula is accurate to about
    > 0.2' for temperatures in the range of -20� to + 40� C and pressures of
    > 970 - 1050 mb.
    >
    > (3)   R = R'((P - 80)/930)/(1 + 8x10 ^ -5(R' + 39) T - 10)))
    >
    >
    >   
    I found another formula for temperature and pressure which I used for 
    correcting the observations with the demi-cross and compared that to (2):
    
    (4)      R = (0.267 x P / T)/(TAN(Altitude + 
    ((4.848x10^-2)/(TAN(Altitude)+0.028))))
    
    In our case (alt = 4m, P = 1018mbar, T = 12�) there was only 0.05 arc 
    minutes difference with (2)
    
    
    
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