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    Re: Refraction
    From: Marcel Tschudin
    Date: 2005 Aug 4, 17:38 +0300

    Thank you Fred and Robert for all your support.
    I found finally the "Table 6 - Refraction", previously Table 8... This sould
    help to derive some sort of table or function.
    I do not quite agree with "black magic", it is actually very basic physics
    which can be calculated by integration over air layers. May be some one out
    there has even a programmed source code to do this. I actually came across a
    Basic program, but it can not handle negative altitudes, where certain air
    layers have to be passed twice. Furtheron, I am unfortunately not in a
    position to adapt that Basic code to my needs.
    So, as for the moment, I try to go on with this Table 6.
    ----- Original Message -----
    From: "Fred Hebard" 
    Sent: Thursday, August 04, 2005 3:56 PM
    Subject: Re: Refraction
    > Marcel,
    > You can download copies of the Air Almanac from
    > Fred
    > On Aug 4, 2005, at 8:40 AM, Marcel E. Tschudin wrote:
    >> Hello Robert
    >> Thank you for your fast reply. Unfortunately I am not in possession of
    >> the Air Almanac; in addition I am for about an other year abroad in a
    >> country where I am not able to order copies from a library. May I
    >> therefore ask you, whether it would be possible for you to derive from
    >> the table you indicated one or the other benchmark value? As you
    >> mentioned this would at least give a clue. Thanks a lot!
    >> Like you I am still hoping that there is some one out there who really
    >> can provide a solution to this problem.
    >> Marcel
    >>> ----- Original Message -----
    >>> From: Robert Eno
    >>> Sent: Saturday, September 03, 2005 2:37 PM
    >>> Subject: Re: Refraction
    >>> Hello Marcel
    >>> Refraction error makes the observed body appear to be higher than it
    >>> actually is, ergo the same correction rules for postitive
    >>> altitudes would apply to negative altitudes; only in this case the
    >>> negative numbers would be added.  If, for example, your sextant
    >>> altitude was minus 5 degrees, and the refraction correction was (I am
    >>> just pulling a random number out of my head) minus 60 minutes
    >>> (refraction correction is always subtracted from the sextant
    >>> altitude), then your corrected altitude would be minus 6 degrees.
    >>> Near as I can reckon, you would only experience negative altitudes
    >>> from an aircraft flying at high altitudes or in the polar regions in
    >>> winter.
    >>> Have a look at Table # 6, HO 249, Sight Reduction Tables for Air
    >>> Navigation and the correction tables at the back of the Air Almanac.
    >>> These give you a bit of a clue as to what to expect. Other than these
    >>> tables, I know of no other tables which provide refraction
    >>> corrections for negative altitudes for observers at sea level.  If
    >>> anyone out there is aware of such tables, do let me know. It is a
    >>> question I have not pondered for some time now.
    >>> Robert
    >>>> ----- Original Message -----
    >>>> From: Marcel E. Tschudin
    >>>> Sent: Thursday, August 04, 2005 6:48 AM
    >>>> Subject: Refraction
    >>>> Hello
    >>>> While searching with Google, I came across this mail list. May be
    >>>> some one out here may be able to help me answering the following
    >>>> question:
    >>>> How do refraction values for negative elevations have to be
    >>>> calculated, such as e.g. the horizon from a plane? (I am interested
    >>>> in the range of 0? to approx. ?5?.)
    >>>> Is Bennett?s approximation also valid for negative elevations? If
    >>>> not, what other approximation formulae should be used, or, where can
    >>>> one find some benchmark values?
    >>>> I am interested in formulae for both, refraction from apparent
    >>>> position and from physical position.
    >>>> Thank you for any input to my problem.
    >>>> Marcel

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