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    Re: Corrections for refraction in deserts
    From: Doug Royer
    Date: 2006 Mar 6, 09:58 -0800

    Dr. Kolbe wrote:
    I am off to the Libyan desert at the end of March to see the total eclipse
    which has its maximum period of totality in the Southern Sahara. I will
    then be spending a couple of weeks wandering around the South Eastern
    Libyan desert with a group looking at various natural wonders, and
    finishing off at Gebel Uweinat on the Libya Egypt border to see some
    Saharan rock art.
    The group will naturally have GPS receivers to keep them found, but that
    need not stop me using celestial navigation to plot our progress - and
    amuse the rest of the group showing them how it was done "in the old days"!
    Of course, due to the high temperatures (40 to 50 Centigrade) corrections
    will have to me made to the standard Altitude Correction tables. But it
    occurred to me that the Refraction Correction Tables for Non Standard
    Conditions in the Nautical Almanac may not be appropriate in the middle of
    the Sahara, where the hot sand will bake the air near the ground.
    Does anyone have any data on altitude corrections for desert conditions?
    I spend a good deal of time in the deserts of the American Southwest in the
    last 2 years. During all seasons. I do play around, when the whim strikes
    me, using CN to find a position when hiking/traveling cross country.
    I use a Palm OS type NA and reduction program where one may input the
    temperature of the time when the cut is taken. The accuracy of my positions
    are relative. Sometimes not so close and sometimes very close to the
    corresponding gps pos. I do this to lighten up the weight of my kit. PDAs
    are smaller and lighter than a paper NA. I carry a wrist and stop watch, a
    small pad of notepaper and pencil, an Astra IIIB sextant for the most part,
    and a home made glass AH.
    Now, in all fairness, I use neither the paper NA nor reduce any of the data
    by hand. I just allow the software to do all the reductions.
    So basically what I do is note the DR pos, the time when the sight is shot
    and of what body (usually the Sun), and input that into the program to
    obtain an LOP.
    Another interesting wrinkle I use that may contribute to the accuracy or
    inaccuracy of the positions obtained is using software to again convert the
    Lat. - Long. Coordinates obtained into a UTM or MGRS coordinate because the
    terrestrial coordinates of the projections I use are in that format. I have
    found that when traveling cross country using this coordinate system instead
    of lat-long I will have more "accurate" position represented on the 1:24,000
    or 1:62,500 scale projections I use.
    Good luck on your trip to the Libyan deserts.

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