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    Re: Accuracy of GPS
    From: Richard B. Langley
    Date: 2010 Nov 29, 00:50 -0400

    And another series of extensive tests of handheld receivers in 2001 by
    a geodesist:
    -- Richard Langley
    Quoting Gary LaPook :
    > There has been some discussion recently of GPS accuracy. I have
    > attached three graphs of testing I have done. The grid is the UTM
    > grid and the units are meters using NAD27 coordinates.
    > The first shows a test I did when Selective Availability was active
    > and the scatter covered 130 meters east to west and 152 meters north
    > to south giving a CEP of 22.22 meters. Plus marks show one minute
    > averages and the dots show single fixes. With SA on averaging did
    > not improve the fix accuracy since SA swamped the effect of
    > averaging since SA was not a random error.
    > The second shows a test done on September 18, 1994 and I was shocked
    > to see how small the scatter was, only 7 meters east to west and 10
    > meters north to south with a CEP of 2.90 meters. I learned that the
    > reason for this was that SA had been turned off because of the U.S.
    > invasion of Haiti. This is a good comment on selective availability
    > which was designed to allow high precision positioning for the U.S.
    > military while denying it to the enemy. But, during Desert Storm and
    > again when invading Haiti, our military did not have enough military
    > GPSs so had to use off the shelf units so SA was turned off to allow
    > greater accuracy. This is humorous since it is exactly during
    > military operations that SA was developed to prevent the enemy using
    > high accuracy GPS against us but, then in practice, SA was not used
    > at exactly the time it was supposed to be used.
    > The third graph was done in 2000 and includes individual fixes and
    > one  minute averages. Without SA, averages were more accurate than
    > single fixes with CEP of 1.50 meters for averages compared to  5.338
    > meters for single fixes.
    > Here are links to two other tests:
    > http://www.madbavarian.org/gps/accuracy.html
    > http://users.erols.com/dlwilson/gpswaas.htm
    > gl
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    > =============================================================================== Richard B. Langley E-mail: lang@unb.ca Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ ===============================================================================

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