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    Re: sextant without paper charts
    From: Scott Owen
    Date: 2008 Nov 11, 00:19 -0600

    bruce hamilton wrote:
    >  I have a friend who often finds himself in the North as a ship's
    > navigator who says the GPS has problems because the satellites get too
    > close to the horizon. Even the the Gyros (both mechanical and FOG) get
    > wonky too. Fortunately, the sun is always up when he is there.
    > Does anyone have any published information on accuracy of GPS at high
    > latitudes?
    Since GPS satellite orbits are at 11,900NM up, GPS high latitude
    reception should be ok.  From the graphs of orbital paths I've seen
    there is coverage at the poles based on the the number of birds and
    constellation footprint.  A good graph of orbital paths is at:
    [the above link will fire off a java applet go "full screen with the
    applet" then from the satellite pull down menu click on select and
    another applet comes up with the names of a lot of satellites select the
    one you want and you will get a rather nice 3d depiction of the orbital
    path] I am hoping that makes sense.
    Clearly if a few satellites are not "healthy" this could easily hurt
    high latitude reception and accuracy.  In my experience in varying parts
    of the world [Pacific and Indian oceans from 40N - 40S] as long as I was
    receiving at least 3 "healthy" satellites my GPS position accuracy was
    good, more satellites was better.  I could receive both SPS and PPS
    signals so I always knew my position very accurately.  The ultimate
    accuracy of the entire GPS constellation is dependent on the current
    status of "selective availability".  A very large or small error can be
    introduced into the system and was when initially brought on line.  It
    is the current policy of the U.S. to leave GPS selective availability
    turned off thereby providing very good accuracy worldwide, well,
    depending on the number of birds your GPS unit can receive.
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