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    Re: Hundreds of millions of GPS receivers
    From: Richard B. Langley
    Date: 2009 Dec 18, 12:04 -0400

    Quoting Bruce Hamilton :
    
    > My tongue was half in my cheek.
    >
    >  I understand that GPS can be very accurate. The satellites even have to
    > account for the time difference from altitude after all and positions are
    > usually extremely accurate BUT this has not always been so The built in
    > error can be turned on again at any time without warning
    
    In theory, with the current constellation, yes (but highly unlikely), but not for the
    future GPS III constellation. Please see .
    
    -- Richard Langley
    
    > and you will not
    > necessarily know it unless you have a differential GPS or are doing
    > celestial fixes to ensure the accuracy of the GPS.  I thought I even read
    > somewhere that the USN keeps doing celestial to have a source of information
    > other than GPS, but that may be history with the total electronic bridge.
    >
    > Jeremy, I only use air sextants on land so I am happy with a fix within 3
    > miles.
    >
    > On Fri, Dec 18, 2009 at 7:21 AM, Richard B. Langley  wrote:
    >
    > > Quoting anabasis75---.com:
    > >
    > > > I can't agree with this Bruce.  GPS isn't any more a single source  than
    > > a
    > > > celestial fix or a visual fix.  GPS takes data from multiple birds  and
    > > > determines position from a minimum of two
    > >
    > > A minimum of three is needed for a horizontal position if height is known
    > > or can be
    > > assumed to be near 0 (sea surface). Why three? The receiver's clock offset
    > > from GPS
    > > Time is generally unknown and changing second to second and therefore is
    > > estimated as
    > > an unknown along with the receiver coordinates.
    > >
    > > -- Richard Langley
    > >
    > > , but usually four or more  signals.
    > > >
    > > > As far as comparison, I'm interested in what kind of difference you are
    > > > willing to accept between celestial and GPS position?  Is a mile good
    > >  enough,
    > > > two, five?  I just wonder what your standard is.
    > > >
    > > > Jeremy
    > > >
    > > >
    > > > In a message dated 12/18/2009 1:25:45 A.M. Eastern Standard Time,
    > > > brucerhamilton---.com writes:
    > > >
    > > > I even  think that a GPS has become an important part of celestial
    > > > navigation. I call  it my AP indicator. :-)
    > > >
    > > > Since GPS is a single source, I don't  take it's data as true until I
    > > > confirm it through something trustworthy like a  celestial shot.  I'm not
    > > sure if
    > > > this GPS technology is always working,  but the moon and stars are pretty
    > > > reliable.
    > > > --
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    > >
    > >
    > >
    > > ===============================================================================
    > >  Richard B. Langley                            E-mail: lang---.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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    >
    > --
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    ===============================================================================
     Richard B. Langley                            E-mail: lang---.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/
    ===============================================================================
    
    
    --
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
    To unsubscribe, email NavList+unsubscribe@fer3.com
    

       
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