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    Re: GPS shortcomings.
    From: Richard B. Langley
    Date: 2005 Jun 9, 09:23 -0300

    On Wed, 8 Jun 2005, Lu Abel wrote:
    
    >Richard:
    >
    >My understanding of WAAS is limited, but I am under the impression that
    >it is similar to differential GPS, ie, a receiver's precisely known
    >position is used to create corrections to GPS's position indication.
    
    WAAS is a wide-area augmentation system which uses a vector of corrections
    in parameter space rather than pseudorange corrections for a specific area as
    in DGPS. This allows a wide area to be served with corrections and the
    corrections are typically delivered by geostationary communications
    satellites.
    
    >What I don't know is how widespread WAAS control receivers are.
    
    There are about 25 WAAS reference stations in the contiguous 48 states, Puerto
    Rico, Hawaii, and Alaska, providing WAAS coverage over a large part of North
    America. Stations are soon to be added in Canada and Mexico.
    
    >I could
    >certainly see that if a satellite's clock went awry over Europe the
    >anomaly would be quickly noted.
    
    Europe is covered by a similar system called EGNOS, soon to be activated.
    
    >But what about notification to someone
    >trying to pick their way through some coral atolls in mid-Pacific?  Or
    >on a passage from Cape Town to Australia?
    
    India and Japan are also working on similar systems, as is Brazil. The plan is
    to eventually have seamless global coverage by WAAS and WAAS look-alikes.
    
    The integrity of GPS itself is being improved through U.S. Air Force
    initiatives.
    
    I don't know much about Loran-C further development plans. As mentioned by
    other list members, consult the USCG NavCen Web site and the publications of
    the ION.
    
    -- Richard Langley
    
    
    >Lu Abel
    >
    >Richard Langley wrote:
    >> GPS IS monitored for integrity but by WAAS! The problem WAS detected by WAAS
    >> and the offending satellite was marked as "do not use." Consequently WAAS
    >> users were protected from this anomaly.
    >> -- Richard Langley
    >>    Professor of Geodesy and Precision Navigation
    >>    Fellow of The Institute of Navigation
    >>
    >> On Wed, 8 Jun 2005, Lu Abel wrote:
    >>
    >>
    >>>George and list:
    >>>
    >>>There's mention of this failure (due to the failure of the atomic clock
    >>>on one of the satellites) in
    >>>www.ion.org/sections/southcalifornia/ lavrakas_civil_gps_monitoring.ppt
    >>>
    >>>I was rather surprised to learn that there is NOT continuous monitoring
    >>>of the GPS system for integrity of its positioning information, given
    >>>that there's a whole new generation of air traffic control that's going
    >>>to be based on GPS...
    >>>
    >>>Lu Abel
    >>>
    >>>George Huxtable wrote:
    >>>
    >>>>I know that references to GPS are discouraged, if not forbidden, on this
    >>>>list. But I wonder if a note about GPS shortcomings would be more
    >>>>acceptable?
    >>>>
    >>>>I've just received a leaflet from the British Lighthouse authorities,
    >>>>pushing for use of the new Loran-C transmitter at Rugby, England.
    >>>>
    >>>>In which is the statement-
    >>>>
    >>>>"GNSS systems themselves also suffer occasionally from undetected failures;
    >>>>for example, a satellite clock error on 1 January 2004 gave rise to errors
    >>>>in measured positions of up to 45 kilometres in Western Europe for a period
    >>>>of some 3 hours, resulting in onboard navigational failures."
    >>>>
    >>>>I hadn't heard about that event, which sounds rather serious. Is it common
    >>>>knowledge? Can anyone offer further details?
    >>>>
    >>>>It wouldn't have affected me. I would have been by the fireside on that day.
    >>>>
    >>>>George.
    >>>>
    >>>>================================================================
    >>>>contact George Huxtable by email at george---.u-net.com, by phone at
    >>>>01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    >>>>Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    >>>>================================================================
    >>>>
    >>>>
    >>>
    >>
    >>
    >> ===============================================================================
    >>  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/
    >> ===============================================================================
    >>
    >>
    >
    
    
    ===============================================================================
     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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