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    Re: GPS shortcomings.
    From: Brooke Clarke
    Date: 2005 Jun 8, 14:19 -0700

    Hi Carl:
    
    The current LORAN-C system is made up of chains of stations, one master
    and a hand full of slaves.  Now the slaves listen for the master pulse
    and after a wait send their pulse.  In the not too distant future all
    stations will just transmit based on a collection of Cesium clocks.
    
    The result will be a more accurate fix.
    
    The older LORAN-C receivers could needed to be programmed for the Group
    Repetition Interval (GRI) of the nearest chain.  The new generation
    receivers are, to borrow a GPS term, "All In View".  These receivers
    know about all the world's stations and use Digital Signal Processing
    technology to receive all the stations at the same time.
    
    It was not only the events on 9/11 but also the realization that it's so
    easy to jam GPS and European and Russian counterparts that kept LORAN-C
    alive.
    
    The U.S. has turned off Selective Availability which makes a GPS fix
    more accurate for civilian users, but to counter this the military is
    developing jamming technology and from time to time tests it.  If you
    happen to be in a test area you position will be either wrong or non
    existent.
    
    Have Fun,
    
    Brooke Clarke, N6GCE
    --
    w/Java http://www.PRC68.com
    w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
    http://www.precisionclock.com
    
    
    
    Carl Herzog wrote:
    > Lu Abel wrote:
    >
    >> Two or three years
    >> ago the US Coast Guard was trying to accelerate their schedule for
    >> shutting down Loran C.  Now it looks like they and the US Department of
    >> Transportation (which includes the US's Federal Aviation Administration)
    >> are concerned about possible problems with GPS in navigation
    >> applications (such as aircraft routing or precision harbor approaches)
    >> that require hyper-reliability and appear to be considering an enhanced
    >> Loran as a backup system.
    >
    >
    > As of 1992, plans were in place to eliminate LORAN by 2015. By 1994, the
    > termination date had been bumped up to the year 2000. Opposition by
    > LORAN users, largely lead by general aviation interests, caused another
    > review of the need for the system.
    >
    > At the same time, in an unrelated effort, the Volpe National
    > Transportation Systems Center, a division of the U.S. Dept. of
    > Transportation, was studying the vulnerabilities of the GPS system. They
    > released their resulting paper on September 10, 2001. This report is
    > available as a pdf online:
    >
    > http://www.navcen.uscg.gov/archive/2001/Oct/FinalReport-v4.6.pdf
    >
    > Needless to say, the events the next day gave the report a higher
    > profile than it may have otherwise had.
    >
    > It was shortly after that the department began looking at refurbishing
    > the existing LORAN infrastructure to backup GPS. Upgrades to the LORAN
    > infrastructure in the U.S. are already well underway. This spring the
    > Coast Guard upgraded all the transmitters and new timing and frequency
    > equipment is being installed this summer.
    >
    > Studies are still underway to determine whether an enhanced LORAN system
    > can completely meet standards for accuracy and other characteristics
    > that would be required for it to serve as a backup to GPS  in aviation
    > and harbor approach navigation. So far the results look promising, but
    > it may be a few more years before you start seeing integrated GPS/LORAN
    > receivers for sale at your local marine supply store.
    >
    > Carl
    >
    >
    >
    
    
    

       
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