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    Re: GPS shortcomings.
    From: Carl Herzog
    Date: 2005 Jun 8, 17:06 -0400

    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:
    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.

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