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    Re: Threats to GPS and introduction of eLORAN
    From: Lu Abel
    Date: 2016 Jan 31, 17:09 -0800

    Before the advent of GPS, I had two Loran-C receivers aboard my 1980
    sailboat (which I still own and love).   The first cost nearly $2000
    because it had the new-fangled feature of displaying one's lat/long
    instead of merely one's hyperbolic coordinates.  It was typically off by
    a quarter-mile because it did not have ASF correction (if I recall
    correctly, "Secondary Factor" correction in Loran-C compensates for the
    fact that radio waves travel more slowly through air than a vacuum,
    while "Additional Secondary Factor" compensates for the fact that the
    conductivity of the earth (Loran-C waves hug the earth) slows them by an
    additional amount; SF can be calculated but ASF has to be based on
    on-the-ground measurements).
    
    However, the error was quite consistent, not only repeatable but
    consistent over 50 to 100 miles.  Many's the time I navigated
    successfully for an entire day's journey in pea-soup fog.   As I used to
    explain to people, "if you break the end of a yardstick, you can still
    measure distance with it, you just have to remember that it starts with
    2 and not 0."   In fact, I recall repeatedly logging the "slightly-off
    as shown by the Loran receiver" position of a sea buoy and wondering why
    I was getting a position that varied by 0.01 minutes or so -- until I
    realized that the buoy was in 100 ft of water and was simply swinging on
    its chain with the tide!
    
    After 15 years of service the first set, filled with transistors and
    coils, gave up the ghost and I bought a brand new one.   This was a
    tenth of the volume and displayed lat/long spot on, thanks to built-in
    ASF correction tables.  I still have it somewhere, waiting for Loran to
    be resurrected.
    
    GPS is a miracle of modern technology.   But I do wish there were a
    reliable electronic backup....
    
    
    

       
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