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    Re: How likely is a GPS shutdown?
    From: Peter Smith
    Date: 2000 Jan 25, 8:27 AM

    On Mon 24 Jan 2000 6:27 PM, Luis Soltero [mailto:lsoltero@GULFCOAST.NET]
    > ...
    > I suspect that the military has its own encrypted version
    > of GPS although this is simply a guess on my part.
    There are two GPS data streams. The Standard Positioning Service (SPS) is
    available to anyone unencrypted and can provide fixes accurate to 100
    meters. Selective Availability (S/A) can be turned on and off to degrade
    the accuracy of SPS fixes. This is what was turned off during the 1990-91
    Gulf War because NATO didn't have enough military receivers deployed, and
    had to buy and distribute civilian receivers.
    The Precise Positioning Service (PPS) is only available to the US Government
    and its friends. It can provide 22 meter accuracy by virtue of containing
    more data bits in its message. When they want to be extra-secure,
    this message is double-encrypted to ensure that no one is transmitting false
    signals -- this is called "anti-spoofing".
    Differential GPS is based on the SPS. Corrections from land-based stations
    are broadcast to allow near-by receivers to correct the SPS to 10 meter
     -- Peter

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