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    Re: GPS time for celestial (longish)
    From: Bob Hardin
    Date: 1997 May 19, 09:40 EDT

    I quote from:
    You can see that a receiver can correct automatically or you can get the 
    difference from the navigation message.
    "GPS Time
    Internally in the Navstar system, time is kept as GPS time. GPS Time began on 
    January 6, 1980 and is referenced in GPS Weeks and seconds. GPS time is a 
    composite time composed of the times of all available satellite and monitor 
    station clocks. It is monitored by the GPS Operational Control System and by 
    the U.S. Naval Observatory and is steered to keep it within 1 microsecond of 
    UTC. However, leap seconds are not inserted so GPS time lags behind UTC. The 
    exact difference is provided as constants in the GPS navigation message. By 
    using the information given in the NAV message, the user can refer time via 
    GPS directly to UTC (as determined by the USNO) with a precision of better 
    than 340 nanoseconds (95% probability) using the standard positioning service 
    and 100 nanoseconds using the precise positioning service."
    "Inexpensive Methods of GPS Time Syncronization
    Windows NT Resource Kit
    Want to syncronize the clock on your PC?
    I was looking through the Windows NT 3.51 Resource Kit the other day and what 
    did I find but a time syncronization utility. On reading through the 
    documentation, I found that it would accept output from a GPS receiver! The 
    docs indicated that it would accept the output of Trimble and Rockwell 
    Tom Clark's Totally Accurate Clock project
    Tom Clark from NASA's (?) has developed a high quality timing module from a 
    low cost GPS receiver. The Totally Accurate Clock (TAC) project began as a 
    home project, but was found to have applications in the scientific community 
    as well. Incidentally, the name "Totally Accurate Clock" is a take-off on the 
    old Heathkit "Most Accurate Clock" that syncronized itself to WWV signals. 
    The TAC uses a Motorola Oncore Basic equipped with Option A (timing) and a 
    small home built PC board containing additional buffering circuitry and 
    signal amplification.
    Details and software for the TAC are <a 
    href="ftp://aleph.gsfc.nasa.gov/GPS/totally.accurate.clock/">available by 
    FTP</a>.  People that have additional questions about the TAC can read the 
    <a href="<A 
    Subject:  Re: [Nml] GPS time for celestial
    I am writing this in the hope that someone can tell me what the
    >"watch error" is for the GPS time signal as received.
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