On Feb 5, 2005, at 4:33 AM, Keean Schupke wrote:
Aaron Denney wrote:
Right. The NTP source code has plenty of other things that can by synced off of, including many fairly common GPS receivers, and radio clocks.
Does this mean I can sync my system time to TAI using a GPS reciever? Sounds like how it should be done...
Many, but not all, GPS clocks can deliver GPS time (i.e., the time based on the number of seconds since the GPS epoch 1980 January 6 00:00 UTC converted to a calendar time using days of 86400 seconds and the standard leap year rule). TAI is ahead of GPS time by 19 seconds, and as GPS time does not have leap seconds, TAI = GPS + 19 s. Raw GPS time (the number of seconds since the GPS epoch) is broadcast in a complicated form in the satellites' navigation message, along with the current number of UTC leap seconds since the GPS epoch. From the raw GPS time and the number of leap seconds UTC can be calculated. The gory details of the GPS navigation messages are available in the signal specification document, http://www.navcen.uscg.gov/pubs/gps/sigspec/gpssps1.pdf The GPS clock in my office, an old TrueTime XL-AK, can be set to output GPS time instead of UTC. (Unfortunately, it can't be forced to output the number of leap seconds directly, you have to calculate leap seconds from the difference between GPS and UTC times.) I have worked with other clocks, in particular an Odetics SatSync unit that would only provide UTC. That clock would not provide low level access to the fields of the navigation message, so we just had to maintain a table of leap seconds manually. Best Wishes, Greg
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