Hello; GPS maintains a set of its own clocks at Falcon AFB and does not really track or steer to TAI - however, they are very close in practice (except that the AF did not know about Leap Seconds when they started out and synced it to UTC in the early 1980's - thus, there is a 19 second offset between the GPS time system and TAI.) Every major time service and most national standards labs maintain a set of clocks of comparable accuracy - US, UK, France, Germany, Russia, Japan, Australia, etc., so there is no shortage of timing info to compare it with. The International GPS Service (IGS - http://igscb.jpl.nasa.gov/ - a collaboration between various geodetic and time service users of GPS - has a rapid service with information including clock offsets with 17 hours latency see http://igscb.jpl.nasa.gov/components/prods_cb.html for data availability. These solutions are NOT based on the official DOD tracking data but instead on the much more accurate carrier phase (and are not affected by either Anti-Spoofing or Selective Availability when these are turned on - see www.timingtechnologies.com/Gpswp1.pdf for a description of these degradations for civilian users). There is no doubt that a major perturbation in the GPS clocks (say, several 100 nanoseconds as is typical with SA) would be detected by the IGS within 24 hours. These was a pilot program set up to use these data for official time transfer - see http://maia.usno.navy.mil/gpst.html for a host of details. I do not know its status since Jim Ray left the USNO. GLONASS maintains another set of clocks and satellites. Of course, once Galileo is launched there will be yet another source of time sync. All of this is important if you need synchronization at 100 nanoseconds or better. LORAN will not give you this by several orders of magnitude - nor will WWVB nor NTP. If you do care about time at this level, get at least a Rubidium clock and sync it to GPS. If you do not, I would not worry about it even at the highest paranoia levels - there are other equally paranoid people who will start screaming well before you notice. On Sunday, June 1, 2003, at 09:57 PM, David G. Andersen wrote:
On Sun, Jun 01, 2003 at 08:13:08AM -0700, Peter Lothberg quacked:
I don't expect GPS to spin out of control soon..
So GPS tracks TAI and the difference is published (2 months after the fact..)
But it's simple to build a 'jamer' that makes GPS reception not work in a limited area, same for Loran-C used in combination with GPS in many Sonet/SDH S1 devices.
but I did wonder how hard it is to find a another reliable clock source of similar quality to GPS to double check GPS.
For NTP purposes, WWVB is actually just fine, as long as you properly configure your distance from the transmitter. The NTP servers list shows several WWVB synchronized clocks. CDMA clocks synch indirectly to GPS, but are typically locally stabalized by a rubidium or ovenized quartz oscillator with decent holdover capabilities for a few days of GPS outages. But they'll suffer the same fate if GPS went just plain wrong.
The NIST timeservers are available over the net, if you can deal with that degree of synch. Lots of them just use ACTS dialup synch to get the offset, and have very good local clocks. ACTS is certainly a good fall-back for GPS, since it uses a wired path instead of a wireless one.
So if you're really paranoid: GPS + WWVB + ACTS + internet to tick/tock or one of the NIST primaries. Ultimately, WWVB, ACTS, and ntp to NIST are all synched from pretty much the same source, but the odds that they'd all go bad are pretty slim. GPS is steered from the USNO time, but the clocks on the satellites are pretty good.
-Dave
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