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 -- work: dga@lcs.mit.edu me: dga@pobox.com MIT Laboratory for Computer Science http://www.angio.net/ I do not accept unsolicited commercial email. Do not spam me.