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Re: Non-GPS derived timing sources (was Re: NTp sources that work in a datacenter)
From: "David G. Andersen" <dga () lcs mit edu>
Date: Sun, 1 Jun 2003 21:57:08 -0400


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/
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