NIST NTP servers

Valdis.Kletnieks at vt.edu Valdis.Kletnieks at vt.edu
Tue May 10 15:23:56 UTC 2016


On Tue, 10 May 2016 08:07:15 -0700, Brandon Vincent said:
> On May 10, 2016 7:59 AM, "Stephane Bortzmeyer" <bortzmeyer at nic.fr> wrote:
> > Yes, but they may switch it off for civilian use (by going encrypted,
> > for instance) at any time, if it is better for *their* operations.
>
> I think you are referring to selective availability which degraded the
> positional accuracy for civilians.

I seem to recall that the positional accuracy is degraded to "on the order
of 200 meters" when selective availability is enabled (and yes, they *can*
do it by geographic area by careful turning on/off on each orbit of each
satellite - devices are told the signal is degraded and can downvote that
signal.  So unless you're in the war zone, you'll just notice your device
reporting a lock on 2-3 fewer signals).

The upshot is that Grace Hopper told us about electricity moving a foot per
nanosecond - which means that the time-domain jitter introduced is all of
600 nanoseconds or so.  In other words, anybody using it to feed an NTP
server will hardly notice on the millisecond level.  If you're trying to
use NTP to get microsecond stability, you'll notice - but you have enough
*other* things to do correctly to do this that it shouldn't be an actual issue...
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