[ntp:questions] NTP architecture recommendation

Wolfgang S. Rupprecht wolfgang.rupprecht+gnus200708 at gmail.com
Sat Aug 18 17:07:30 UTC 2007

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
> I suspect that the precision desired was that of position rather than
> time.  A GI in combat generally couldn't care less about time to the
> microsecond but exactly where he is and exactly where he wants an
> artillery strike could be of the greatest importance.

The X,Y,Z,T position and time are pretty well intertwined.  Much of
the GPS calculation's math just treats things as 4 different 4-space
equations with 4 unknowns.  That's why you need to have at least 4
satellites to get a "3-d fix" (solving for all 4 of x,y,z,t).  If you
know your altitude (or the time to the low ns accuracy) you can
eliminate one variable and get by with one less satellite observation.

In any case, under "selective availability" they used to claim that
they added enough noise to cause a 100meter wandering offset in the
position.  That by itself would put a 330ns upper limit on the time
offsets (assuming the speed of light is ~30cm/ns).  Even that wildly
pessimistic number is still so small that ntp timing wouldn't be
noticeably effected.

Wolfgang S. Rupprecht                http://www.wsrcc.com/wolfgang/
IPv6 on Fedora 7 http://www.wsrcc.com/wolfgang/fedora/ipv6-tunnel.html

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