[time-nuts] A question about GPS pps quality.

John Ackermann N8UR jra at febo.com
Thu Jul 26 12:57:58 EDT 2007

Robert Sexton said the following on 07/26/2007 11:38 AM:

> 1.  If I interface one of these to my closet-based server, do I really 
> have a Stratum-1 source? 1us accuracy doesn't sound all that great, but 
> I'm nut so sure that the server can take advantage of a pulse with 
> greater accuracy.

The GPS 20/25 aren't the best time sources around, but you'll probably
do a bit better than the quoted 1us since NTP will average over time and
hopefully get rid of some of the jitter.

> 2.  Would I get significantly better performance if I shelled out $75 
> for one of the new trimble timing GPS modules, or found an old oncore 
> timing module on ebay?

Yes, in that even the oldest Motorola units are at least an order of
magnitude better than the GPS-20/25 series.  They will give you time
down to 100 nanoseconds or better.  As a practical matter, though, your
NTP performance may not be all that much better after getting through
jitter added by the serial port interface, kernel latency, etc.

Unless of course, you go with a very inexpensive ($150) but much better
hardware platform like the Soekris net4501 (see


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