[ntp:questions] stupid, simple question about precision
Richard B. Gilbert
rgilbert88 at comcast.net
Tue Nov 21 01:46:52 UTC 2006
Simple Simon wrote:
> rgilbert88 at comcast.net wrote...
>>David J Taylor wrote:
>>>Simple Simon wrote:
>>>>My understanding is the best accuracy I might get, on a generic PC
>>>>clock, is about 0.050 ms--and that's with SSP and the kernel clock.
>>>My really old 133MHz Pentium PC with FreeBSD and a PPS source (with an
>>>incomplete view of the sky) does a little better than than. I'm sure more
>>>modern hardware and a better-positioned GPS antenna would do substantially
>>I haven't tried it with a PC but a ~10 year old Sun Ultra 10 with a 440
>>MHz processor and Solaris 8 keeps time within a couple of microseconds.
>> I do have a good view of the sky. Something blazingly fast with a
>>lower interrupt latency might do still better but my modest hardware
>>meets and exceeds my very modest needs.
> Is your GPS directly updating your hardware, or is your SPARC an NTP
> client of a Stratum 1 timeserver connected to the GPS, or... ?
My GPS is connected directly to serial port "B" on the Sun Ultra 10.
The PPS signal is connected to the DCD Pin. The Sun also uses a bunch
of Internet servers as a backup and sanity check. Normally, the GPS is
the selected synchronization source. This gives me a platform from
which I can evaluate internet servers and NTP over internet.
I find that servers/network connections are of highly variable quality.
I have ONE internet server that is solid as a rock and maintains an
approximately 2 millisecond offset from my GPS. I assume this is due to
an asymmetry in the network somewhere. A couple of the others bounce
around +/- ten to fifteen milliseconds. The quality of some of the
others varies with the time of day; the hours of darkness seem to
provide the best performance while normal business hours in the Eastern
time zone (US) seem provide the worst. I expect that this correlates
strongly with network traffic.
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