[ntp:questions] Accuracy of GPS satellite signal ?

Eugen COCA ecoca at eed.usv.ro
Sun Jan 28 16:43:31 UTC 2007

Please read the following documents:


We may find some diagrams with GPS to NIST time transfer error at:


We are very happy to see error in the order on magnitude of tens of 

I'm sure, on the road to the Internet, there are many other errors: 
the RS232 port, PC motherboard and it's *bad* 14, 318182 MHz clock, 
the NTP daemon, the network card, the switches, the routers, etc.

What I obtained with my servers are errors around 1 to 10 microseconds 
from the GPS receiver to the motherboard but no sub microsecond error.

https://ecoca.eed.usv.ro/mrtg/ntp1usvro_offset_10.html - the best 
possible results with the best possible PC I found (P2/333MHz - 
Fujitsu/Siemens / FreeBSD 5.4 / Garmin GPS 18) - (no, P4, P4 Duo, P4 
dual core are not good for timekeeping - I made tens of tests with 
different machines and different brands and configurations)

and a more common result:

https://ecoca.eed.usv.ro/mrtg/ntp2usvro_offset.html (any other PC 
brand and the same hardware / software configuration).

I have now two http://www.thinksrs.com/products/PRS10.htm and I plan 
to use them and I also plan to hack the motherboards 14MHz clocks and 
replace them with http://www.febo.com/time-freq/hardware/ICS525 boards 
(driven by the rubidium 10MHz outputs).

I'll post here the results and I am sure there wiil be the best we may 
obtain in a non-professional environment.

On Jan 28, 5:59 pm, "Richard B. Gilbert" <rgilber... at comcast.net> 
> John Allen wrote:
> > Richard B. Gilbert wrote:
> >> It should not matter which satellites your site is using, they all
> >> keep the same time to within nanoseconds!
> > To satisfy my curiosity without effort on my part, can someone tell me
> > where the GPS satellites actually get their time from?
> > John
> > --
> > John Allen
> > Bofferdange, Luxembourg
> > allen{at}vo{dot}lu
> >http://www.homepages.lu/allenThe Navstar (GPS) satellites have atomic clocks (cesium) on board.
> These clocks are monitored and controlled by a ground station.

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