[time] GPS Time Server Survey

Björn Gabrielsson bg
Fri Apr 25 05:46:32 UTC 2008


On Fri, 2008-04-25 at 00:01 +0200, Jan Hoevers wrote:

> > I agree with your conclusion that 30ns is meaningless for NTP use.
> > 
> > However why do you say the GPS system itself does not have 30ns
> > accuracy.
> > 
> > The signal in space (SIS) accuracy quoted for the current constellation
> > is between 1 and 2 meters, which is well below 10ns. 
> 
> Well, maybe you're right and my information is outdated.
> This page (http://tycho.usno.navy.mil/gpstt.html) speaks about "a few
> hundred nanoseconds". I recall 340 ns but I forgot where I read it.

340ns is quoted for L1 CA with SA dithering of the SV clocks active.
Together with a 156 meter horizontal accuracy. The SA intentional error
generation was turned off 7/8 years ago.

http://igscb.jpl.nasa.gov/components/prods.html

> Anyhow, a 30 ns accuracy cannot be transferred over an ethernet, let
> alone the internet. It cannot be maintained without a cesium time base.

What is the accuracy of PTP?  100ns?

A good OCXO maintain it plenty good in a GPSDO.

> This may be interesting for a well equipped lab, but not for us.

Like I wrote before, I agree with you conclusions wrt NTP use.

> Jan
> 
> 
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