[ntp:questions] help with setting up NTP on windows with a USB GPS

David J Taylor david-taylor at blueyonder.not-this-bit.nor-this-part.co.uk.invalid
Fri Nov 27 19:11:19 UTC 2009

> Just to let everyone know that the u-blox GPS works well now. The
> offset is 0.02ms with jitter at 0.002ms. My other GPS with only USB
> out has an offset of 5ms and jitter of 2.5ms.
> Jack

Good news, Jack.  For comparison, my own tests with a serial to USB 
converter (Sitecom) which did include the DCD line (we think) I saw a 
minimum averaged jitter of about 45 microseconds, whereas with a LAN 
connection it had been 110-140 microseconds.  A direct serial port 
connection (yes, you /ca/n still get computers with them, but it's 
becoming more difficult) showed minimum averaged jitter in the order of 
2 - 3 microseconds (with Windows XP).

The kernel-mode PPS driver allowed a minimum averaged jitter of just over 
3 microseconds to be reduced to a much steadier just over 2 microseconds, 
but had almost no effect on the offset or frequency variation.

With Windows-7 and a direct serial GPS/PPS connection, the offset is far 
less, but the minimum averaged jitter is around 25 microseconds (PC 

It looks like that you need a direct serial connection if you want the PCs 
to be synced to within the millisecond.  Perhaps much better to have 
completely separate time stamp to which both PCs refer.


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