[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 
Stamsund).

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.

Cheers,
David 




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