[ntp:questions] Assistance with PPS on Windows

Chris Albertson albertson.chris at gmail.com
Thu May 5 19:59:02 UTC 2011

On Thu, May 5, 2011 at 11:46 AM, unruh <unruh at wormhole.physics.ubc.ca> wrote:

>> If you need better buy a GPS.  You can get setup with one that does
>> works at the nanosecond level for under $100.
> Uh, no. There is no way of getting the signal into the computer with
> nanosecond accuracy. 1usec is about it.

You are right.  While A $60 GPS can produce a PPS with 1 sigma error
on the order of a hand full of nanoseconds, the computer's interrupt
handing has about 1 or 2 uSec resolution.  So nanoseconds are wasted
on an NTP server.

But there are other things one can do with a PPS signal.  For example
you can discipline a local frequency standard.  As long as you are
installing a gps, may as well get one that works at this level as long
as the cost is still reasonable.  You can pay a lot more for a GPS
that is literally 1,000 times worse.   I have an older Motorola unit I
paid $25  on ebay and it does about 50 nanoseconds one sigma.

An interesting question is, If the computer's time resolution is 1
uSecond, how good must the GPS' PPS signal be so that the majority
(90%) of the error is in the computer and not GPS.  Off hand I think
you want the GPS to be about one order of magnitude better or on order
of 100 ns.   But I've not done the math.

Chris Albertson
Redondo Beach, California

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