[ntp:questions] Assistance with PPS on Windows

Richard B. Gilbert rgilbert88 at comcast.net
Thu May 5 20:45:56 UTC 2011

On 5/5/2011 3:59 PM, Chris Albertson wrote:
> 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.

My Motorola GPS claims +/- 50 NS on one of the edges of the PPS signal. 
  I don't recall *which* edge.

Getting the time into the computer takes X microseconds where X depends 
on the hardware and software.  Windows' clock ticks every 17 
milliseconds or something like that.  Other operating systems also take 
a toll!

If you *really* care about the nanoseconds, you don't feed them to the 
computer!  If you must, please don't step in what comes out the other 
end! ;-)

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