[ntp:questions] fudge time1 for gps-18x-LVC?

David J Taylor david-taylor at blueyonder.delete-this-bit.and-this-part.co.uk.invalid
Sun Feb 7 17:21:57 UTC 2010

"Dave Hart" <davehart at gmail.com> wrote in message 
news:264438b3-7b1e-4808-8bc0-602a7220360c at t34g2000prm.googlegroups.com...
> If, like me, you have your GPS
> emitting a single sentence each second, configure NMEA without fudge
> flag1, and have PPS/ATOM on the same port using ntpd on Windows with
> serialpps.sys, the result is the NMEA driver is showing the "user
> mode" PPS timestamps taken by ntpd when it notices the DCD transition
> via normal Windows serial APIs (which do not depend on serialpps.sys
> functionality), while the PPS/ATOM driver is showing the PPSAPI
> timestamps originating in the serialpps.sys interrupt handler:
> *GPS_NMEA(1) .uPPS.  0 l    8   16  377    0.000   -0.042   0.002
> oPPS(1)      .kPPS.  0 l    6   16  377    0.000    0.003   0.002
> The ~40us difference is what I typically see, but it varies with the
> load on the machine.
> Cheers,
> Dave Hart

OK, Dave, understood, I think.  It is a requirement of using the 
serialpps.sys (for its kernel-mode timestamps) that that you also use the 
ATOM driver if you want the kernel-mode timestamps to be used.  Right?

I also see the GPS time as indicating some tens of microseconds behind the 
PPS time. with more offset on the slower PC, as would be expected. 
However, my PPS line is the first in the billboard, and not the second. 
Does that matter?  Or is it just because in my ntp.conf I have:

  server  minpoll 4         # PPS - serialpps.sys
  server  minpoll 4  prefer    # NMEA serial port

Would it make any difference if they were in the reverse order - I'm 
guessing that it would not.


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