[LinuxPPS] Offset between GENERIC and PPS

[Thorsten Mühlfelder]

Hi

I'm using 2 ntp servers with Meinberg DCF and GPS cards.
http://www.meinberg.de/english/products/gps170pci.htm
http://www.meinberg.de/english/products/pci511.htm

Additional to the Meinberg NTP driver these cards provide a PPS signal, which is connected to /dev/ttyS1. But as you can see in the following ntpq outputs, there is an offset between GENERIC driver and PPS (~1.7 ms for gps, ~0.6 ms for dcf). Does anybody have an idea how this is caused?
Additional question: Why is jitter 0.015?

gps:
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        .LOCL.          12 l   31   64  377    0.000    0.000   0.015
+GENERIC(0)      .GPSi.           0 l   10   64  377    0.000    1.664   0.015
oPPS(0)          .PPS.            0 l   18   64  377    0.000   -0.046   0.015

dcf:
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        .LOCL.          12 l   54   64  377    0.000    0.000   0.015
+GENERIC(0)      .DCFi.           0 l   17   64  377    0.000    1.391   0.866
oPPS(0)          .PPS.            0 l   34   64  377    0.000    1.944   0.870

PS: I'm using a 2.6.26.2 kernel with patched glibc (nano patches) and ntp build against this glibc (status 0x2001 (PLL,NANO)).