[ntp:questions] NTP and PPS (and gpsd etc..)

Jason Rabel jason at extremeoverclocking.com
Wed Apr 25 23:22:56 UTC 2007


David,

I have a Jupiter 8 unit that I am using the Jupiter Refclock and it works
perfect. However I looked over the specs for the Jupiter 21 and it only
supports SiRF and NMEA. AFAIK there is no SiRF refclock.

With the older Jupiter modules you have to use them in their Binary mode,
using them in NMEA can cause a 1-2 second offset because of the delay in
processing NMEA output from the module. I don't know if that is still true
for the newer modules, but I would assume it's entirely possible. Even if
the time info was wrong you can use just the PPS signal from the GPS via the
PPS/ATOM refclock, then use another NTP server for the general time info.

The catch here is: exactly what OS are you running? You said "linux embedded
machine", which by default linux does not support a PPS signal. You have to
patch the kernel to get PPS support. On the other hand FreeBSD supports PPS
and also has nanosecond precision.

Jason




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