Hello,
I have ntpd set up with two time sources, GPS_NMEA and a PPS pulse, and I have
a couple
of perhaps basic questions.
NMEA sentences are not read directly from a physical serial port but a
"virtual"
/dev/gps0 that is fed by a separate process.
PPS pulse comes from the gps module and is handled by custom Linux kernel
driver.
With no time1 fudge values set for either of the drivers, I let the system run
over night and it settled so that 'ntpq -p' shows
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 53 64 377 0.000 335.809 20.427
oPPS(0) .PPS. 0 l 6 16 377 0.000 0.005 0.004
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 9 64 377 0.000 331.531 16.355
oPPS(0) .PPS. 0 l 10 16 377 0.000 0.005 0.004
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 29 64 377 0.000 331.531 16.355
oPPS(0) .PPS. 0 l 14 16 377 0.000 0.003 0.00
running 'ppstest /dev/pps0' shows
source 0 - assert 1325148282.999996125, sequence: 985573 - clear 0.000000000,
sequence: 0
source 0 - assert 1325148283.999996770, sequence: 985580 - clear 0.000000000,
sequence: 0
source 0 - assert 1325148284.999995073, sequence: 985587 - clear 0.000000000,
sequence: 0
source 0 - assert 1325148285.999996346, sequence: 985594 - clear 0.000000000,
sequence: 0
source 0 - assert 1325148286.999995188, sequence: 985601 - clear 0.000000000,
sequence: 0
Does that assert time of +/- ~5 usec within a second change tell that the
system
clock is running in sync (and in the same phase) with the pps pulse, or do I
understand that completely wrong?
Other thing is the GPS_NMEA offset in ntpq output; looking at the output from
last
night it varies between 300 and 350 ms. Does that matter, since the pps pulse
tells when the second starts so having the time from the gps within the correct
second should be enough, no? Or should I tweak fudge time1 value for the NMEA
driver to 350 or so to get is near zero? (NMEA jitter stays under 20 ms most of
the time, some larger peaks every now and then.)
Thanks much,
Tomi
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