[ntp:questions] ntpd syncs to NMEA message but not PPS
Unruh
unruh-spam at physics.ubc.ca
Sun Jan 25 23:56:41 UTC 2009
cstankevitz at toyon.com (Chris Stankevitz) writes:
>Hello,
>I believe ntp is syncing to NMEA messages and not my PPS. I have some
>specific questions below.
>Facts:
>1. My kernel is patched to support LinuxPPS
Not needed.
You can just use something like gpsd to use the shm hardware clock to run
the pps.
>2. ntpd-4.2.4 patched to support LinuxPPS
>http://ftp.enneenne.com/pub/misc/linuxpps/refclocks/nmea/
??? Patched?
>3. I am using a Garmin GPS18 LVC with PPS tied to DCD
>4. ntpd was configured with --disable-all-clocks --disable-parse-clocks
>--enable-NMEA --enable-LOCAL-CLOCK
Just why did you disable all clocks?
>5. ntp.conf contains:
>server 127.127.20.0 minpoll 4 prefer
>server 127.127.20.0 flag3 1 flag2 0
>6. PPS signals are received by the kernel evidenced by entries in
>/var/log/messages:
>Jan 24 16:57:04 gps kernel: [ 136.314349] PPS event at 4294902783
>Jan 24 16:57:05 gps kernel: [ 136.947295] PPS event at 4294903033
>7. The NMEA clock is offset from a stratum 2 server by 600ms:
>root at gps:~/ntp-4.2.4p6# ntpq -p
> remote refid st t when poll reach delay offset
>jitter
>==============================================================================
> europium.canoni 193.79.237.14 2 u 13 64 1 154.790 548.684
> 0.001
> GPS_NMEA(0) .GPS. 0 l 11 16 1 0.000 -63.337
> 0.001
YOu must have set up your nmea to issue a huge bunch of sentences. Why not
use the fudge to get the gps at least near the right ballpark.
>8. ntpdc -c kern makes no reference to pps:
>root at gps:~/ntp-4.2.4p6# ntpdc -c kern
>pll offset: -0.017965 s
>pll frequency: 82.619 ppm
>maximum error: 0.012304 s
>estimated error: 1.6e-05 s
>status: 0040 unsync
>pll time constant: 4
>precision: 1e-06 s
>frequency tolerance: 512 ppm
>root at gps:~/ntp-4.2.4p6#
>9. According to ntpdc_ops.c, the pps variables are displayed only if
>ik_shift evaluates to true.
>Questions:
>a) Why doesn't ik_shift evaluate to true in ntpdc_ops.c?
>b) How can I confirm that ntp is aware of my PPS signal?
>c) How can I get ntp to use PPS?
I would just use the shmpps set of routines to set up pps using the shm
refclock.
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