[ntp:questions] ntpd syncs to NMEA message but not PPS
Chris Stankevitz
cstankevitz at toyon.com
Sun Jan 25 02:09:44 UTC 2009
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
2. ntpd-4.2.4 patched to support LinuxPPS
http://ftp.enneenne.com/pub/misc/linuxpps/refclocks/nmea/
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
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
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?
Thank you,
Chris
More information about the questions
mailing list