[ntp:questions] NTP, GPSD & PPS

Sander Smeenk ssmeenk at freshdot.net
Tue Dec 9 14:29:45 UTC 2014


Hi,

I run a stratum 1 server which has a Garmin LVC 18x connected to its ttyS0.
The GPS provides a PPS signal via serial and i use gpsd to provide the
NMEA sentences and pulse data in shared memory to NTP.

This partly works. NTP syncs against the PPS signal but the NMEA signal
is always marked as falseticker even though i managed to bring down the
offset to -1.5ųsec average by fudging the time a bit. The NMEA signal
offset fluctuates a lot. From ~ -65ųsec to ~ +75ųsec.

The GPS provides 9600bps serial comms. Would it help to speed this up to
19200bps? I've already disabled all NMEA sentence output for sentences that
"aren't useful for timekeeping" but at this moment i have to use external
clocks to sync against.

Few questions:

1) Can i get a 'true PPS sync' with this setup?
Eliminating gpsd so 'ntpq -p' shows 'oSHM(1)' instead of '*SHM(1)' ?

2) What could i possibly do to get NTP to sync/accept the NMEA data,
other than set it as a truechimer in the configuration?

3) Why does last_event show clock_alarm for the PPS SHM signal in assoc?


At this moment my 'ntpq -p' looks like:

| # ntpq -np
|      remote           refid      st t when poll reach   delay   offset jitter
| ==============================================================================
|  127.127.28.0    .NMEA.           0 l   10   16  377    0.000  -52.910 12.585
| *127.127.28.1    .PPS.            0 l    9   16  377    0.000   -0.002 0.002
| -193.67.79.202   .PPS.            1 u   50   64  377    3.690    0.331 0.073
| +193.79.237.14   .PPS.            1 u   48   64  377    2.753    0.128 0.725
| +80.94.65.10     .PPS.            1 u   63   64  377    3.226    0.005 0.498
| -130.89.0.19     103.52.146.131   2 u   30   64  377    5.417   -0.100 0.041

| # ntpq -c rv
| associd=0 status=0415 leap_none, sync_uhf_radio, 1 event, clock_sync,
| version="ntpd 4.2.6p5 at 1.2349-o Wed Oct  9 19:08:06 UTC 2013 (1)",
| processor="x86_64", system="Linux/3.13.0-39-generic", leap=00, stratum=1,
| precision=-23, rootdelay=0.000, rootdisp=0.386, refid=PPS,
| reftime=d8318503.7827c0d3  Tue, Dec  9 2014 15:26:11.469,
| clock=d831850e.45ff40f3  Tue, Dec  9 2014 15:26:22.273, peer=53550, tc=4,
| mintc=3, offset=0.001, frequency=-19.125, sys_jitter=0.003,
| clk_jitter=0.001, clk_wander=0.000

| # ntpq -c as
| ind assid status  conf reach auth condition  last_event cnt
| ===========================================================
|   1 53549  9024   yes   yes  none    reject   reachable  2
|   2 53550  964b   yes   yes  none  sys.peer clock_alarm  4
|   3 53551  931d   yes   yes  none   outlyer              1
|   4 53552  9424   yes   yes  none candidate   reachable  2
|   5 53553  9424   yes   yes  none candidate   reachable  2
|   6 53554  931d   yes   yes  none   outlyer              1


Thanks for any input!

-Sander.
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