GPS Sync Outage

Wed Jan 1 09:48:16 UTC 2020

The following is from one ntpd instance in Chennai, India. We use Garmin
GPS 18x LVC receivers with our ntpd instances. ntpd interfaces directly
to the receiver using its NMEA driver and also receives a PPS signal
from the GPS receiver.

During last night, the ntpd process suddenly dropped its GPS reference
clock. This is the first time we've observed this happening.  ntpq -np
showed an 'x' in the remote field where an 'o' should be. 'x' means "Out
of tolerance, do not use (discarded by intersection algorithm);"
(https://nlug.ml1.co.uk/2012/01/ntpq-p-output/831)

It was noticed in the morning. A drift is observed in
/var/log/peerstats.20191231. Corresponding to the log times of where the
drift starts, the $GPRMC sentences from /var/log/clockstats.20191231
show the following:

58848 73710.974 127.127.20.0 $GPRMC,202830,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73726.974 127.127.20.0 $GPRMC,202846,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73742.974 127.127.20.0 $GPRMC,202902,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73758.974 127.127.20.0 $GPRMC,202918,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73774.974 127.127.20.0 $GPRMC,202934,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73790.974 127.127.20.0 $GPRMC,202950,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73806.974 127.127.20.0 $GPRMC,203006,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73822.974 127.127.20.0 $GPRMC,203025,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73838.973 127.127.20.0 $GPRMC,203041,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73854.973 127.127.20.0 $GPRMC,203057,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73870.973 127.127.20.0 $GPRMC,203113,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73886.974 127.127.20.0 $GPRMC,203129,A,____.____,_,_____.____,_,000.0,029.5,311219,001.5,W,D*__
58848 73902.973 127.127.20.0 $GPRMC,203145,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 73918.973	127.127.20.0 $GPRMC,203201,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 73934.973 127.127.20.0 $GPRMC,203217,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 73950.973 127.127.20.0 $GPRMC,203233,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 73966.973 127.127.20.0 $GPRMC,203249,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 73982.973 127.127.20.0 $GPRMC,203305,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 73998.973 127.127.20.0 $GPRMC,203321,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74014.973 127.127.20.0 $GPRMC,203337,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74030.973 127.127.20.0 $GPRMC,203350,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74046.973 127.127.20.0 $GPRMC,203406,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74062.973 127.127.20.0 $GPRMC,203422,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74078.973 127.127.20.0 $GPRMC,203438,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74094.973 127.127.20.0 $GPRMC,203454,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74110.973 127.127.20.0 $GPRMC,203510,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,A*__
58848 74126.973 127.127.20.0 $GPRMC,203526,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 74142.974 127.127.20.0 $GPRMC,203542,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__
58848 74158.973 127.127.20.0 $GPRMC,203558,A,____.____,_,_____.____,_,000.0,004.7,311219,001.5,W,D*__

These messages are logged once every 16 seconds. The $GPRMC time (UTC)
jumped from 203006 to 203025 (delta 19 seconds), and then corrected back
from 203337 to 203350 (delta 13 seconds). Note that these $GPRMC lines
are the (derived) output of the GPS receiver.

ntpd was restarted this morning, and appears to use the GPS timesource
properly.