[ntp:questions] Single GPS/PPS time source gets marked as a falseticker

unruh unruh at wormhole.physics.ubc.ca
Tue Jul 12 19:29:00 UTC 2011


On 2011-07-12, Chris Albertson <albertson.chris at gmail.com> wrote:
> The simplest explanation is that everything is working as expected but
> your GPS receiver is just not very good.  What you might try is
> turning off all the NMEA sentences you are not using.   I don't know
> if the SHM driver degrades the accuracy or not but you might try
> removing it. (See what others say about SHM)  I saw this same thing
> but only worse when I tried using a hand held GPS for NTP.

That may be simple to you, but highly unlikely. 
The PPS almost certainly does deliver highly accurate times.
Unfortunately it is useless for determining which second the pulse if
for. For that you need either the nmea or some net source of time. 
The shm driver is fine. It works to microsecond accuracy, assuming it is
fed properly. 

>
> On Tue, Jul 12, 2011 at 9:26 AM, Michael Eder <meder at whoi.edu> wrote:
>> Below is the NTP.conf file.
>>
>> The system runs great but the status does not seem to like our sources.
>>
>> ? ? remote ? ? ? ? ? refid ? ? ?st t when poll reach ? delay ? offset
>> jitter
>> ============================================================================
>> ==
>> xSHM(0) ? ? ? ? ?.NMEA. ? ? ? ? ? 0 l ? 13 16 377 0.000 ? 43.755
>> 66.491
>> *SHM(1) ? ? ? ? ?.PPS. ? ? ? ? ? ?0 l ? ?9 16 377 0.000 ? ?2.466
>> 0.255
>> ?IP-208-40-229-1 .STEP. ? ? ? ? ?16 - ? ?- ?256 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>> ?64.73.32.135 ? ?.STEP. ? ? ? ? ?16 - ? ?- ?256 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>> ?meander.cross-l .STEP. ? ? ? ? ?16 - ? ?- ?512 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>> ?nist.netservice .STEP. ? ? ? ? ?16 - ? ?- ?512 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>>
>> assID=0 status=09d4 leap_none, sync_telephone, 13 events,
>> event_peer/strat_chg,
>> version="ntpd 4.2.2p4 at 1.1585-o Sun Nov 22 17:01:18 UTC 2009 (1)",
>> processor="armv4tl", system="Linux/2.6.21-ts", leap=00, stratum=1,
>> precision=-14, rootdelay=0.000, rootdispersion=3.164, peer=6590,
>> refid=PPS, reftime=d1c6f2ab.4689f159 ?Tue, Jul 12 2011 16:19:55.275,
>> poll=7, clock=d1c6f2b4.5ede7151 ?Tue, Jul 12 2011 16:20:04.370, state=4,
>> offset=2.466, frequency=-119.343, jitter=0.255, noise=0.950,
>> stability=0.032, tai=0
>>
>> synchronised to modem at stratum 1
>> ? time correct to within 3 ms
>> ? polling server every 128 s
>> ? ? remote ? ? ? ? ? refid ? ? ?st t when poll reach ? delay ? offset
>> jitter
>> ============================================================================
>> ==
>> xSHM(0) ? ? ? ? ?.NMEA. ? ? ? ? ? 0 l ? ?- ? 16 ?377 ? ?0.000 ? 47.508
>> 22.302
>> xSHM(1) ? ? ? ? ?.PPS. ? ? ? ? ? ?0 l ? 11 ? 16 ?377 ? ?0.000 ? ?2.466
>> 0.255
>> ?IP-208-40-229-1 .STEP. ? ? ? ? ?16 - ? ?- ?256 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>> ?64.73.32.135 ? ?.STEP. ? ? ? ? ?16 - ? ?- ?256 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>> ?meander.cross-l .STEP. ? ? ? ? ?16 - ? ?- ?512 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>> ?nist.netservice .STEP. ? ? ? ? ?16 - ? ?- ?512 ? ?0 ? ?0.000 ? ?0.000
>> 0.000
>>
>> assID=0 status=00e4 leap_none, sync_unspec, 14 events, event_peer/strat_chg,
>> version="ntpd 4.2.2p4 at 1.1585-o Sun Nov 22 17:01:18 UTC 2009 (1)",
>> processor="armv4tl", system="Linux/2.6.21-ts", leap=00, stratum=1,
>> precision=-14, rootdelay=0.000, rootdispersion=3.194, peer=0, refid=PPS,
>> reftime=d1c6f2ab.4689f159 ?Tue, Jul 12 2011 16:19:55.275, poll=7,
>> clock=d1c6f2b6.a96e3689 ?Tue, Jul 12 2011 16:20:06.661, state=4,
>> offset=2.466, frequency=-119.343, jitter=0.255, noise=0.950,
>> stability=0.032, tai=0
>>
>> synchronised to unspecified at stratum 1
>> ? time correct to within 3 ms
>> ? polling server every 128 s
>>
>>
>>
>> # /etc/ntp.conf, configuration for ntpd
>>
>> driftfile /var/lib/ntp/ntp.drift
>> statsdir /var/log/ntpstats/
>>
>> statistics loopstats peerstats clockstats
>> filegen loopstats file loopstats type day enable
>> filegen peerstats file peerstats type day enable
>> filegen clockstats file clockstats type day enable
>>
>> #For testing GPS w/ shared-memory
>> server 127.127.28.0 minpoll 4
>> fudge 127.127.28.0 time1 0.680 refid NMEA
>> server 127.127.28.1 minpoll 4 prefer
>> fudge 127.127.28.1 refid PPS
>>
>> # pool.ntp.org maps to more than 300 low-stratum NTP servers.
>> # Your server will pick a different set every time it starts up.
>> # ?*** Please consider joining the pool! ***
>> # ?*** <http://www.pool.ntp.org/join.html> ***
>> server 0.debian.pool.ntp.org iburst
>> server 1.debian.pool.ntp.org iburst
>> server 2.debian.pool.ntp.org iburst
>> server 3.debian.pool.ntp.org iburst
>>
>> # By default, exchange time with everybody, but don't allow configuration.
>> # See /usr/share/doc/ntp-doc/html/accopt.html for details.
>> restrict -4 default kod notrap nomodify nopeer noquery
>> restrict -6 default kod notrap nomodify nopeer noquery
>>
>> # Local users may interrogate the ntp server more closely.
>> restrict 127.0.0.1
>> restrict ::1
>>
>>
>> -----Original Message-----
>> From: Michael Eder [mailto:meder at whoi.edu]
>> Sent: Tuesday, July 12, 2011 8:01 AM
>> To: 'questions at lists.ntp.org'
>> Subject: Single GPS/PPS time source gets marked as a falseticker
>>
>> Hi Folks,
>>
>> For our buoy deployments we have GPS and a highly accurate oscillator
>> conditioned by the GPS PPS. ?Unfortunately when we let this run for longer
>> periods we see both the GPS and PPS being marked as false tickers. ?There is
>> a lot of information on the web about this, but I have not found a
>> definitive solution. ?What is the best way to configure NTP when you only
>> have a GPS NMEA string and pulse per second? ? In our situation it is
>> feasible that the NMEA may not be always be available while the PPS is, we
>> are seeing a problem when both the NMEA and pulse seem to be working
>> properly.
>>
>> I have also seen some discussion about issues regarding what needs to happen
>> when NTP first comes up in this scenario. ?Is it a problem to have the PPS
>> always available even before the NMEA string can fully set the time?
>>
>> I did try setting minclock and minsane, to two and one respectively, but
>> this did not seem to change the situation.
>>
>> Any suggestions on how to configure for this situation and/or things I
>> should be aware of would be much appreciated.
>>
>> Thanks in advance,
>>
>> Kind Regards,
>>
>> Michael Eder
>>
>> Added this to the cfg file
>> #Look for two clocks instead of three for falsetickers - tos minclock 2
>> minsane 1 but still get the following.
>>
>> ce000:/etc# ntpq -p 127.0.0.1
>> ? ? remote ? ? ? ? ? refid ? ? ?st t when poll reach ? delay ? offset
>> jitter
>> ============================================================================
>> ==
>> xSHM(0) ? ? ? ? ?.NMEA. ? ? ? ? ? 0 l ? 12 16 377 0.000 ?-407.31
>> 42.404
>> xSHM(1) ? ? ? ? ?.PPS. ? ? ? ? ? ?0 l ? ?8 16 377 0.000 ?-256.43
>> 2.003
>>
>>
>> Seems like one clock (pps) does better than pps and gps.
>>
>> ce000:/etc# ntpq -p 127.0.0.1
>> ? ? remote ? ? ? ? ? refid ? ? ?st t when poll reach ? delay ? offset
>> jitter
>> ============================================================================
>> ==
>> *SHM(1) ? ? ? ? ?.PPS. ? ? ? ? ? ?0 l ? 12 16 377 0.000 ? -6.243
>> 0.238
>>
>>
>> Michael Eder
>> Woods Hole Oceanographic Institution
>> AOP&E/OOI
>> www.whoi.edu/programs/ooi
>>
>>
>>
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>
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