Since the old gpskit gps was showing pro ... / 2014-02-16

Since the old gpskit gps was showing problems in ntp tests earlier I decided now that the weatherstation computer is up and running on the alix.1c board to try a different gps unit: The Holux GR-213 GPS I still have from earlier wardriving.

Not much of a succes sofar. First the GPS did not get a lock at all. I was expecting a delay in acquiring a lock since it hadn't been used in over a year but after a day and a half it still wasn't locking. So I moved it a bit which led to a lock (blinking led). But ntpd was still not using the GPS_NMEA driver. When I had time to have more of a look than just the graphs at NTP server ritchie.idefix.net stats I noticed ntpd was still seeing GPS_NMEA as a falseticker. Which is about right, when I look at the peer stats the GPS_NMEA clock has an offset of about 500 milliseconds(!!) compared to the rest.

To my best knowledge I can find the right offset with 'enable calibrate'. But documentation is very minimal on this matter: Reference clock drivers - ntp 4.0.99k documentation has:

The recommended procedure is to enable the function, let it run for an hour or so, then edit the configuration file using the time1 values displayed by the ntpq utility and clockvar command.

With 'enable calibrate' on I see after a long run:

ntpq> peer
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+greenblatt.idef 131.211.8.244    2 u  137  512  377    1.012    1.780  87.722
*metronoom.dmz.c .PPS.            1 u  166  512  377   18.297   -1.207  46.401
+auth1.xs4all.nl 193.67.79.202    2 u  119  512  377   16.604   -1.104  27.267
xGPS_NMEA(0)     .GPS.            0 l    4   16  377    0.000  -529.94   3.286
ntpq> clockvar
associd=0 status=0000 , no events, clk_unspec,
device="NMEA GPS Clock",
timecode="$GPGGA,210752.000,5206.6230,N,00507.0976,E,1,06,1.5,-0.1,M,47.1,M,,0000*7F",
poll=762, noreply=0, badformat=0, baddata=0, fudgetime1=0.000, stratum=0,
refid=GPS, flags=0

So even after running for a long time with clearly an offset between the other clocks and the reference clock there is no change in the suggestion for the time1 factor, still showing 0.000.

Remarks in [ntp:questions] enable calibrate? suggest 'enable calibrate' will only work when there is a PPS signal available, and confirm the lack of documentation and samples I found.

The Holux GR-213 also does not have a PPS signal to the outside at all, so I can't use a PPS signal anyway.

Update: Some sleep, thinking and reading later: first of all, time1 is the PPS time offset and time2 is the gps message offset, found by reading ntpd documentation Generic NMEA GPS driver.

So I started looking for the right offset with the 127.127.20.0 driver in noselect mode. After some testing I found a reasonable answer with:

# GPS as time source without pps
server 127.127.20.0 minpoll 1 maxpoll 4
fudge 127.127.20.0 time2 +0.544

And now things look better:

ntpq> peer
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+greenblatt.idef 131.211.8.244    2 u    2   64   77    0.956   -2.252  41.400
*metronoom.dmz.c .PPS.            1 u    5   64   77   17.921   -2.236   1.190
-auth1.xs4all.nl 193.67.79.202    2 u    3   64   77   16.254   -2.750   0.880
+GPS_NMEA(0)     .GPS.            0 l    2    8  377    0.000   -5.916   1.100