[ntp:questions] NMEA ref.clock better than my ISP's timeserver?

David Lord snews at lordynet.org
Sun Jun 28 09:54:23 UTC 2009


Hal Murray wrote:
>> I'm not sure whether the NMEA driver attempts to send anything /to/ the 
>> GPS device.  If not, three lines might to (ground, TX from GPS, PPS), 
>> otherwise four lines are required.  If you want to take USB power from the 
>> server (as I now do), it's five lines minimum.
> 
> The driver expects the data to arrive without any help.  That
> works with all the NMEA devices I've worked with.  You set them
> up ahead of time by sending them a sequence of commands which
> differ from device to device.
> 
> The driver does send "$PMOTG,RMC,0000*1D\r\n" every poll interval,
> with a comment about some Motorola device needing it.  (That's one
> per poll interval rather than one per second.  I don't expect it
> to work very well.)

I used input to disable non required NMEA sentences but I'm not
sure if any apparent improvement is due to that, temperature or
system load or other effects.

This system doesn't have network connection or mrtg. Ntp offsets
taken at 30 minute intervals with/without PPS and  with default
NMEA sentences vs RMC only. Garmin gps18x-lvc

nmea_std.dat
*      +/-offset     Num     Tot       %
*           (ms)
*           0.12       5      25      56
*           0.16       2      40      89
*           0.17       5      45     100

pps_std.dat
*      +/-offset     Num     Tot       %
*           (ms)
*          0.004      25      79      52
*          0.010       7     148      97
*          0.014       2     153     100

nmea_rmc.dat
*      +/-offset     Num     Tot       %
*           (ms)
*           0.07       5      27      51
*           0.16       3      49      92
*           0.20       1      53     100

pps_rmc.dat
*      +/-offset     Num     Tot       %
*           (ms)
*          0.002      49      76      54
*          0.005       7     140      99
*          0.006       1     141     100


David




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