Monitoring the accuracy of the system clock on a real-time data acquisition system provides useful information about the performance the system. Hence this long discussion.
Data System Clock, NTP and GPS
The turbulence tower data system at the Manitou Forest Observatory (aka, the DSM) uses a GPS receiver and the NTP (Network Time Protocol) software to set the system clock, which, in addition to the normal uses of a system clock, is used to time-tag the data samples.
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On April 12, 2011 the old Garmin GPS 25-HVS at the tower was replaced with a much newer Garmin 18x-LVC model. The model numbers are shown in the $PGRMT messages in the archive, where the time is UTC:
| Code Block |
|---|
data_dump -i 1,30 -A manitou_20110412_120000.bz2 | fgrep GPSPGRMT ... 2011 04 12 16:41:39.6568 0.15 49 $PGRMT,GPS 25-HVS VER 2.50 ,P,P,R,R,P,,23,R*08\r\n 2011 04 12 16:42:50.4248 0.1249 51 $PGRMT,GPS 18x-LVC software ver. 3.10,,,,,,,,*6D\r\ |
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The periodic spikes in GPSdiff_max up to 1 second that occur at 23:00 local time and last about an hour, are simultaneous with the network transfer of the day's data files from the DSM to the RAL server. These suggest that increased sampling sample buffering and latency is happening at these times, which needs to be investigated and improved.
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In July, the clock behaviour during the file transfer is similar, but the initial increase in NTPClockOffset and a rising slope in NTPFreqOffset , might be due to increased heating of the system clock oscillator, due to increased CPU load during the file transfers. Wild conjecture? After a quick scan of the web plots of 5 minute averages, I think these positive bumps in NTPFreqOffset seem to occur during file transfers when the outside air temperatures are above 0 C, and don't occur in colder conditions.
ppstest and ntpq
On the DSM, the ppstest program is helpful for gaining an understanding of the system and GPS clocks. It displays the system clock value when the interrupt function is called at the time of the assertion and the clear of the PPS signal. Do ctrl-C to terminate ppstest.
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