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- NTPClockOffset: the estimated offset of the GPS time from the data system time. A positive value indicates that NTP is estimating that the GPS clock is ahead of the system clock, i.e. the GPS showing a later time than the system clock.
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On a warmer 3 day period in July, where the temperatures were all above 5 degC, the temperature effect on the system clock oscillator is very evident.
PPSTEST
On the DSM, the ppstest program is helpful for gaining an understanding of the system and GPS clocks. It displays the system clock value at the time of the assertion and clear of the PPS square pulse. Do ctrl-C to terminate.
| Code Block |
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root@manitou root# ppstest /dev/ttyS3
trying PPS source "/dev/ttyS3"
found PPS source #3 "serial3" on "/dev/ttyS3"
ok, found 1 source(s), now start fetching data...
source 0 - assert 1315494544.999995675, sequence: 37249847 - clear 1315494544.099998000, sequence: 37249862
source 0 - assert 1315494544.999995675, sequence: 37249847 - clear 1315494545.099995000, sequence: 37249863
source 0 - assert 1315494545.999994675, sequence: 37249848 - clear 1315494545.099995000, sequence: 37249863
source 0 - assert 1315494545.999994675, sequence: 37249848 - clear 1315494546.099993000, sequence: 37249864
source 0 - assert 1315494546.999994675, sequence: 37249849 - clear 1315494546.099993000, sequence: 37249864
ctrl-C
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The above sequence shows that the GPS is ahead of the system clock. The interrupt function is being called 5 microseconds before each second (1.0 - 0.999995). This corresponds to a NTPClockOffset of 5 microsecond, which is confirmed with the ntpq program (which reports its offset in milliseconds):
| Code Block |
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ntpd -p
root@manitou root# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
xral 38.229.71.1 3 u 34 64 377 0.320 3.804 0.031
LOCAL(0) .LOCL. 10 l 93d 64 0 0.000 0.000 0.000
oGPS_NMEA(0) .GPS. 2 l 6 16 377 0.000 0.005 0.031
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The ntpq output indictes that NTP is using the GPS as the system's reference clock. It also displays the offset of the RAL server's clock of 3.804 milliseconds. The RAL server uses NTP over a WIFI connection to adjust its clock, so it is not as accurate as the DSM.
The loopstats file also shows the 5 usec offset at this time:
| Code Block |
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55812 54504.454 0.000005000 39.301 0.000030518 0.001408 4
55812 54520.455 0.000006000 39.302 0.000030518 0.001415 4
55812 54536.454 0.000005000 39.303 0.000030518 0.001392 4
55812 54552.454 0.000005000 39.305 0.000030518 0.001372 4
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