[ntp:questions] PSYCHO PC clock is advancing at 2 HR per second
David J Taylor
david-taylor at blueyonder.co.uk.invalid
Wed Mar 21 06:49:34 UTC 2012
"Ron Frazier (NTP)" <timekeepingntplist at c3energy.com> wrote in message
news:4F692255.2020304 at c3energy.com...
> On 3/20/2012 11:21 AM, David J Taylor wrote:
>> You /will/ see variation in the serial output from the Sure device, as
>> you will in many NMEA devices. For the Sure device, one measurement is
>> under the heading "NMEA Latency". The graph here is 100 milliseconds
>> full scale.
> That's funny, there's this line in the text.
> "In a 15 minute run (900 seconds) the mean latency was 350.2 ms with a
> standard deviation (jitter) of 10.7 ms. "
> Then there's the graph, which seems to show a variance in NMEA start
> time of 75 ms or so. The two seem to contradict each other.
How so? Are you taking the peak-to-peak figures from the graph and
comparing it to the standard deviation? SD isn't a peak-to-peak value.
> You already mentioned the Garmin previously, and the Sure now, and I
> have reports of similar NMEA drifting behavior from other SIRF units.
> So, it appears that most, if not all GPS's exhibit a variance in the
> timing of NMEA data of 50 to 120 ms or so. That would definitely put a
> limit on what you could do with NMEA only data.
Yes, in typical GPS receivers the NMEA data is only accurate to a second -
it says where the unit was at the UTC second preceding the data. I don't
believe that jitter in NMEA output time is limited to one particular
(Is jitter RMS, SD, peak-to-peak?).
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