[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 
>> here:
>>
>>  http://www.leapsecond.com/pages/MG1613S/
>>
>> under the heading "NMEA Latency".  The graph here is 100 milliseconds 
>> full scale.
>>
>>  http://www.leapsecond.com/pages/MG1613S/nmea-jitter-1.gif
>>
>
> 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 
chipset.

(Is jitter RMS, SD, peak-to-peak?).

Cheers,
David 



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