[ntp:questions] Jitter on $GPGGA of Sure board

unruh unruh at invalid.ca
Wed Mar 21 17:44:06 UTC 2012


On 2012-03-21, Gom <gom at gom.invalid> wrote:
> However if you use NTP, it will average the values, hence reducing the 
> jitter by sqrt(number of seconds of polling).
>
> So if you use 8 seconds, jitter of the Sure NMEA clock will be 
> 10/sqrt(8)= 3,5 ms.
>
> This is still waaaay jigher than the jitter of the Sure PPS : 22 ns

You are comparing apples to oranges. The Sure nmea jitter is far far
higher than that. The Sure PPS jitter is far higher than that if you use
the RS232 level conversion. The Sure PPS jitter is far higher than that
if you run it into your computer's interrupt routines. 

It is like comparing the nmea sentences with fundamental timing
limitations of the GPS (3m is about 3ns and differeential at 1 cm is
about .03 ns.) but there is no way of getting that into your computer. 


Note that ntp does NOT take the average. it takes the median of about
60% of the past 8 inputs (as I understand it). 


>
> Le 21/03/2012 16:54, Gom a ?crit :
>> Hi,
>>
>> The thread priority is already maximum.
>>
>> The program has no problem disciplining the clock, as it's able to keep
>> the calculated offset to under 30 ?s with the precise PPS.
>>
>> What I wanted to show is that if you use the basic NMEA you will get
>> about 50ms error on the arrival of the first NMEA sentance.
>>
>> That is confirmed here : http://www.leapsecond.com/pages/MG1613S/
>> Check paragraph "NMEA latency" , you will see the dots are scattered in
>> about a 50 ms windows. If you do the math on the distribution this
>> represents a 10 ms jitter (standard deviation).
>>
>>



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