[ntp:questions] System time ramps forcing step correction?

David Lord snews at lordynet.org
Mon Sep 10 11:40:20 UTC 2012


A C wrote:
> On 9/9/2012 15:41, E-Mail Sent to this address will be added to the 
> BlackLists wrote:
>> A C wrote:
>>> Has anyone seen a system with GPS/PPS have its system
>>>   time ramp/slew ahead by over a second requiring a step
>>>   correction?
>>
>> Prefer the PPS source?
>>
>> GPS time is usually only second relative without the PPS,
>>   and will sometimes drift around a lot within the second
>>   on non timing GPSes.
>>
>>   I have GPS chips on embedded systems that the serial sentence
>>    always seems to wander around 20ms to 50ms.
>>
> 
> NTP doesn't let me prefer the PPS source (ATOM).  I'm forced to prefer 
> some other source and let PPS activate on its own.

When atom driver was working reliably I had both type-20
and type-22 drivers specified, the type-20 NMEA gave the
"second" and PPS sync was from type-22 atom.

Because of the wander of the nmea value it was also
required that I had "tos mindist 0.5"  and  used a large
fudge time for the nmea. Both required values depended on
the particular make and firmware revision of the GPS unit.


David



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