[ntp:questions] Single GPS/PPS time source gets marked as a falseticker

Rob nomail at example.com
Thu Jul 14 14:26:07 UTC 2011


unruh <unruh at wormhole.physics.ubc.ca> wrote:
> On 2011-07-14, Rob <nomail at example.com> wrote:
>> Michael Eder <meder at whoi.edu> wrote:
>>> We have looked at our GPS on a scope, the PPS it is dead on and the NMEA
>>> (just one sentence) is also reliable with about a 680 ms latency and 10 ms
>>> jitter.
>>
>> Again, are you using gpsd?
>>
>> If so, you may want to try removing the (huge) 680ms offset inside gpsd
>> instead of in ntpd.
>> There is a place in the code where a fixed offset is added to time obtained
>> using NMEA (because the gpsd author does not want configurable items) and
>> it cannot be correct for every possible receiver.
>>
>> Again, it is better to switch from NMEA to the native binary protocol
>> of the receiver.
>
> Why? That native binary will also suffer from large latency (the serial link
> is slow no matter what is being sent over it) and probably also fairly
> large jitter. And it suffers from the huge downside of your having to
> learn a whole new language.

The native binary protocol usually has a SPECIFICATION about the time
field in the messages (like: it is the current time at the moment the
beginning of the message is leaving the receiver).  The NMEA protocol
has a "time of fix", and that tells you nothing about what time it is
now.

There is no language to be learned as those details are handled by the
drivers inside gpsd.  That is what gpsd is for.

(assuming he is using gpsd.  he is very secretive about that)




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