[ntp:questions] UTC Time from NMEA receiver one second behind DCF?

Harald Brinkmann hb7267 at gmx.de
Tue Aug 12 21:02:30 UTC 2008


David J Taylor wrote:

> Venu Gopal wrote:
>> David J Taylor wrote:
>>> Venu Gopal wrote:
>>>> I have a couple of similar experiences !
>>>> I observed that the NMEA sentences are not generated in sync with
>>>> PPS. Theres lot of jitter in these sentences resulting in 1 second
>>>> offsets. Its fine if the jitter is within few milliseconds. But
>>>> sometimes it exceeds a second and thats really painful.
>>>>
>>>> This observation was discussed earlier and the solution is to go for
>>>> a better GPS receiver!
>>>
>>> Use the shortest GPS sentences, and the highest baud rate, to keep
>>> the total message time as short as possible.
>>>
>> The issue is not with the length of the messages and the baud rate.
>>
>> Venu
> 
> Well, I found that if you had a bad configuration (sentences too long or
> baud rate too low), the sentences could exceed one second, and therefore
> be useless for NTP.
> 
> However, I have not made any measurements showing jitter versus sentence
> length, so I would appreciate any pointers to measurements you have made
> or know about.
> 
> Cheers,
> David

But even problems caused by a slow data transmission wouldn't explain why
the observed offset is small compared to one second most of the time (and
in my case continuously for well over a year) and suddenly and only on
certain days there is an offset of almost exactly one second, unless the
messages are suddenly and only on those days extraordinarily long and just
happen to delay everything by one second. I'll have a lookout for the raw
NMEA messages the next time this happens. *If* I were using a PPS signal, I
could understand the system being confused about which NMEA message belongs
to which PPS pulse, so one would get an offset of (maybe multiples of) one
second. But that's not the case here.

Harald




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