[ntp:questions] 1s offset

David Taylor david-taylor at blueyonder.co.uk.invalid
Thu Mar 7 09:29:47 UTC 2013


On 06/03/2013 20:22, unruh wrote:
[]
> That should all be in the same second. The nmea sentence cannot come
> before the PPS, so the "one second" should all be between the PPS pulse
> and the next one. The problem occurs if the end of the sentence comes
> after the next pulse occurs.
>
> The code should probably use the beginning of the sentence, not the end,
> to mark the time. It can always throw away the timestamp if it is not
> needed. Or at least once it knows that the appropriate sentence is
> coming. (the first four or 5 characters received).
[]

Yes, it /should/ be the same second, but it depends on how NTP may, or 
may not, round the value 0.6, for example.  I haven't looked at the code 
to check the actual behaviour.

The problem of going into the "next" second, requiring a fudge of over 1 
second, did occur with one lot of Garmin firmware, but was fixed.

Yes, it was whether the beginning or end of the sentence was used that 
made me wonder about the different behaviour of different NTP versions. 
I recall that in the Windows version it is the end of the first sentence 
received which has the PPS timestamp substituted as the time of 
reception.  I am less familiar with the UNIX versions.
-- 
Cheers,
David
Web: http://www.satsignal.eu



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