[ntp:questions] nmea and initial large offset
unruh at wormhole.physics.ubc.ca
Thu May 6 14:36:18 UTC 2010
On 2010-05-05, Kalle Pokki <kalle.pokki at iki.fi> wrote:
> On Wed, May 5, 2010 at 16:44, Andy Helten <andy.helten at dot21rts.com> wrote:
>> Yes, this is exactly right. ?We worked around this problem in our system by
>> grabbing the time from the refclock, setting system time with the refclock
>> time, quick sync NTP, and then finally start the NTP daemon.
> This is probably the only possible workaround without modifying NTP
> code itself. However, in a vehicle that boots and starts all
> operations in a place without GPS coverage, this also becomes quite
> impossible to arrange properly.
> The funny thing is that the exact date is provided by e.g. the SHM
> refclock driver, but the year information is intentionally discarded
> when the timestamp propagates up towards the NTP core code. Then the
> code does all kinds of heuristics to guess the correct year, but is
> unable to do that if the clock is off by more than four hours.
I do not understand this. The ntp timestamp provides the date as well as
the time to ns. While the date has an ambiguity of soemthing like
100years, the assumption is that the century is the current one.
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