[ntp:questions] help - clock jumps up to 5000 ms startup ntpd with usb gps linux

Dave Hart davehart at gmail.com
Sun Feb 19 23:13:26 UTC 2012

On Sun, Feb 19, 2012 at 16:32, unruh <unruh at invalid.ca> wrote:
> On 2012-02-19, Rob <nomail at example.com> wrote:
>> Don't try to use NMEA for timesyncing!  It will only drive you crazy.
> No. It is fine.
>> The timestamp in NMEA is a "time of last fix", not the current time.
> And ntpd allows you to alter the offset timestamp to account for the
> delay in the end of the nmea message coming through. That it uses the
> end of the message rather than the beginning causes excess jitter, but
> that is never anywhere near 5000ms.

To elaborate a wee bit, each NMEA sentence parsed by the driver
effectively has two timestamps.  One is the timestamp implied by the
sentence text, which typically has 1s precision.  The other is the
timestamp the complete line was received, which has higher precision
but reflects the transmission time and benefits from fudge time2 being
calibrated to match.

Dave Hart

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