[ntp:questions] what happens when sys.peer turns stratum 16?
nomail at example.com
Thu Jun 3 07:29:38 UTC 2010
Hal Murray <hal-usenet at ip-64-139-1-69.sjc.megapath.net> wrote:
>>Apparently their firmware is crap.
>>But have you also seen this with bare GPS receivers?
>>Wrong time but indication of lock to GPS (2D or 3D).
> I've seen output from a Garmin GPS-18 that was off by a second.
> I don't have details. It's on my list to play with.
> There is fine print in some documentation that says something
> about needing 4 seconds of good before believing the PPS,
> or something like that. (Or maybe I'm confusing it with
> something else.)
NMEA should not be trusted for time messages. The time in an
NMEA message is the "time of fix", i.e. the time at which the last
fix calculation was made in the receiver, and at which moment the
position values in the message were valid. There is no guarantee
that this is the CURRENT time, as the fix may be transmitted to
the outside world delayed by an unspecified amount of time.
And indeed, there often is a noticable and variable delay between
the time in the message and the moment it is sent. In certain situations
the delay may be more than a second, and you see the "off by a
For the binary protocol this situation often is a bit better. There
usually exist special messages to send the current time, and the
delay between fix and message may be specified better. Also, the
bitrate is often higher (rather than the default 4800 bps for NMEA).
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