[ntp:questions] NMEA ref.clock better than my ISP's timeserver?

Rob nomail at example.com
Fri Jun 12 18:00:27 UTC 2009


Unruh <unruh-spam at physics.ubc.ca> wrote:
> Well, I suppose you could somehow set the thing up to grab the rise time
> of the first bit sent along the serial line, but there is no evidence
> that the maker of the GPS made even that first bit at all accurate. Ie,
> the time when that first bit is sent could well vary by milliseconds.
> NMEA was never intended for accurate timeing.

The time field in the NMEA packet is the time of the fix that the data
in the packet is related to, presumably the most recent fix the unit
has calculated.
Interpreting it as "the current time" is quite risky.  The delay between
the calculation of the fix and the transmission of the packets depends
on implementation details, and on the amount of data being transmitted.
(an NMEA command can be issued to instruct the unit to transmit fewer
or more different types of data packets)

When you want more reliable time information, it often is better to
put the unit into the binary protocol mode.  However, this is different for
every manufacturer, so it complicates the writing of drivers.

gpsd tries to put the unit in binary mode.  it often provides more
detail as well.
however, the timing remains tricky.  to get reliable timing, you really
need PPS in addition to serial data.




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