[ntp:questions] best gps receiver for time synchronization

Rob nomail at example.com
Mon May 11 16:00:00 UTC 2009


Hal Murray <hal-usenet at ip-64-139-1-69.sjc.megapath.net> wrote:
> It uses the SiRF chip set.  There is no PPS.  The timing
> on the NMEA strings is bad to horrible.
>
> If anybody has figured out how to get good timing out of the
> SiRF units, please clue me in.

The NMEA protcol is unsuitable for accurate timing.  There often is
not even a spec for the delay between calculating a fix and sending
the NMEA data for a given device, and this delay often varies according
to internal priorities and scheduling.
Also note that the time field in NMEA packets is a "time of fix" and
not a "current time".

There are SiRF units that output PPS and the timing is no problem
with those.  But they do not seem to exist with USB interface.
(probably because there is no way to send the PPS signal over USB
with accurate timing)

You need an RS232 model, that puts the PPS on DCD or CTS.  At least the
SiRF chips send reasonably long PPS pulses, that can be transmitted
over RS232 without external pulse stretcher (as some Trimble devices
do require)




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