[Lowfer] Use of NMEA strings for GPS 1 pps time tagging

[Bill de Carle]

At 01:40 AM 2/15/2004 +1300, Dave, ZL3FG wrote:
>I am only just coming up to speed on this but-
>It appears that time tagging the1 pps output of a GPS rx from data in the
>NMEA string is not always as straightforward as it may seem. Not all rxs
>behave the same in the way - most, but not all, tag the immediately previous
>1 pps leading edge. As well, a particular rx can have a variable delay
>between the 1 pps leading edge and the corresponding NMEA string output,
>depending on several factors that can/will change, even at a fixed site.
> So the possibility exists for there to be varying integral second
>differences between the GPS derived time at various QTHs-depending on the
>hardware used. Clever software can evidently sort these issues and allow a
>reliable 'time' output to be produced.
>This is proving to be a rather interesting subject and I would welcome
>feedback from others who may have more input to offer. In particular this
>has ramifications for Africam -any comment, Bill?

Hi again Dave:

I found that Garmin manual at last.  Here's what it says:

"Regardless of the selected baudrate, the information transmitted by
the GPS25 sensor board is referenced to the pulse immediately
preceding the NMEA 0183 RMC sentence."

Bill VE2IQ