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

[Bill de Carle]

At 01:40 AM 2/15/2004 +1300, you 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?

AFRICAM was written primarily for my own use, and am using a Garmin GPS
receiver.
The Garmin manual was very specific about the time of day each pps pulse was
associated with.  Unfortunately it was a few years ago that I did this and I
can't put my hand on that manual at this moment.  But AFRICAM works every time
for me with that GPS Rx.  I just assumed they would all be the same.  If
anyone
has a unit that always seems to be 1 second off, please let me know the
details
and I'll set up a special parameter you can use when invoking the program so
it will do it the right way for that particular GPS unit.

Bill VE2IQ