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

[Bill de Carle]

With the Garmin unit, I don't think the NMEA messages ever run over
one second.  It issues several sentences each second and still doesn't
run into the next second.  I'm only interested in one of 'em, ($GPRMC)
so if needed we could restrict output to just that one sentence, but
I've never found it necessary to do that.  If it ran longer than one
second to get all its messages out, over time it would run right past
the next 1-pps, etc.  I've never seen that.

Bill VE2IQ


>Hi Bill
> Yes. That seems to be the usual arrangement- I don't know what NMEA
>sentences that unit uses, maybe only one, but regardless of the baudrate, if
>a fix is slow to get generated (low number of SVs etc) then the NMEA string
>can, timewise, overrun the next 1 pps leading edge.  So the information in
>the string IS always referencing the immediately prior 1 pps, BUT ONLY at
>the time the string starts.  If overrun occurrs, then all bets are off as to
>what  time data gets included in the next NMEA output. I guess it depends on
>how the firmware in the rx is set up to handle the situation.
>
>One possibility may be to NOT use the NMEA output on a continuous basis, but
>just sample it continuously and check its content to ensure it appears valid
>before passing on an updated time tag  at eg 10 second intervals.
>I suspect that most of the recent crop of GPS rxs have addressed this sort
>of issue as best they can in the rx firmware but older (and therefore
>cheaper!) units, such as we may likley be using, probably have not.
> Another aspect that maybe contributes to this uncertainty in the NMEA
>output is that the 'native' output spoken by some GPS rxs is NOT NMEA- it is
>some other format-and the NMEA output is an option-so there is maybe more
>delay in generating the NMEA output, as its probably done after the main
>output message has been set up and started being sent on its way. But this
>is only speculation and needs to be tested to verify.
>I'm presently thinking about making a GPS antenna  feedline splitter to feed
>multiple rxs from one antenna- gets to be a pain having to put up another
>GPS antenna for each rx !
>73
> Dave, ZL3FJ