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

[Dave Brown]

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

----- Original Message ----- 
From: "Bill de Carle" <ve2iq@magma.ca>
To: <lowfer@mailman.qth.net>
Sent: Sunday, February 15, 2004 2:53 PM
Subject: Re: [Lowfer] Use of NMEA strings for GPS 1 pps time tagging


> 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
>
>
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