[time-nuts] My Garmin 18x, Ver 3.50, currently 1 second slow to UTC

[paul swed]

In my various tinkering it always seems that I have to recreate a software
clock that accounts for the behavior thats advanced by 1 second. Have done
that on 2-3 projects the most recent being the GOES DC468 simulator.
Regards

On Thu, Dec 23, 2010 at 5:49 AM, Kiwi Geoff <geoff36 at gmail.com> wrote:

> Hal Murray wrote:
> > I assume it was working correctly previously.  If not...   At least one
> GPS
> > receiver I've used is off by a second.  There is an ambiguity as to
> whether
> > the time applies to the previous or next PPS.
>
> You were correct Hal, the problem with the new firmware for the 18x,
> is the time relationship between the 1PPS transition and the serial
> data.
>
> My application required the RMC and GGA sentence at 4800 BAUD.
>
> So I measured the start and end times of the serial data (with respect
> the 1PPS).
>
> Using version 3.00 firmware on the Garmin 18x, the NMEA data starts
> 450 ms after the 1PPS transition, and finishes at 740 ms.
>
> However using the latest firmware version (3.50) for the 18x, the NMEA
> data starts at 750 ms and finishes 39 ms into the "following" second.
> That is why my software was loading the wrong time.
>
> These start and end times of the NMEA serial data change quite a bit as the
> workload of the GPS changes.
>
> There is no mention in all the update notes, that Garmin have changed
> the timing of the NMEA data with respect the 1PPS transition.
>
> Running the same test with my GPS 18, the NMEA starts about 50 ms and
> finishes 350 ms after the 1PPS transition - nice and reliable!
>
> I think the reason my 18x started (in the last few days) to show the
> wrong time, is that the trees around my house are at the peak of
> growing leaves (Summer in the Southern Hemisphere) and so this would
> have caused the GPS to work harder, and so must have delayed the NMEA
> data enough (with version 3.50) to give the wrong time on the serial
> data.
>
> So the moral of the story, don't expect that a device will be "better"
> if you upgrade to the latest version of software.
>
> Thanks Hal for your kind pointer - I now understand what is going on,
> and not to be so excited about new firmware updates.
>
> Kiwi Geoff (Christchurch, New Zealand).
>
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