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

[Kiwi Geoff]

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).