[time-nuts] Garmin 18x GPS WNRO

[Kiwi Geoff]

Hi Tom,

SNAP - we were doing the same experiment at the same time - mine at 43
South in NZ.

However a word of caution for all Garmin 18x owners, you may no get
the same results as what Tom and I get !

Here is what my 18x GPS (using firmware v3.90) showed as the week rolled over.

I temporarily enabled sentence type "RMF" and "RMT" so I could see
what was going on.

The first few fields of RMF are:

GPS Week Number (0 to 1023) , GPS Seconds of Week (0 to 604799) , UTC Date,
Time of Fix (HHMMSS) , GPS Leap second count

$PGRMF,1023,604799,060419,235941,18
$PGRMF,0,0,060419,235942,18
$PGRMF,0,1,060419,235943,18
$PGRMF,0,2,060419,235944,18
$PGRMF,0,3,060419,235945,18
$PGRMF,0,4,060419,235946,18
$PGRMF,0,5,060419,235947,18
$PGRMF,0,6,060419,235948,18
$PGRMF,0,7,060419,235949,18
$PGRMF,0,8,060419,235950,18
$PGRMF,0,9,060419,235951,18
$PGRMF,0,10,060419,235952,18
$PGRMF,0,11,060419,235953,18
$PGRMF,0,12,060419,235954,18
$PGRMF,0,13,060419,235955,18
$PGRMF,0,14,060419,235956,18
$PGRMF,0,15,060419,235957,18
$PGRMF,0,16,060419,235958,18
$PGRMF,0,17,060419,235959,18
$PGRMF,0,18,070419,000000,18
$PGRMF,0,19,070419,000001,18
$PGRMT,GPS 18x-LVC software ver. 3.90,,,,,,,,*65

So using version 3.9 of the firmware, the 18x coped well with the
"Week Rollover" without any time or date issues.

However I then started getting comments from 18x users who said that
both the time and date were incorrect after the latest Sunday
Roll-over !

Because I had saved OLD versions of the 18x firmware, I was keen to
see if I could duplicate what these folks were (post rollover) getting
by my "backtracking" my 18x to version 2.8 (circa 2008 vintage)

$PGRMT,GPS 18x-LVC software ver. 2.80,,,,,,,,*65
$PGRMF,0,27840,220899,074346,14
$PGRMF,0,27841,220899,074347,14
$PGRMF,0,27842,220899,074348,14

Notice how the date now reads 22-08-99 and the delta between GPS and
UTC is 14 seconds (and hence the 4 seconds time error that was being
noticed).

I thought the 14 seconds delta may change with a new "almanac", but I
waited 15 minutes, cycled the power, but the time was STILL 4 seconds
out and the date 22-08-99.

However I had succeeded in duplicating what others were seeing.

I then upgraded my 18x firmware to the "latest" (21st March 2019) version 4.2

$PGRMT,GPS 18x-LVC software ver. 4.20,,,,,,,,*69
$PGRMF,0,29839,070419,081701,18

Both the time and date are now correct ;-)

For the GPS 18x PC/LVC, Ver. 4.20, as of Mar 21, 2019,
https://www8.garmin.com/support/agree.jsp?id=4055

-----------------------------------------------------------
Garmin 18x Firmware History
Changes made from version 4.00 to 4.20:

    Fixed GPS week number rollover for devices using older hardware.
    Improved PPS (pulse per second) reliability.

Changes made from version 3.90 to 4.00:

    Improved time keeping.

Changes made from version 3.80 to 3.90:

    Better handling for leap second data storage especially over long
periods of non-use.
------------------------------------------------------------

So after all this time, it appears that at last the 18x has resolved
some of the issues we have tried to report to Garmin over the years !

So if you have a Garmin 18x, you may want to upgrade the firmware,
which will hopefully tell us the correct time and date for the next
1023 weeks !

Regards, Geoff (Christchurch, New Zealand).