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

Kiwi Geoff geoff36 at gmail.com
Wed Jan 5 03:46:05 UTC 2011

Joseph Gray wrote:
> If I am using the NMEA data (as in the Garmin is the only time
> source), what firmware is recommended by all those using a GPS18x-LVC?

I thought I would run some tests for you Joe, using Version 3.0 and 3.2

Although I myself have run version 3.0 for a number of months (when it
first came out in 2009) without issues, I take note of what Kasper
Pedersen wrote about his experience  - so I have now loaded version
3.2 into my 18x, rather than any newer version (until Garmin fix the
NMEA latency issue).

I ran my tests for 2 hours each (7,200 one second samples) with GPS
sitting on an inside window sill with a very limited view of the sky,
so my readings should be taken as perhaps worse case rather than from
a perfect view of the sky. The GPS was using 9 - 10 satellites for the
fix solution during the test.

The RF sensitivity of the 18x is much better than the Garmin 18.

I measured the time of the "START" character ($) of the $GPRMC
sentence and  the "LAST" 0x10 (Linefeed) character, GPS 18x set to
4800 Baud.

Following times are in milliseconds from the leading edge of the 1PPS marker.

Version 3.00 firmware for Garmin 18x
START Min =  365
START Avg =  427 (N = 7200)
START Max =  499
LAST Min =   513
LAST Avg =   576 (N = 7200)
LAST Max =   648

Version 3.20 firmware for Garmin 18x
START Min =  370
START Avg =  433  (N = 7200)
START Max =  518
LAST Min =  519
LAST Avg =  582  (N = 7200)
LAST Max=  667

So if you use Version 3.20 firmware for the 18x, at 4800 baud the NMEA
RMC sentence will give the time of the current UTC second and will be
received by an average of 582 ms after the leading edge of the 1PPS
transition (with a maximum jitter of plus 85 ms / minus 63 ms).

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