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

Joseph Gray jgray at zianet.com
Tue Jan 4 23:29:43 EST 2011


Geoff,

Thanks a lot for comparing firmwares. That hard information helps tremendously.

Joe Gray
W5JG

On Tue, Jan 4, 2011 at 8:46 PM, Kiwi Geoff <geoff36 at gmail.com> wrote:
> 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).
>
> From what I have said earlier in the thread, and from tests that
> Kasper Pedersen has done, I think we can say that currently version
> 3.2 is the "best" version if you want to use the 18x NMEA timestamps
> to be given early in the second (rather than much later as per version
> 3.3 and 3.5).
>
> Joe, you can download    GPS18xPC_LVC_320.exe     from here:
>
> http://www.gawisp.com/perry/oem_sensor/
>
> The above file is identical to the one I downloaded in 2009 from Garmin.
>
> Hope that helps.
>
> Regards, Geoff (Christchurch, New Zealand).
>
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