[ntp:questions] garmin 18x and linux

Chris Albertson albertson.chris at gmail.com
Wed Sep 7 15:12:46 UTC 2011


2011/9/7 Miguel Gonçalves <mail at miguelgoncalves.com>:
> On 7 September 2011 05:07, unruh <unruh at physics.ubc.ca> wrote:
>
>>  > I've tried Garmin 18 LVC and Sure. Not want to start a war here but for
>> the
>> > specifications and price Oncore beats both. :-)
>>
>> Beats them how? What measurements have you made of those two units in
>> comparison with the oncore?
>> I am not disputing but would like evidence rather than mystical
>> feelings.

Simply read the manufacture's specifications.    The most notable
difference is the one sigma error on the time of the PPS.   The newest
Oncore units (sold new by Synergy) are at the 2nS level while Garmin
claims 1uS.  That is a 500X difference.  The older UT+ version on eBay
typically com in at about 55nS (one sigma) and these sell for under
$20.

In the specific case we have here the Garmin unit lacks any PPS at
all.  That is a HUGE difference.

The other huge difference is the ability of the Oncore units to
self-survey their location, typically to within a third of a foot over
a 2 hour period.  They then use this known fixed location in their
time solutions and greatly reduce uncertainty.


They use a binary protocol of the serial wire that dumps a lot of
detailed information and the timing is "good" bt not measured  One
thing that comes over this bnary stream is the exact time relative to
UTC of the last pulse.  And an error estimate.  Som for example it
might say "xxx.00002314 plus minus 35nS"   This is functionality not
present in any NMEA based GPS.


Chris Albertson
Redondo Beach, California



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