[ntp:questions] garmin 18x and linux

Bill Unruh unruh at physics.ubc.ca
Wed Sep 7 15:36:04 UTC 2011

On Wed, 7 Sep 2011, Chris Albertson wrote:

> 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

I do not believe manufacturer's specs, especially not when they seem to badly
overstate the performance.

> 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

Yes, and I simply do not believe this. Note that Oncore has claimed this
accuracy since at least 2003 ( ie old units).

> claims 1uS.  That is a 500X difference.  The older UT+ version on eBay

I believe we were comparing Oncore to Sure, not Oncore to GPS18LVM

> typically com in at about 55nS (one sigma) and these sell for under
> $20.

55ns I might believe.

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

No argument. But there the argument was not the PPS timing but whether or not
it met the OP's needs. It did. That is all that is required.

> 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.

That may, or may not, be an advantage, depending on how they do it.

> 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.

Agreed, but also irrelevant. 
And I am confused. You were claiming that the pulses were within 2ns of utc,
and your example here is 23us out from utc. That is 10000 times different.

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