[time-nuts] Accuracy/drift of Garmin GPS 16 HVS 1 PPS output under invalid fix conditions...

[Bob kb8tq]

Hi

Any of the “typical” surplus GPSDO’s should be able to hold 10 us over 24 hours. That’s the CDMA spec
that the market was looking for when they were manufactured.  With a big hunky OCXO pulling lots of
watts (and a few other issues), I doubt they are going to work in this case.

====

15 minutes is 900 seconds. 1 ms at 1,000 seconds is (quite nicely) a 1 ppm frequency offset. That gets you 
out of the world of a bare crystal oscillator. It does not present much of a challenge for a typical TCXO. 
Unfortunately a lot of low end GPS gizmos have a crystal rather than a TCXO in them. They might  “learn”
the frequency offset while locked. Who knows if they do that ….It seems like a lot of effort on their part for
a feature that nobody ever looks at. 

Bob

> On Mar 11, 2019, at 11:48 AM, David J Taylor via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi All,
> 
> Just a basic question for an application with low accuracy
> requirements.  I only need an accuracy of +/- 1 ms on the 1 PPS signal.
> 
> Does anyone have any data on the behaviour of a Garmin GPS 16 HVS (or be
> able to point me in the right direction to find such data) w.r.t. the 1
> PPS output when a valid fix is not available.
> 
> I need this to determine whether the 1 PPS hold-over accuracy is
> sufficient to maintain the 1 PPS within +/- 1 ms of GPS time during a
> loss of GPS lock of, say, 15 minutes.
> 
> I know that different manufacturers might handle the hold-over
> differently - so I need some experience of others with Garmins.
> 
> The only information I can find about Garmins is that the 1 PPS holds
> within the +/- 1 us specification for up to 4 seconds of invalid fix
> conditions.
> 
> Cheers
> 
> Steve Olney
> 
> HawkRAO
> 
> http://hawkrao.joataman.net/pulsar/daily_obs/index.html
> =============================================
> 
> Steve,
> 
> Not a lot to add, but I have also seen Garmin GPS devices where the PPS disappears almost immediately.
> 
> This device claims "Hold over on loss of GPS signal (is) typically 2ms in 24 hours", so well within you requirements.  I don't know whether that applies just to its NTP serving function or to the PPS output as well.  Worth asking....
> 
> http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=272
> 
> Cheers,
> David
> -- 
> SatSignal Software - Quality software for you
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
> Twitter: @gm8arv 
> 
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