[time-nuts] Low cost GPS module for < 100ns timestamping error

Chris Albertson albertson.chris at gmail.com
Tue May 6 01:24:14 UTC 2014

On Mon, May 5, 2014 at 6:55 AM, Tony <tnuts at toneh.demon.co.uk> wrote:

> Yes - that is exactly what I intended. The problem though is maintaining
> sufficient accuracy during periods when the GPS clock is unavailable or
> unreliable (perhaps due to local interference), but I don't have any handle
> on how long that may be or how often it occurs. Clearly there are no
> absolute guarantees - eg. the GPS selective availability could be turned on
> again in exceptional circumstances, so I accept that 100ns accuracy can't
> be absolutely guaranteed.

I assumed you were making these measurements at a fixed location.    You
don't loose GPS signal often.  Onece you have the antenna in a location
that works it continues to work, most of the time.  Drop outs are rare in a
fixed system after you gt it working.    It's different in a moving vehicle.

> The question then is, in the experience of users of GPS timing references,
> for a decent but low cost receiver with a reasonably well sited antenna and
> assuming there is no significant interference, how long and how frequently
> is time synchronisation lost? If for example it's only 2 or 3 seconds every
> few weeks, then there isn't much of a problem. If 5 minute outages occur
> every few days then the holdover performance of the local oscillator is
> much more critical.

As said above, once it works it pretty much continues to work.  With a very
good antenna site (mine is on a 4 foot above the roof line with a 360
degree view of the sky) I've never had a loss of signal except as a test.
But then I don't look for them either.

If you do get a loss of signal then all that happens is my GPSDO controller
never updates the local oscillator. It sticks at the last setting.  So the
drift depends on how good is the local oscillator.   I have two.  One is a
$15 crystal.  It can run for "hours" before I can detect any drift (I my
case that is a few ns of phase drift)  Certainly your example of 5 minutes
per day of GPS outage would be no problem at all even for a moderate
quality OCXO.

My other oscillator is a Rubidium.  It is the $40 FE-5680 from eBay and it
can go for "days" with no GPS corrections (at the few ns level)

> What about in more difficult circumstances - eg. in urban environment with
> an antenna that has a restricted view of the sky? Not that I expect to
> operate in such circumstances but it would be interesting to get a feel for
> how good or bad timing is maintained in less favourable situations.

It all depends on the quality of the oscillator.  But again you would
fiddle with the antenna until it worked as best it could then you don't se
much change in a fixed location system.

The other thing that "saves" you is that for timing at a fixed location the
GPS only needs ONE satellite.  With any reasonable setup yo are likely to
have one sat visible at all times.


Chris Albertson
Redondo Beach, California

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