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

Tony tnuts at toneh.demon.co.uk
Fri May 9 13:46:05 EDT 2014


On 06/05/2014 02:24, Chris Albertson wrote:
> 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.

Thanks Chris - that's just the information I was looking for. Yes it would be at a fixed location; it wouldn't be a problem checking that it had good reception during installation.

>> 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)
That's interesting. What model is the $15 oscillator? Is it an OXCO? 
Unfortunately the power consumption of the OXCOs I've looked at are much 
too high at < 1W. However this TCXO is both cheap and remarkably 
comprehensively specified:

http://media.digikey.com/pdf/Data%20Sheets/NDK%20PDFs/NT2016SA-16.368000_MHZ-NTG1.pdf 


Its a 16.368MHz oscillator for less than $2 and uses <1.5mA . Unusually 
the data sheet specifies not only the max temperature stability at +/- 
.5ppm from -10 to +70C, but also the max frequency/temperature slope at 
+/- .05ppm/C . It also specifies short term stability at max 1ppb over .1s.

Quite a remarkable datasheet for a low cost part - I've not found any 
other low cost oscillator with either of those specifications, and even 
some (most?) of the OXCO don't specify the freq/temp slope. Having said 
that, I can't find the same datasheet anywhere else - those on NDK's 
website are less comprehensive. Perhaps those on Digikey's site are out 
of date, NDK not wanting to guarantee those specs for such a low cost part.

I intend to try one and see how it performs in a box, with some 
insulation, when moved into a sunny spot after being shaded for a while.

>> 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.

But isn't that only supported by 'timing' GPS modules that allow you to 
specify the location? But they are rather more expensive than the common 
navigation type modules - are there sub $15 modules that support that 
single-satellite timing feature?

Thanks, Tony H




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