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

Tom Van Baak tvb at LeapSecond.com
Mon May 5 10:38:21 EDT 2014


> The data loggers will be continuously powered, in fixed locations 
> and should have reasonably good views of the sky so the use of a low 
> cost GPS module should be feasible.

Hi Tony,

Ah, now you are asking a completely different question. When you started this thread you didn't mention anything about reliability of GPS signal. Now you are talking about lost signals, holdover duration, urban canyons, local interference. That totally changes the problem.

The first thing you need to do is replace fuzzy adjectives with hard numbers. In your paragraphs below you use words like "sufficient accuracy", "during periods", "decent", "low cost", "reasonably well", "no significant", etc. It is impossible to solve your problem until you create a specification using real numbers instead of words. This could be a $50 solution or a $500 solution or a $50,000 solution, depending on what those words mean.

If you implement holdover, the choice of oscillator and packaging is completely determined by the holdover spec you have to meet -- what's the worst case duration, what's the ambient temperature variation, and how many nanoseconds or microseconds of error can you tolerate during holdover.

Do you have any choice where these sensors will be placed? I mean, if there is restricted sky view or too much local interference what will you do. Can you accept or reject locations based on a 1-day or 2-week performance validation trial? How many sensors are being deployed? Is this a one-off project or something commercial?

I think it might be best to tell the group exactly what your project is; you may get many useful suggestions. Maybe GPS is not the most robust solution.

/tvb

> Tom,
> 
> 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.
> 
> 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.
> 
> 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.
> 
> Thanks, Tony H





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