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

Tony tnuts at toneh.demon.co.uk
Mon May 5 13:55:20 UTC 2014

On 03/05/2014 18:41, Tom Van Baak (lab) wrote:
> Tony, Chris, Bert,
> Since all you want is a 10 ns time stamp / data logger you do not need a GPSDO, or OCXO, or VCXO.
> The solution is cheap and very simple.
> Your GPS receiver provides a 1PPS to the microprocessor. Use a plain XO or TCXO; the frequency does not need to be accurate, just stable to about 1e-9 (many $1 xtals do this). Each second your code [re]computes the drift between the clock and GPS. You may average over 10 to 100 seconds if you wish.
> Even though your clock is off-time and off-frequency your software knows what the offset is. Therefore, you can simply adjust the time stamp reading by the current clock error.
> This "software GPSDO" gives equal or actually slightly better performance than a real GPDSO but it is much simpler: no DAC, no EFC, no OCXO, no VCXO, no PLL.
> /tvb (i5s)
> _______________________________________________

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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