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

Tony tnuts at toneh.demon.co.uk
Fri May 2 22:54:25 UTC 2014

Hi, I'm new here so please be gentle!

I'm considering designing and building some dataloggers, probably ARM 
Cortex based (eg. STM32F4xx), which record the time of infrequent 
events, preferably to better than 100ns and if possible better than 
50nS. 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. I believe it shouldn't be too 
difficult to resolve the PPS timing to +/- 5ns or better with a 100MHz+ 
microcontroller clock, but obviously jitter would add to the error 
requiring the GPS to be better than perhaps 90ns or so worst case.

Inevitably cost and power constraints apply - ideally the GPS would cost 
less than $20 (in quantities of 100), and < $15 would be good, but it 
doesn't seem easy to find very lost cost receivers with timing outputs 
that are properly specified, presumably because of the relative market 
volumes. The power consumption of most timing receivers also seem to be 
higher than navigation units - eg. the Trimble SMT-x spec is 100mA 
compared to the ADAfruit MTK3339-based module which draws 20mA (but they 
are a bit too expensive at $24 apiece).

There are several cheap modules that have PPS outputs but no accuracy 
specification; it's possible that these could be used with sufficient 
averaging/filtering of the PPS output. Actually repeatability is the 
important requirement rather than accuracy as they could be calibrated. 
Perhaps even a PPS o/p is not absolutely necessary - could the NEMA 
output timing be used given enough averaging and a sufficiently stable 
oscillator? Compromising the timing accuracy requirement a bit to say 
150ns may be acceptable if the GPS device is cheap enough.

I understand that the PPS outputs of some cheap modules sometimes become 
ill-behaved, but in this application the time stamp can be adjusted (or 
anomalous clocks ignored) post-event if necessary to correct for 
temporary disturbances.

This also raises questions about the short term stability of the 
microcontroller oscillator required to maintain sufficient accuracy when 
GPS timing is temporarily lost for some reason - but how long would that 
need to be? 30s? 5 minutes? 30 minutes? An OCXO or a Stratum-3 TXCO 
would be too expensive, but oscillator manufacturers don't seem to 
publish short term frequency stability specifications for low cost/low 
power oscillators, and finding such information isn't easy. Can anyone 
point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz?

I did find this study, http://tf.nist.gov/general/pdf/2276.pdf, 
measuring the stability of some low cost quartz wristwatches which gives 
some interesting data of 20 to 65ppb Allan deviation over 100s. That, 
but a 32kHz oscillator might give rise to jitter problems when 
multiplied up to a suitable frequency.

Some oscillator datasheets specify Allan deviation values, but I guess 
what I need for estimating worst case timestamp error during holdover 
periods are actually MTIE values. Is there any way to estimate the 
latter from Allan deviations specs? Would an ADev of 65 x 10^-9 over 
100s imply up to 6.5us of error after 100s?

Any thoughts? Thanks,
            Tony H

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