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

EWKehren at aol.com EWKehren at aol.com
Fri May 2 19:59:14 EDT 2014

Welcome to the nuts Tony
You are not specifying exactly how accurate time has to be but in my book  
and based on tests the most reasonable priced GPS with 1 pps is a Ublox 6M 
that  you can get with antenna for less than $ 22 antenna included from 
_www.DX.com_ (http://www.DX.com) . They have volume discount. Shipping is  very 
slow but included. They seem to be presently out of the 1 pps version but  
all ublox units have a 1 pps output and I use with and without and all I do is 
 solder a wire to pin 3.
Bert Kehren
In a message dated 5/2/2014 7:02:57 P.M. Eastern Daylight Time,  
tnuts at toneh.demon.co.uk writes:

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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