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

Edesio Costa e Silva time-nuts at tardis.net.br
Fri May 2 21:07:36 EDT 2014


Take a look at NavSpark from SkyTraq (http://www.skytraq.com.tw/). They had
an Indiegogo
campaign recently and should deliver real soon now. The NavSpark chip has an
trigger pin for time capture, a feature suggested by a fellow time-nut and a
100 MHz clock.


On Fri, May 02, 2014 at 07:59:14PM -0400, EWKehren at aol.com wrote:
> 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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