[time-nuts] Best GPS 1PPS Accuracy

Ulrich Bangert df6jb at ulrich-bangert.de
Thu Dec 14 14:07:22 EST 2006


Björn,

thank you for the link! Looks very interesting!

Regards
Ulrich Bangert, DF6JB

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von bg at lysator.liu.se
> Gesendet: Donnerstag, 14. Dezember 2006 13:30
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Best GPS 1PPS Accuracy
> 
> 
> Ulrich,
> 
> +-3 sigma is 99% on a gaussian distribution.
> 
> If you want to look at other algorithms on this topic, here is one:
> 
>  http://www.control.isy.liu.se/books/adfilt/
> 
> br,
> 
>    Björn
> 
> On Thu, December 14, 2006 12:55, Ulrich Bangert said:
> > Hello Said,
> >
> > from my own experience in detecting and removal of outliers i can 
> > asure you that it is a challenging and ambitious task in 
> statistical 
> > math. I fear you expect too much from a receiver's TRAIM 
> firmware to 
> > compute the necessary statistic math on a pulse to pulse base. 
> > Nevertheless you can do it on your own if the microcontroller that 
> > compares the LO to the GPS's 1pps has enough RAM and 
> processing power.
> >
> > It involves computing the MEDIAN over a number measurements 
> (say 60) 
> > to get a reliable estimate of the measurement's MEAN value in that 
> > time. Note that the normal MEAN computation is susceptible 
> to outliers 
> > while the MEDIAN is not. On the other hand computing the MEDIAN 
> > involves sorting the measurements in ascending order, much 
> more stuff 
> > than simple MEAN adding/dividing. Once you have the MEDIAN of your 
> > measurements you can compute the distances between the single 
> > measurements and the median. But in contrast to the simple STANDARD 
> > DEVIATION which is susceptible to outliers itself, you now 
> compute the 
> > MEDIAN over the distances between the measurements and 
> their MEDIAN to 
> > give you something that is called the MAD (MEDIAN ABSOLUTE 
> DEVIATION. 
> > As the STANDARD DEVIATION the MAD is a measure of the witdh of the 
> > measurement's distribution in the sense of a 'sigma' but not 
> > susceptible to outliers. Once you have that number you can easily 
> > decide whether the last measurement is within or without a range of 
> > say +/- 5 * sigma. I guess +/- 5 * sigma resembles 99% of normal 
> > distributed values, so you are on the safe side to throw 
> away anything 
> > outside +/- 5 * sigma. Note that you have to perform the 
> above steps 
> > for every new measurement, i.e. every second removing the oldest in 
> > your data buffer and adding the latest measurement to you 
> data buffer. 
> > It can be done and i do so in my DIY GPSDO but actually it needs 
> > exactly this complexity.
> >
> > Best regards
> > Ulrich Bangert, DF6JB
> >
> >> -----Ursprüngliche Nachricht-----
> >> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] 
> >> Im Auftrag von SAIDJACK at aol.com
> >> Gesendet: Donnerstag, 14. Dezember 2006 01:56
> >> An: time-nuts at febo.com
> >> Betreff: Re: [time-nuts] Best GPS 1PPS Accuracy
> >>
> >>
> >> Hi guys,
> >>
> >> slightly off-topic, but has anyone done stability tests 
> with the m12+ 
> >> or M12M when the antenna goes bad in TRAIM-enabled mode (e.g.
> >> less than 4 good sats
> >> being received by i.e. disconnecting the antenna)?
> >>
> >> I see that the 1PPS output goes away only after about 5 or so  
> >> additional pulses have been generated, so these 5 pulses are  
> >> "free-running".
> >>
> >> These pulses are somewhat inaccurate, and I am wondering if anyone 
> >> has done a max/min analysis on the errand pulse's offsets?
> >>
> >> I wonder if the TRAIM limits can be set to say +-50ns 
> effectively to 
> >> kill these pulses and improve overall accuracy?
> >>
> >> I know lot's of folks run the M12 TRAIM limits at 1us, that seems 
> >> excessive on a receiver that is supposed to do 10ns 
> 1-Sigma, doesen't 
> >> it? ...
> >>
> >> Thanks for your feedback,
> >> Said
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> >>
> >
> >
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> 
> 
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