[time-nuts] Best GPS 1PPS Accuracy
df6jb at ulrich-bangert.de
Thu Dec 14 14:07:22 EST 2006
thank you for the link! Looks very interesting!
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
> +-3 sigma is 99% on a gaussian distribution.
> If you want to look at other algorithms on this topic, here is one:
> 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
> > 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
> > 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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