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
Said,
The software does allow several seconds of free running pulses to be
sent out while it verifies the alarm. My guess is that the time error of
the "free running" pulses would not be significant (unless you are a ns
hunter ;-) ). Many users do "flywheel" from these pulses just to keep
some sort of clock source.
I HAVE attached one of my favorite screenshots that shows that you
REALLY have to be careful who you believe when it comes to time
accuracy. We have to remember that the 1PPS is placed at the top of the
UTC second as best the receiver can determine. It can be absolute
garbage, but if one doesn't mind the error flags and alarms, one can get
a lot of useless time data.
Anyway, as with the screenshot I sent previously, this one shows the
sawtooth data from an M12+T. The only difference is that I disconnected
the antenna about a third of the way through the window. Note how
consistent and pretty the sawtooth is? Too bad it's garbage.
Based on some old Motorola stuff about the VP TRAIM, the problem with
severely reducing the TRAIM alarm limit is simply one of irritation. In
their experiments they found that if you tried to reduce the TRAIM limit
to less than 300ns the number of false alarms rose drastically. Remember
this was with an old 8 channel receiver and that SA was on. I think the
plain fact is that 1us works fine for 99.99% of the applications out
there. The telecoms would much rather give the TRAIM alarm lots of room
so that they are not getting constantly bombarded with errant TRAIM
alarms.
I do have a copy of this old doc that I could send out so that it could
be hosted on one of the timing sites. The information is really
outdated, but it does do a good job of explaining some of the original
reasoning behind TRAIM and timing performance in general. Just keep in
mind that it is historical in nature and not state of the art.
Randy
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of SAIDJACK@aol.com
Sent: Wednesday, December 13, 2006 4:56 PM
To: time-nuts@febo.com
Subject: 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
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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@febo.com
[mailto:time-nuts-bounces@febo.com] Im Auftrag von SAIDJACK@aol.com
Gesendet: Donnerstag, 14. Dezember 2006 01:56
An: time-nuts@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
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
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@febo.com
[mailto:time-nuts-bounces@febo.com] Im Auftrag von SAIDJACK@aol.com
Gesendet: Donnerstag, 14. Dezember 2006 01:56
An: time-nuts@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
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Björn,
thank you for the link! Looks very interesting!
Regards
Ulrich Bangert, DF6JB
-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] Im Auftrag von bg@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@febo.com
[mailto:time-nuts-bounces@febo.com]
Im Auftrag von SAIDJACK@aol.com
Gesendet: Donnerstag, 14. Dezember 2006 01:56
An: time-nuts@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
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
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.
I second Ulrich's comments. A certain amount of
sanity checking on your end is worthwhile. You can
get GPS 1PPS glitches, OCXO glitches, and TIC
glitches. No need to decide which is which; just do
outlier detection and you solve them all.
The beauty of a GPSDO is that you're only changing
the DAC every few minutes so there's plenty of time
to look carefully at the quality of your 1 Hz samples
before you commit the DAC update.
It would be a more difficult problem if you needed to
update the DAC in realtime.
/tvb