M12+T sawtooth correction and additional filtering
OK, so before I commit to my Sawtooth correction design, I had a thought about adding
additional filtering to the sawtooth corrections using a Kalman filter.
The Kalman filter would be driven using the @@Hn data coming from the M12+T reciever.
Is this something worth doing or am I expecting too much from the GPS system timing or
the M12+T receiver?
Advice and comments welcomed.
Thanks
Jerry
To use the Kalman filtering technique you need a model of the sawtooth
error... the model of the error is simply the quantization of the UTC PPS
done by the M12 clock, so I think that in the sawtooth correction output by
the M12 you have already all you need.
On Wed, May 9, 2012 at 12:20 AM, Jerry Mulchin jmulchin@cox.net wrote:
OK, so before I commit to my Sawtooth correction design, I had a thought
about adding
additional filtering to the sawtooth corrections using a Kalman filter.
The Kalman filter would be driven using the @@Hn data coming from the
M12+T reciever.
Is this something worth doing or am I expecting too much from the GPS
system timing or
the M12+T receiver?
Advice and comments welcomed.
Thanks
Jerry
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Jerry,
It depends on what your goal is. Hardware sawtooth correction is
good at improving the short-term jitter of the 1PPS output.
But if you're using the M12+T with a TIC simply to measure the
performance of some time/frequency standard, then software
correction is easier and gives slightly better results. This assumes
you have a PC or equivalent, and that you like coding more than
soldering.
You'll note that both Trimble's Tboltmon and Rick's Tac32 programs
allow you to combine sawtooth corrections and TIC readings into
a single value by using two serial ports.
Note there is a point where averaging lots of raw 1PPS data and
sawtooth corrected 1PPS data is pretty much the same. Before
you jump into this -- try a minute, an hour, and a day of data and
see what you get.
But there's more...
As long as you're playing with @@Hn messages, Kalman filters,
and software sawtooth correction, you might want to consider
looking at the per-SV "fractional GPS local time estimates" in the
Hn message. It's the mean of these values that determines the
virtual 1PPS (= the physical 1PPS + sawtooth correction).
It's my guess that taking a number of factors into account -- such
as lat/lon, az/el, svn, snr, and fractional stddev -- that one could
have a history-driven "intelligent weighted mean" instead of the
simple realtime simple mean that is currently used in the M12.
The M12 uses a fixed global elevation mask to decide if a SV is
in the solution; so it's all or nothing. With your Kalman filter, over
a few days or weeks, your software would have a more general
equation with which to solve for the 1PPS each second. Each
SV clock would have its own weight, each Az/El square degree
of each SV would have its own weight. You can also weight by
SNR or DOP. Or even temperature. Not only should this give a
more precise virtual 1PPS but you also get wonderful 2D or 3D
plots showing how weight varies with each factor.
I have some data on this I can share; contact me off-line.
/tvb
----- Original Message -----
From: "Jerry Mulchin" jmulchin@cox.net
To: time-nuts@febo.com
Sent: Tuesday, May 08, 2012 3:20 PM
Subject: [time-nuts] M12+T sawtooth correction and additional filtering
OK, so before I commit to my Sawtooth correction design, I had a thought about adding
additional filtering to the sawtooth corrections using a Kalman filter.
The Kalman filter would be driven using the @@Hn data coming from the M12+T reciever.
Is this something worth doing or am I expecting too much from the GPS system timing or
the M12+T receiver?
Advice and comments welcomed.
Thanks
Jerry
Tom,
very interesting. The Kalman filtering, as long as I know, requires a
model: you apply the model, make the measurements, compute the delta
between the model and the measurements and then the new gain to take the
next step. Let me ask: is this model to be determined? From your message it
seems to me that this model is already available waiting for someone to run
it. Don't look at me for this, only curious about the availability of such
a mathematical model. Actually I'm more interested on how to model an OCXO
to apply the Kalman filtering on such a device.
On Wed, May 9, 2012 at 1:19 AM, Tom Van Baak tvb@leapsecond.com wrote:
Jerry,
It depends on what your goal is. Hardware sawtooth correction is
good at improving the short-term jitter of the 1PPS output.
But if you're using the M12+T with a TIC simply to measure the
performance of some time/frequency standard, then software
correction is easier and gives slightly better results. This assumes
you have a PC or equivalent, and that you like coding more than
soldering.
You'll note that both Trimble's Tboltmon and Rick's Tac32 programs
allow you to combine sawtooth corrections and TIC readings into
a single value by using two serial ports.
Note there is a point where averaging lots of raw 1PPS data and
sawtooth corrected 1PPS data is pretty much the same. Before
you jump into this -- try a minute, an hour, and a day of data and
see what you get.
But there's more...
As long as you're playing with @@Hn messages, Kalman filters,
and software sawtooth correction, you might want to consider
looking at the per-SV "fractional GPS local time estimates" in the
Hn message. It's the mean of these values that determines the
virtual 1PPS (= the physical 1PPS + sawtooth correction).
It's my guess that taking a number of factors into account -- such
as lat/lon, az/el, svn, snr, and fractional stddev -- that one could
have a history-driven "intelligent weighted mean" instead of the
simple realtime simple mean that is currently used in the M12.
The M12 uses a fixed global elevation mask to decide if a SV is
in the solution; so it's all or nothing. With your Kalman filter, over
a few days or weeks, your software would have a more general
equation with which to solve for the 1PPS each second. Each
SV clock would have its own weight, each Az/El square degree
of each SV would have its own weight. You can also weight by
SNR or DOP. Or even temperature. Not only should this give a
more precise virtual 1PPS but you also get wonderful 2D or 3D
plots showing how weight varies with each factor.
I have some data on this I can share; contact me off-line.
/tvb
----- Original Message ----- From: "Jerry Mulchin" jmulchin@cox.net
To: time-nuts@febo.com
Sent: Tuesday, May 08, 2012 3:20 PM
Subject: [time-nuts] M12+T sawtooth correction and additional filtering
OK, so before I commit to my Sawtooth correction design, I had a thought
about adding
additional filtering to the sawtooth corrections using a Kalman filter.
The Kalman filter would be driven using the @@Hn data coming from the
M12+T reciever.
Is this something worth doing or am I expecting too much from the GPS
system timing or
the M12+T receiver?
Advice and comments welcomed.
Thanks
Jerry
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
I've gotten myself confused ...easily done when you're not a physicist (just
a lowly EE.) :-)
I am studying cesium clock design and trying to learn how these complex
instruments actually work. I seem to be getting the understanding on the
'workings' of the clock ...the tube, the counters, disciplining, etc.
But I have a higher level question I need help understanding. (Remember
...not a physicist here.)
Who decided 9,192,631,770 cycles of 'light' constitute one second? I don't
mean who the person was, or which company or institution or when.
I ask, why not ...771 cycles ...or ...669 cycles????
What I need is the understanding of what a 'second' is defined to be without
having the definition of a 'second' to begin with.
This seems all so recursive to me.
Thank you. (remember - - you guys said there were no dumb questions - - or
at least, we should ask them anyway.)
Who decided 9,192,631,770 cycles of 'light' constitute one second? I don't
mean who the person was, or which company or institution or when.
I ask, why not ...771 cycles ...or ...669 cycles????
Hi Don,
It was the result of 4 astronomical measurements made over several years.
9,192,631,761
9,192,631,767
9,192,631,772
9,192,631,780
You take the mean and get 9,192,631,770 +/- 20. That's how the atomic
second was "calibrated" against the astronomical second. Clearly the
number is only a rough estimate; the earth is a poor timekeeper.
Details here:
http://www.leapsecond.com/history/1958-PhysRev-v1-n3-Markowitz-Hall-Essen-Parry.pdf
/tvb
Because the length of the second is essentially arbitrary. There is
nothing 'fundamental' or 'universal' about it. It is essentially derived
from the average rotational period of the earth, which is a random number.
When the number of cycles was defined, they picked the nearest whole
number to an integral number of cycles, and re-defined the second to be
that.
-John
================
I've gotten myself confused ...easily done when you're not a physicist
(just
a lowly EE.) :-)
I am studying cesium clock design and trying to learn how these complex
instruments actually work. I seem to be getting the understanding on the
'workings' of the clock ...the tube, the counters, disciplining, etc.
But I have a higher level question I need help understanding. (Remember
...not a physicist here.)
Who decided 9,192,631,770 cycles of 'light' constitute one second? I
don't
mean who the person was, or which company or institution or when.
I ask, why not ...771 cycles ...or ...669 cycles????
What I need is the understanding of what a 'second' is defined to be
without
having the definition of a 'second' to begin with.
This seems all so recursive to me.
Thank you. (remember - - you guys said there were no dumb questions - -
or
at least, we should ask them anyway.)
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Tom,
So I'm thinking that as long as I'm putting in sawtooth correction, and I'm using a
0.25ns/step delay device (DS1123L part), then it seems to me that at some point
in the kalman filtering algorithm I should be able to decide that instead of delaying
1ns all the time, that I should be able to delay 0.5ns, or maybe 0.25ns. Giving a
smoother sawtooth correction to the PRS10.
But does that really improve anything over 1ns delay corrections given by the M12+T
receiver. I don't want to go off and try and convince myself that this is a real improvement
when in fact it may not be. So the real question is; is this going to work or am I
wasting my time?
Jerry
At 04:19 PM 5/8/2012, you wrote:
Jerry,
It depends on what your goal is. Hardware sawtooth correction is
good at improving the short-term jitter of the 1PPS output.
But if you're using the M12+T with a TIC simply to measure the
performance of some time/frequency standard, then software
correction is easier and gives slightly better results. This assumes
you have a PC or equivalent, and that you like coding more than
soldering.
You'll note that both Trimble's Tboltmon and Rick's Tac32 programs
allow you to combine sawtooth corrections and TIC readings into
a single value by using two serial ports.
Note there is a point where averaging lots of raw 1PPS data and
sawtooth corrected 1PPS data is pretty much the same. Before
you jump into this -- try a minute, an hour, and a day of data and
see what you get.
But there's more...
As long as you're playing with @@Hn messages, Kalman filters,
and software sawtooth correction, you might want to consider
looking at the per-SV "fractional GPS local time estimates" in the
Hn message. It's the mean of these values that determines the
virtual 1PPS (= the physical 1PPS + sawtooth correction).
It's my guess that taking a number of factors into account -- such
as lat/lon, az/el, svn, snr, and fractional stddev -- that one could
have a history-driven "intelligent weighted mean" instead of the
simple realtime simple mean that is currently used in the M12.
The M12 uses a fixed global elevation mask to decide if a SV is
in the solution; so it's all or nothing. With your Kalman filter, over
a few days or weeks, your software would have a more general
equation with which to solve for the 1PPS each second. Each
SV clock would have its own weight, each Az/El square degree
of each SV would have its own weight. You can also weight by
SNR or DOP. Or even temperature. Not only should this give a
more precise virtual 1PPS but you also get wonderful 2D or 3D
plots showing how weight varies with each factor.
I have some data on this I can share; contact me off-line.
/tvb
----- Original Message ----- From: "Jerry Mulchin" jmulchin@cox.net
To: time-nuts@febo.com
Sent: Tuesday, May 08, 2012 3:20 PM
Subject: [time-nuts] M12+T sawtooth correction and additional filtering
OK, so before I commit to my Sawtooth correction design, I had a thought about adding
additional filtering to the sawtooth corrections using a Kalman filter.
The Kalman filter would be driven using the @@Hn data coming from the M12+T reciever.
Is this something worth doing or am I expecting too much from the GPS system timing or
the M12+T receiver?
Advice and comments welcomed.
Thanks
Jerry
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Jerry Mulchin
Yes, but eventually the time must be corrected with leap seconds because of all the
randomness that accumulates. It is interesting that the leap seconds correction is
always a positive number. We never seem to gain time that I'm aware of. argh....
Jerry
At 05:35 PM 5/8/2012, you wrote:
Because the length of the second is essentially arbitrary. There is
nothing 'fundamental' or 'universal' about it. It is essentially derived
from the average rotational period of the earth, which is a random number.
When the number of cycles was defined, they picked the nearest whole
number to an integral number of cycles, and re-defined the second to be
that.
-John
================
I've gotten myself confused ...easily done when you're not a physicist
(just
a lowly EE.) :-)
I am studying cesium clock design and trying to learn how these complex
instruments actually work. I seem to be getting the understanding on the
'workings' of the clock ...the tube, the counters, disciplining, etc.
But I have a higher level question I need help understanding. (Remember
...not a physicist here.)
Who decided 9,192,631,770 cycles of 'light' constitute one second? I
don't
mean who the person was, or which company or institution or when.
I ask, why not ...771 cycles ...or ...669 cycles????
What I need is the understanding of what a 'second' is defined to be
without
having the definition of a 'second' to begin with.
This seems all so recursive to me.
Thank you. (remember - - you guys said there were no dumb questions - -
or
at least, we should ask them anyway.)
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Jerry Mulchin
It's unlikely you'll ever see a negative leap second - the earth's rotation is slowing down, not speeding up.
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On Behalf Of Jerry Mulchin
Sent: 09 May 2012 01:56
To: jfor@quikus.com; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Why 9,192,631,770 ??
It is interesting that the leap seconds correction is always a positive number.
Jerry