M12+T sawtooth correction and additional filtering

Discussion:

Jerry Mulchin

2012-05-08 22:20:53 UTC

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

Azelio Boriani

2012-05-08 22:36:34 UTC

Permalink

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.

Post by Jerry Mulchin
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.
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 unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Tom Van Baak

2012-05-08 23:19:14 UTC

Permalink

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-***@public.gmane.org>
To: <time-nuts-***@public.gmane.org>
Sent: Tuesday, May 08, 2012 3:20 PM
Subject: [time-nuts] M12+T sawtooth correction and additional filtering

Azelio Boriani

2012-05-08 23:44:20 UTC

Permalink

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.

Post by Tom Van Baak
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...
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
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

Post by Jerry Mulchin
about adding
additional filtering to the sawtooth corrections using a Kalman filter.
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 unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Don Lewis

2012-05-09 00:14:13 UTC

Permalink

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.)

Tom Van Baak

2012-05-09 00:24:58 UTC

Permalink

Post by Don Lewis
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

J. Forster

2012-05-09 00:35:45 UTC

Permalink

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

================

Post by Don Lewis
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.)
_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Jerry Mulchin

2012-05-09 00:56:12 UTC

Permalink

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

Post by J. Forster
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
================

_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Jerry Mulchin

David C. Partridge

2012-05-09 01:14:36 UTC

Permalink

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-***@public.gmane.org [mailto:time-nuts-bounces-***@public.gmane.org] On Behalf Of Jerry Mulchin
Sent: 09 May 2012 01:56
To: jfor-9KbcMkp5tujQT0dZR+***@public.gmane.org; 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

paul swed

2012-05-09 01:19:06 UTC

Permalink

Thanks downloaded the paper. I assume the hyper fine transition happened to
be the one nearest that frequency? You can tune to either side of the peak
and get a lock on the next transition.
Regards
Paul.

On Tue, May 8, 2012 at 9:14 PM, David C. Partridge <

Post by David C. Partridge
It's unlikely you'll ever see a negative leap second - the earth's
rotation is slowing down, not speeding up.
Dave
-----Original Message-----
Behalf Of Jerry Mulchin
Sent: 09 May 2012 01:56
Subject: Re: [time-nuts] Why 9,192,631,770 ??

It is interesting that the leap seconds correction is always a positive

number.
Jerry
_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

David McGaw

2012-05-09 01:28:33 UTC

Permalink

It is my understanding that they actually got it a bit wrong, which is
why we seem to have a lot of leap seconds.

David

Post by paul swed
Thanks downloaded the paper. I assume the hyper fine transition happened to
be the one nearest that frequency? You can tune to either side of the peak
and get a lock on the next transition.
Regards
Paul.
On Tue, May 8, 2012 at 9:14 PM, David C. Partridge<

It is interesting that the leap seconds correction is always a positive

number.
Jerry
_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Eric Lemmon

2012-05-09 01:32:43 UTC

Permalink

No, leap seconds have to do with the gradual slowing of the Earth's
rotation, and nothing to do with Cesium.

73, Eric Lemmon WB6FLY

-----Original Message-----
From: time-nuts-bounces-***@public.gmane.org [mailto:time-nuts-bounces-***@public.gmane.org] On
Behalf Of David McGaw
Sent: Tuesday, May 08, 2012 6:29 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Why 9,192,631,770 ??

It is my understanding that they actually got it a bit wrong, which is
why we seem to have a lot of leap seconds.

David

It is interesting that the leap seconds correction is always a positive

number.
Jerry
_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

_______________________________________________
To unsubscribe, go to

https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

Post by paul swed
and follow the instructions there.

_______________________________________________
time-nuts mailing list -- time-nuts-***@public.gmane.org
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Poul-Henning Kamp

2012-05-09 05:43:51 UTC

Permalink

Post by Eric Lemmon
No, leap seconds have to do with the gradual slowing of the Earth's
rotation, and nothing to do with Cesium.

Not quite true.

When they decided the 9,192,631,770 they did so with astronomical
observations which were half a century old. If they had used a
more up to date astronomical result, we would have had a lower
rate of leap seconds, essentially getting rid of the "one leap
second every 18 months" average value.

In the long term it doesn't make a difference of course, there
earths slowing down will dominate.

--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk-***@public.gmane.org | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Majdi S. Abbas

2012-05-09 03:16:34 UTC

Permalink

Post by David McGaw
It is my understanding that they actually got it a bit wrong, which
is why we seem to have a lot of leap seconds.

The atom is a much better timekeeper than the Earth. They
could have chosen differently for convenience, but there's nothing really
wrong with the choice as it was made.

The issue is what to do with it over time as the Earth continues
to slow.

--msa

Tom Van Baak

2012-05-09 06:22:04 UTC

Permalink

Hi Paul,

The definition is the second specifies which hyperfine transition to use; it also requires zero temperature and zero magnetic field. Since you can't actually achieve the latter two in a beam tube, good cesium standards accurately compensate for the slight frequency shift with synthesizers.

Yes, when you apply the magnetic field you do get multiple peaks. You'll see this in most technical descriptions of how cesium clocks work; see this old one by hp:
http://leapsecond.com/museum/hp5062c/theory.htm

The center peak is the one to use because it's most immune from changes in magnetic field and also the tallest, sharpest peak. This is clear in the energy level diagram. You can also see each of the 7 peaks in detail here:
http://leapsecond.com/pages/cspeak/
Or the tall wall-poster(!) version:
Loading Image...

John Miles joined the 9192631770 club:
http://www.ke5fx.com/cs.htm

Yes, older cesium standards would allow you to deliberately (or accidentally) lock to the wrong peak. In that case the clock runs many ppm fast or slow. Easy to detect.

The difference between the peaks is the "Zeeman frequency"; it allows one to cleverly indirectly calibrate the C-field so that the cesium standard more accurately ticks SI seconds. Modern cesium standards (e.g., 5071A) do this automatically. Here is a tall plot showing how much the other 6 peaks shift when the magnetic field is manually changed (e.g., hp 5061A):
Loading Image...

For more information on the Zeeman frequency and cesium beam tubes goggle: zeeman frequency site:febo.com

/tvb

Jerry Mulchin

2012-05-09 00:52:10 UTC

Permalink

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

_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Jerry Mulchin

Poul-Henning Kamp

2012-05-09 05:47:19 UTC

Permalink

Post by Tom Van Baak
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).

See also:
http://phk.freebsd.dk/raga/sneak/

E***@public.gmane.org

2012-05-09 10:10:49 UTC

Permalink

Jerry
If you want to contact me off list I will share with you our low cost M12+
saw tooth correction circuit.
Bert

In a message dated 5/9/2012 5:28:16 A.M. Eastern Daylight Time,
jmulchin-***@public.gmane.org writes:

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-***@public.gmane.org
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

WarrenS

2012-05-09 16:15:01 UTC

Permalink

It is interesting that the leap seconds correction is always a positive number.

But less so now than 40 years ago according to:
http://en.wikipedia.org/wiki/Leap_second
So does that mean the earth is speeding up? Maybe the cause is all the DARK ENERGY out there speeding everything up :)

A good and simple explanation of why they CHOOSE to make the second wrong is at:.
http://www.nist.gov/pml/div688/leapseconds.cfm

"There are two main reasons that cause leap seconds to occur.
The first is that the duration of the atomic second was measured and defined by comparing cesium clocks to the Ephemeris Time (ET) scale, an obsolete time scale that defined the second as a fraction of the tropical year. The duration of the ephemeris second was slightly shorter than the mean solar second and this characteristic was passed along to the atomic second.
If the atomic second had been defined with respect to the mean solar second, it is likely that leap seconds would have been required much less frequently.
The second reason for leap seconds is that the speed of the Earth's rotation is not constant. It sometimes speeds up, and sometimes slows down,
but when averaged over long intervals the trend indicates that it is gradually slowing.
This gradual decrease in the rotational rate is causing the duration of the mean solar second to gradually increase with respect to the atomic second."

J. Forster

2012-05-09 16:30:50 UTC

Permalink

This issue came up when I was involved in SETI work. One big question was
"How do you know what frequency to listen at?"

Any earth-centric system is essentially arbitrary. There is nothing
'universal' about the Meter, Kilogram, or Second. (or any of their
equivalents in other systems of units).

And, all other units are derived from those fundamental units.

The only things, thought to be anything like universal, are fundamental
properties of atoms, like hyperfine transitions. Hence the defined second.

-John

=================

Post by WarrenS

It is interesting that the leap seconds correction is always a positive number.

http://en.wikipedia.org/wiki/Leap_second
So does that mean the earth is speeding up? Maybe the cause is all the
DARK ENERGY out there speeding everything up :)
A good and simple explanation of why they CHOOSE to make the second wrong is at:.
http://www.nist.gov/pml/div688/leapseconds.cfm
"There are two main reasons that cause leap seconds to occur.
The first is that the duration of the atomic second was measured and
defined by comparing cesium clocks to the Ephemeris Time (ET) scale, an
obsolete time scale that defined the second as a fraction of the tropical
year. The duration of the ephemeris second was slightly shorter than the
mean solar second and this characteristic was passed along to the atomic
second.
If the atomic second had been defined with respect to the mean solar
second, it is likely that leap seconds would have been required much less
frequently.
The second reason for leap seconds is that the speed of the Earth's
rotation is not constant. It sometimes speeds up, and sometimes slows
down,
but when averaged over long intervals the trend indicates that it is gradually slowing.
This gradual decrease in the rotational rate is causing the duration of
the mean solar second to gradually increase with respect to the atomic
second."
_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Don Lewis

2012-05-09 22:45:06 UTC

Permalink

Thanks, all ...this thread has helped me a lot.

It reinforces my beliefs of what a 'second' is ...what we were taught from
day 1 in school:

One solar day/86,400 (A 'solar' day being an earth solar day)

So as I try to rationalize all this in my mind, ...a good cesium unit is
just a sophisticated 'counter'. The counter gate is just the solar day.

It's interesting to note (to ask?): When did someone get smart enough to
start measuring 1/86thousthanth of a day, ....That starts needing a standard
in itself.

-Don
Austin, TX

(When we go to Mars we'll need a few more cesium 'counts' , btw.)

----------------------------------------------------------

-----Original Message-----
From: time-nuts-bounces-***@public.gmane.org [mailto:time-nuts-bounces-***@public.gmane.org] On
Behalf of J. Forster
Sent: Wednesday, May 09, 2012 11:31 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Why 9,192,631,770 ??

This issue came up when I was involved in SETI work. One big question was
"How do you know what frequency to listen at?"

Any earth-centric system is essentially arbitrary. There is nothing
'universal' about the Meter, Kilogram, or Second. (or any of their
equivalents in other systems of units).

And, all other units are derived from those fundamental units.

The only things, thought to be anything like universal, are fundamental
properties of atoms, like hyperfine transitions. Hence the defined second.

-John

=================

Post by WarrenS

It is interesting that the leap seconds correction is always a positive number.

David

2012-05-09 23:26:06 UTC

Permalink

James Burke did a whole episode of Connections on that subject:

https://www.youtube.com/playlist?list=PL777ACC303F62C744

The oldest example of an automated phase locked loop I have heard of
was used for calibrating newly built pendulum clocks to a master
pendulum clock.

On Wed, 9 May 2012 17:45:06 -0500, "Don Lewis"

Post by Don Lewis
Thanks, all ...this thread has helped me a lot.
It reinforces my beliefs of what a 'second' is ...what we were taught from
One solar day/86,400 (A 'solar' day being an earth solar day)
So as I try to rationalize all this in my mind, ...a good cesium unit is
just a sophisticated 'counter'. The counter gate is just the solar day.
It's interesting to note (to ask?): When did someone get smart enough to
start measuring 1/86thousthanth of a day, ....That starts needing a standard
in itself.

J. Forster

2012-05-09 23:35:39 UTC

Permalink

"Connections" is a wonderful series, as is "The Day The Universe Changed".
Thanks for the YouTube link.

-John

================

Post by David
https://www.youtube.com/playlist?list=PL777ACC303F62C744
The oldest example of an automated phase locked loop I have heard of
was used for calibrating newly built pendulum clocks to a master
pendulum clock.
On Wed, 9 May 2012 17:45:06 -0500, "Don Lewis"

_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Charles P. Steinmetz

2012-05-09 23:27:39 UTC

Permalink

Post by Don Lewis
It's interesting to note (to ask?): When did someone get smart enough to
start measuring 1/86 thousandth of a day

That is generally considered to be the 10th/11th century Persian
Muslim mathematician and astronomer, Abu al-Rayhan Muhammad ibn Ahmad
al-Biruni (a.k.a. Alberonius and Al-Biruni). His eclipse data was
accurate enough that it was used in the late 18th century to help
quantify the acceleration of the moon, and is still used by astronomers today.

Best regards,

Charles

Jim Lux

2012-05-10 01:03:52 UTC

Permalink

Post by Don Lewis
It's interesting to note (to ask?): When did someone get smart enough to
start measuring 1/86 thousandth of a day

That is generally considered to be the 10th/11th century Persian Muslim
mathematician and astronomer, Abu al-Rayhan Muhammad ibn Ahmad al-Biruni
(a.k.a. Alberonius and Al-Biruni). His eclipse data was accurate enough
that it was used in the late 18th century to help quantify the
acceleration of the moon, and is still used by astronomers today.

as for why a second?

That's because of the popularity of base 60. Dividing the day into 24
hours (twice 12) relates a day to a year to zodiac constellations. it's
also got a lot of factors (2,3,4,6,etc)

Then dividing hours into 60 minutes, and minutes into 60 seconds makes
sort of sense.

1 second is also close to the human heartbeat period (as opposed to, say
using 1/100 hour or something like that)

The French Revolution did try to decimalize things, of course.

Tom Van Baak

2012-05-10 14:27:20 UTC

Permalink

Hi Don,

This was true until the 1960's. Hopefully the school curriculum has been updated since then. The definition of a pound, or a foot, or a second has progressed over centuries. We now have a very accurate and convenient definition of the second. And it is no longer 1/86400 of a day; it is no longer 1/31556925.9747 of a year either.

The problem with "day" is you have to ask which one; each day has a different length when you measure down in the milli- or microsecond level; the earth doesn't spin very consistently. See:
http://www.leapsecond.com/museum/earth/

Post by Don Lewis
So as I try to rationalize all this in my mind, ...a good cesium unit is
just a sophisticated 'counter'. The counter gate is just the solar day.
It's interesting to note (to ask?): When did someone get smart enough to
start measuring 1/86thousthanth of a day, ....That starts needing a standard
in itself.

Exactly. A ~1-meter pendulum can conveniently be made to tick seconds. You can adjust the length until each day you count 86400 ticks. The earth is the standard and the pendulum clock is being measured.

What was found in the early 1900's is that some precise pendulum clocks kept better time compared to themselves than to the earth. The conclusion was the earth is less stable than the best pendulum clocks. At that point the pendulum clocks became the standard and the earth became the thing to be measured. This was the beginning of the end for a second being 1/86400 of a day.

The same thing is happening today with the cesium definition of the second. Optical clocks are so good that they are being used to measure the cesium clocks. So in years to come it is likely that the definition of the second will again be redefined; this time in terms of an optical frequency.

/tvb

Peter Monta

2012-05-11 05:14:02 UTC

Permalink

Are there better estimates of the ET second nowadays (relative to the
SI second)? It would be interesting to know what the cesium frequency
"should have been" if much better estimates of the ephemeris-time
second were available at the time. One would think that with all the
solar-system data JPL and others have had at their disposal since the
1970s, a very good ET-second number could be cooked up; better than
1950s Moon cameras at any rate.

For that matter, what are the inherent long-term limits on ET as a
timescale? I gather the observation noise is very high on short
timescales, but what is the situation for, say, tau>1year? It's not
as if the Earth's orbit is randomly perturbed very much, I'm guessing,
and any deterministic perturbations or relativistic corrections would
be compensated for; it's the noiselike processes that would be
interesting. (Solar wind?)

Cheers,
Peter

mike cook

2012-05-11 06:30:34 UTC

Permalink

Post by Peter Monta
Are there better estimates of the ET second nowadays (relative to the
SI second)? It would be interesting to know what the cesium frequency
"should have been" if much better estimates of the ephemeris-time
second were available at the time. One would think that with all the
solar-system data JPL and others have had at their disposal since the
1970s, a very good ET-second number could be cooked up; better than
1950s Moon cameras at any rate.

There are various refs in the pedia to later estimates. Markowitz (1988)
calculated an agreement to 1x10-10. but looking at the article I see
there were still some uncertainty in terms used to calculate ET and
depending on what was chosen gave 2x10-11 . Accordingly he concludes
conservatively that ET has been equal to Si within 1x10-9.
The uncertainties will have been reduced since then but not eliminated
and so "should have been" is a moving target but it would appear from
the above that the chosen SI value would still be preferred if the
decision was to be reappraised.

paul swed

2012-05-11 13:34:10 UTC

Permalink

OK I have learned a lot and absolutely fantastic news.
No matter what my aged CS says I can claim its accurate now. Its simply the
world has not caught up to or slowed down to it.
Regards
Paul

Post by Peter Monta
Are there better estimates of the ET second nowadays (relative to the

Post by Peter Monta
SI second)? It would be interesting to know what the cesium frequency
"should have been" if much better estimates of the ephemeris-time
second were available at the time. One would think that with all the
solar-system data JPL and others have had at their disposal since the
1970s, a very good ET-second number could be cooked up; better than
1950s Moon cameras at any rate.

There are various refs in the pedia to later estimates. Markowitz (1988)
calculated an agreement to 1x10-10. but looking at the article I see there
were still some uncertainty in terms used to calculate ET and depending on
what was chosen gave 2x10-11 . Accordingly he concludes conservatively
that ET has been equal to Si within 1x10-9.
The uncertainties will have been reduced since then but not eliminated and
so "should have been" is a moving target but it would appear from the
above that the chosen SI value would still be preferred if the decision was
to be reappraised.
______________________________**_________________
To unsubscribe, go to https://www.febo.com/cgi-bin/**
mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>
and follow the instructions there.

Tom Van Baak

2012-05-11 18:21:36 UTC

Permalink

Hi Peter,

Everyone should take ten seconds and look at this animated GIF:
Loading Image...

It shows what would have happened to "earth time" vs. "atomic time" if the cesium frequency had been defined to be other than 9192.631770 MHz. As you can see a slightly higher number would have meant less deviation between the two timescales.

However it should also be clear, even with this short 40-year plot, that no number is the best or correct or right choice. It all depends on which year(s) you choose to base your earth rotation rate calibration on (the astronomers doing the calibration in the 1950's selected the year 1900 as their baseline).

To see each page at your own pace here is it as a multi-page PDF file:
http://leapsecond.com/pages/ut/ut-ani-v2.pdf

If you wanted a near perfect match between atomic time and the rotation of the earth during the 1970's hindsight tells you the frequency should have been 9192.632080 MHz. Similarly if your crystal ball said to use 9192.632010 you would have been very close for three decades. If you wanted the best time accuracy from the year 1972 to present you should have picked 9192.631950.

/tvb

paul swed

2012-05-11 18:53:42 UTC

Permalink

So what you are saying is every 30 years select a new leap CS reference.
Dispense with everything in between.

Post by Tom Van Baak

Hi Peter,
http://leapsecond.com/pages/ut/ut-ani-v2.gif
It shows what would have happened to "earth time" vs. "atomic time" if the
cesium frequency had been defined to be other than 9192.631770 MHz. As
you can see a slightly higher number would have meant less deviation
between the two timescales.
However it should also be clear, even with this short 40-year plot, that
no number is the best or correct or right choice. It all depends on which
year(s) you choose to base your earth rotation rate calibration on (the
astronomers doing the calibration in the 1950's selected the year 1900 as
their baseline).
http://leapsecond.com/pages/ut/ut-ani-v2.pdf
If you wanted a near perfect match between atomic time and the rotation of
the earth during the 1970's hindsight tells you the frequency should have
been 9192.632080 MHz. Similarly if your crystal ball said to use
9192.632010 you would have been very close for three decades. If you
wanted the best time accuracy from the year 1972 to present you should have
picked 9192.631950.
/tvb
_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

J. Forster

2012-05-11 18:53:17 UTC

Permalink

IMO, that would be a disaster for all areas of physics and engineering,
except for possibly some aspects of astronomy.

When they 'redefined' the Volt some years ago it was a goat rodeo.

-John

===================

Post by paul swed
So what you are saying is every 30 years select a new leap CS reference.
Dispense with everything in between.

Post by Tom Van Baak

_______________________________________________
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Peter Monta

2012-05-11 21:49:46 UTC

Permalink

Post by Tom Van Baak
http://leapsecond.com/pages/ut/ut-ani-v2.gif

Very nice. I guess a comparable plot with ephemeris time would be a
lot noisier and sparser, something like the graphs in the Markowitz
1988 paper.

I wonder if there's some sort of IERS-like official repository for
ephemeris-time data (or, for that matter, other non-Earth-rotation
timescales like the pulsars). Probably not much call for it, being
mostly of academic interest.

Cheers,
Peter

Bob Camp

2012-05-11 21:58:35 UTC

Permalink

Hi

BIH

Bob

Post by Peter Monta

Post by Tom Van Baak
http://leapsecond.com/pages/ut/ut-ani-v2.gif

Very nice. I guess a comparable plot with ephemeris time would be a
lot noisier and sparser, something like the graphs in the Markowitz
1988 paper.
I wonder if there's some sort of IERS-like official repository for
ephemeris-time data (or, for that matter, other non-Earth-rotation
timescales like the pulsars). Probably not much call for it, being
mostly of academic interest.
Cheers,
Peter
_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.