The current definition of the second is based on Caesium, as that was what
was the best then. However, with work on the newer optical standards, and
Rubidium Fountains, I'm pretty certain that in the next few years the second
will be re-defined. Attached document from UK national Physical Laboratory
(NPL) may be of interest.

Rob K

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Tom Clark, K3IO (ex W3IWI)
Sent: 20 April 2006 06:59
To: time-nuts@febo.com
Subject: Re: [time-nuts] time-nuts Digest, Vol 21, Issue 22

VE2VM commented

But Isn't Cesium drift-free? Since the SI second is standardized as de
duration of 9192631770 oscillation of the hyperfine transition of the atom
133Cs?

If Cesium drifts, theren should be a more formal definition of the second
(Such as density, maximum C-field or level of purity). Does anyone here has
it?

The achieved frequency can be pulled by external factors. The official
SI second definition is "9,192,631,770 cycles of the ground-state
hyperfine splitting of the unperturbed cesium atom." The problem is
making it be unperturbed by the effects of the finite microwave
cavity, wall effects from the containment bulb, the length of time
that the Cesium atom is "stored" in its excited state, etc.
A good recent review is the paper by Diddams (Science, November 2004)
available from the NIST web site at
[1]http://tf.nist.gov/timefreq/general/generalpubs.htm
Getting the cesium atom unperturbed has led to larger and longer
Cesium standards at NIST (for pictures see
[2]http://tf.nist.gov/cesium/atomichistory.htm); with the longest of
the Cs tubes (NIST-7), the interaction time (the length of time the
Cesium atoms "live" in their excited state is ~10 msec. The newest
generation of Cesium fountain clocks use Laser cooling to contain a
cloud of cesium atoms at at temperature near 0º Kelvin to minimize the
wall effects and to increase the storage time to ~1 second (see
[3]http://tf.nist.gov/cesium/fountain.htm and
[4]http://tycho.usno.navy.mil/clockdev/cesium.html).

And also, something else i don't understand: Why do the newer GPS satellites
rely on Rb standards rather than Cs standards? Since Rubidium is known as
less precise than cesium? Is there a reliability issue there (Rb clocks are
more reliable / longer MTBF tha Cesium clocks). I don't know...

The Cesium clocks in GPS have been less reliable (probably because
they are more complicated) than the Rb clocks in early GPS satellites;
[5]for some information see the FAQ on this USAF web site. The 16
Boeing Block II & IIA satellites have 2 Cs and 2 Rb. The 7 LockMart
(your One-Stop Defense Contractor!) Block IIR satellites launched
(plus 8 more awaiting launch) have 3 Rb.
The Rb/Cs mix will change will change again with the IIF & 3rd
generation GPS satellites ([6]see Symmetricom propaganda here) and
with the European Galileo series (Galileo is planning on H-Masers).
There is an interesting article in [7]The Space Review on the clocks
planned for these next generation navigation spacecraft.
73, Tom

References

1. http://tf.nist.gov/timefreq/general/generalpubs.htm
2. http://tf.nist.gov/cesium/atomichistory.htm
3. http://tf.nist.gov/cesium/fountain.htm
4. http://tycho.usno.navy.mil/clockdev/cesium.html
5. http://gps.losangeles.af.mil/jpo/gpsoverview.htm
6. http://www.symmsda.com/about_us/index.asp
7. http://www.thespacereview.com/article/534/1

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So Loran isn't really dead yet!

In message: 16160.1145561192@critter.freebsd.dk
"Poul-Henning Kamp" phk@phk.freebsd.dk writes:
: In message 7.0.1.0.0.20060420152108.03151988@wordsnimages.com,
John Day write
: s:
: >So Loran isn't really dead yet!
:
: Not quite.
:
: The draft European Radio Navigation plan shows that Loran-C gives
: 22% of the benefit for 7% of the money.
:
: But who knows if that translates to political decisions in its favour.

The US is keeping Loran-C going as a backup for GPS as well.  Its been
recently expanded to have data embedded in the pulse streams so that
messages about things like leap seconds and GPS issues can be
disseminated.  A few years ago they upgraded their transmitters and
the savings in electricity from old tube transmitters to new solid
state ones paid for the upgrade.  They upgraded the entire US/Canada
system, except for the chains that extend into Russia.  The new data
channel is being rolled out as well.

One of the interesting things about working on Loran-C was learning
that no one really knows how many current, real users of Loran-C there
are.  Millions of receivers have been sold, but there's evidentially
no good estimates of the number of users (at least there weren't a few
years ago).

Warner

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time-nuts@febo.com
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The current definition of the second is based on Caesium, as that was
what was the best then. However, with work on the newer optical
standards, and Rubidium Fountains, I'm pretty certain that in the next
few years the second will be re-defined. Attached document from UK
national Physical Laboratory
(NPL) may be of interest.

Rob K

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Tom Clark, K3IO (ex W3IWI)
Sent: 20 April 2006 06:59
To: time-nuts@febo.com
Subject: Re: [time-nuts] time-nuts Digest, Vol 21, Issue 22

VE2VM commented

But Isn't Cesium drift-free? Since the SI second is standardized as de
duration of 9192631770 oscillation of the hyperfine transition of the
atom 133Cs?

If Cesium drifts, theren should be a more formal definition of the
second (Such as density, maximum C-field or level of purity). Does
anyone here has it?

The achieved frequency can be pulled by external factors. The official
SI second definition is "9,192,631,770 cycles of the ground-state
hyperfine splitting of the unperturbed cesium atom." The problem is
making it be unperturbed by the effects of the finite microwave
cavity, wall effects from the containment bulb, the length of time
that the Cesium atom is "stored" in its excited state, etc.
A good recent review is the paper by Diddams (Science, November 2004)
available from the NIST web site at
[1]http://tf.nist.gov/timefreq/general/generalpubs.htm
Getting the cesium atom unperturbed has led to larger and longer
Cesium standards at NIST (for pictures see
[2]http://tf.nist.gov/cesium/atomichistory.htm); with the longest of
the Cs tubes (NIST-7), the interaction time (the length of time the
Cesium atoms "live" in their excited state is ~10 msec. The newest
generation of Cesium fountain clocks use Laser cooling to contain a
cloud of cesium atoms at at temperature near 0º Kelvin to minimize the
wall effects and to increase the storage time to ~1 second (see
[3]http://tf.nist.gov/cesium/fountain.htm and
[4]http://tycho.usno.navy.mil/clockdev/cesium.html).

And also, something else i don't understand: Why do the newer GPS
satellites rely on Rb standards rather than Cs standards? Since
Rubidium is known as less precise than cesium? Is there a reliability
issue there (Rb clocks are more reliable / longer MTBF tha Cesium clocks).

The Cesium clocks in GPS have been less reliable (probably because
they are more complicated) than the Rb clocks in early GPS satellites;
[5]for some information see the FAQ on this USAF web site. The 16
Boeing Block II & IIA satellites have 2 Cs and 2 Rb. The 7 LockMart
(your One-Stop Defense Contractor!) Block IIR satellites launched
(plus 8 more awaiting launch) have 3 Rb.
The Rb/Cs mix will change will change again with the IIF & 3rd
generation GPS satellites ([6]see Symmetricom propaganda here) and
with the European Galileo series (Galileo is planning on H-Masers).
There is an interesting article in [7]The Space Review on the clocks
planned for these next generation navigation spacecraft.
73, Tom

References

1. http://tf.nist.gov/timefreq/general/generalpubs.htm
2. http://tf.nist.gov/cesium/atomichistory.htm
3. http://tf.nist.gov/cesium/fountain.htm
4. http://tycho.usno.navy.mil/clockdev/cesium.html
5. http://gps.losangeles.af.mil/jpo/gpsoverview.htm
6. http://www.symmsda.com/about_us/index.asp
7. http://www.thespacereview.com/article/534/1

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No, LORAN is most definitely not dead. Good news, as I used to sell LORAN-C
timing from Austron back in the 80's. Worked well then, and works well now.

Rob K

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of John Day
Sent: 20 April 2006 20:21
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] time-nuts Digest, Vol 21, Issue 22

So Loran isn't really dead yet!

John

No, LORAN is most definitely not dead. Good news, as I used to sell LORAN-C
timing from Austron back in the 80's. Worked well then, and works well now.

Rob K

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of John Day
Sent: 20 April 2006 20:21
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] time-nuts Digest, Vol 21, Issue 22

So Loran isn't really dead yet!

John