Rule of thumb: quartz is best short term, Rb or H-maser mid-term, and Cs by far the best long-term.

For GPS clocks the long-term doesn't matter that much since each space clock is monitored and updated against the GPS master clock(s) on the ground.

/tvb (iPhone4)

On May 4, 2013, at 11:40 AM, Attila Kinali attila@kinali.ch wrote:

In message BB05041D-F03A-42AC-85C6-467110FC3466@leapsecond.com, "Tom Van Baak
(lab)" writes:

I have never seen a technical description of the Cs used in the
early GPS satellites, but I have seen many references to all sorts
of troubles with them, including a much shorter lifetime than
was hoped for.

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

Note also Galileo uses Rb and H-maser only; no Cs.

/tvb (iPhone4)

On May 4, 2013, at 1:53 PM, "Poul-Henning Kamp" phk@phk.freebsd.dk wrote:

On Sat, 4 May 2013 12:36:20 -0700
"Tom Van Baak (lab)" tvb@leapsecond.com wrote:

Ah.. so it's a fundamental limitation. And i was looking for something
GPS specific.

Any references i could read on those limitations? A quick google
did not produce any good results.

		Attila Kinali

--
The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
-- unknown

On 05/05/2013 10:05 AM, Attila Kinali wrote:

There is a handful of references but picking up a book like "Quantum
Leap" is a good start.

Quartz is a bit of (syntetic) rock, cut at some angle(s), cleaned,
mounted in some hermetic sealed chamber with residue dirt, and mounting
that over time shifts the stress of the crystal. The various processes
create a long-term frequency drift (plots over 5 years shows the same
systematic drift, non-linear). Oh, and if you shift temperature of the
crystal, it has to re-align. The parameters of this systematic behaviour
is individual, and by itself it is not able to handle these.

For rubidium gas-cell, there is a bunch of systematics, including
darkening of the pumping Rb lamp, which causes shift in light intensity
pull, temperature miss-alignments changes, causing the lamp and
filtering cell to shift, causing de-pumping to change over time as well
as light intensity. The rubidium reference cell has buffer gas, which
leaks, and the buffer gas is there to reduce the speed of the rubidium
atoms, such that they don't hit the glas wall with inevidable shift in
frequency that has. Oh, the wall shift changes with temperature, the
buffer gas causes a shift, which also shifts with temperature... the
smart guys make them more or less cancel, but this cancellation shifts
over time as some of the buffer gas escapes, especially Helium. Oh, and
the resonator around the rubidium reference cell pulls the frequency,
and that changes with temperature as well. No wonder that things are
being temperature stabilized using ovens, which in itself is the major
power consumption source of a rubidium gas-cell.

The caesium atomic beam does not have wall-shifts, but rather it has
much lower systematics. One of the major onces being magnetic field.
Assuming that the tube is degaussed, the C-field adjustment is
troublesome. Old caesiums had them rather arbitrary set, but you could
adjust them to national references, so first generations where really
secondary sources. By looking at side-bands, this can be servoed and
C-field steered and hence the shift understood, and by that basis for a
primary reference can be found. Then, there is wear mechanisms that
kicks in on the physical package.

The above is a summary of things collected from a variety of sources,
but I think this coarse walk-through of issues gives some insight as to
what issues pops up where and the milage vary a lot within each group.
Modern high-performance rubidium gas-cells outperform the early
caesiums, high-performance crystals outperform several rubidiums.
The HP5065A is an example of an old clock with really good performance,
so modern is not everything, and the modern compact telecom rubidiums
and for that mater CSAC is more space/power oriented than ultimate
performance of the technology as such.

Cheers,
Magnus

On 5/5/13 1:48 AM, Magnus Danielson wrote:

I wonder where mercury ion fits in the scheme of things, since that's
where we're spending some money for spacecraft applications right now.
It's supposed to be orders of magnitude better than Rb.

Hi

Mercury was tried very early on as a vapor standard. They had some significant problems with it in the 1950's. It's not surprising that after 60 years somebody might want to take another swing at it.

Bob

On May 5, 2013, at 9:59 AM, Jim Lux jimlux@earthlink.net wrote:

At HP in the 1990's, Len Cutler's group built some experimental
mercury ion standards for USNO (IIRC).  They were of the
trapped ion type.  BTW, it is important to understand that
the architecture is the key factor, not the flavor of atom.
When people say rubidium is inferior to cesium, they really
mean a gas call is inferior to an atomic beam, etc.

Rick Karlquist N6RK

On 5/5/2013 8:20 AM, Bob Camp wrote:

On 5/5/13 8:42 AM, Richard (Rick) Karlquist wrote:

OK, that's interesting..

So the Hg ion they're building for Deep Space Atomic Clock (DSAC) is a
trapped ion type. The whole thing is, as I recall, 1 liter, 1 kilo,
including both the physics package and the electronics.

It's targeting USO type applications.

Hi Jim,

On 05/05/2013 03:59 PM, Jim Lux wrote:

Mercury standards is of the ion trap variety. It has about 40,5 GHz of
frequency, very high Q due to long unperturbed observation-time and also
cool due to ion locking which reduces doppler shifts as well as thermal
shift. If you also cool the ion trap physics to low temperature, the
black body radiation shift can be reduced significantly. Ion traps can
also be combined with laser cooling.

Ion traps is the new and fancy stuff in a historical perspective. HP did
an attempt to build a commercial device as I recall it. Would not mind
having one.

I think it is an interesting addition to the traditional mix. It
achieves very high stability for the size. The CSAC is really a gas cell
like rubidium, with similar issues. The ion trap takes a different route
to the size issue. It has it's own set of challenges, but toss in a bit
of engineering and they can be mastered. It has the potential to replace
caesium beams in many applications.

It would be interesting to see if your effort on space qualified ion
traps spills over to the commercial market. If you get spare samples, I
can give you an address to send them. ;-)

Cheers,
Magnus

On 05/05/2013 06:50 PM, Jim Lux wrote:

They have been targeting this goal for a very long time. Several
interesting papers is to be found at PTTI, NIST etc.

Getting good performance with small volume and power is definitely
interesting, and it does not have to be on the rice-grain level.

Cheers,
Magnus

On 5/5/13 10:05 AM, Magnus Danielson wrote:

Yeah.. some years (6 or 7?) ago, John Prestage had a prototype of the
physics package working on the bench. Getting from there to a repeatably
manufacturable space flight qualified has been a few years. Not to
mention making flight qualified electronics to go around it.  I think
the first flight will be next year or the year after as a hosted payload
on something.

On 5/5/13 10:01 AM, Magnus Danielson wrote:

Hah.. getting just one made is a chore.. I've not worked on the project,
but it's in the same general program as the stuff I do, so we all see
each others' presentations at the semi-annual reviews.  It took
significantly more time than expected to get the physics package
manufacturing worked out.

Then there's whole thing of making 40 GHz electronics that are small,
low power, radiation tolerant, etc.; I seem to recall that there's a
tiny PMT in the system too, so that means HV, which is no easy feat
either.

In message 51867DF4.4010006@karlquist.com, "Richard (Rick) Karlquist" writes:

Well, somewhat.

Some flavours of atoms don't work with some architectures, so for
most of the stuff in reach for us, the atoms do indeed equate an
architecture.

The exception seems to be fountains, which can run on pretty much
any alkali atom you care to feed it, and some even able to use
both Rb og Cs (Built to nail the Rb frequency firmly down, as
I understand it).

However, there seems to be actual differences between the flavours
of atoms in fountains, and USNO have picked Rb over Cs because
they get better results that way.

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

Hi Jim,

On 05/05/2013 07:33 PM, Jim Lux wrote:

Here is a starting-point:

33rd PTTI 2001:
http://www.pttimeeting.org/archivemeetings/2001papers/paper4.pdf

38th PTTI 2006:
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40340/1/07-0487.pdf

39th PTTI 2007:
http://www.pttimeeting.org/archivemeetings/2007papers/paper25.pdf

NIST:
http://tf.nist.gov/general/pdf/742.pdf

JPL:
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40206/3/04-2783FN.pdf

That was only a quick search, so it is easy to find those and more.

Would be interesting to see how the lasers could be made affordable and
compact. It's a bit difficult frequency/wavelength.

Cheers,
Magnus

On Sun, 05 May 2013 18:29:53 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:

The alkali atoms are pretty much interchangable.
There have been masers from Rb as well as Cs, beam standards
from Rb and H, and Rb fountains. I have not read of any H fountain
yet, but i guess it's pretty difficult to build given that the
laser light needed for cooling is in the 120nm range (UV) and
lasers in that range are pretty difficult to build (there have
been laser spectroscopy experiments using 120nm lasers nevertheless)
and that H is very light, ie the fountain would get quite long.

I also have never seen a H gas cell standard, probably for the same reason
of needing UV light.

Other classes are charged ions, these are mostly positive charged atoms
where the outermost shell becomes an s orbital with a single electron.
Prime examples are alkaline earth metals (Be, Mg, Ca, Sr,..), but also
group 12 metals (Zn, Cd, Hg) and some lanthanide/actinide (e.g. Yb).
AFAIK these are mostly interchangable as well.

I do not know what the general property of laser cooled neutral atom
frequency standards is.

One property seldom explicitly mentioned is the nuclear spin. The alkali
metal standards seem to depend on the spin being half-integral, while
laser cooled ion standards seem to be possible with both integer (including 0)
and half integer spins.

		Attila Kinali

--
The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
-- unknown

In message 20130505205257.8497f166abb1e4918695353d@kinali.ch, Attila Kinali w
rites:

Hydrogen is very hard to contain.

The way you filter hydrogen is to press it through a palladium film,
and that doesn't take a particular high pressure.

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

Hi Poul-Henning,

On 05/05/2013 08:29 PM, Poul-Henning Kamp wrote:

Well, the basic rule is that you want a single electron, preferably in
the S state in the outermost shell. You can have that either as neutral
atom or ionized. Depending on neutral or ionized you go into the
"classic" or "ion" classes. This is why alkali atoms is so popular, as
group 1 fits the description well in its neutral state.

Another aspect is how you achieve the state imbalance which can be
achieved either by state selection magnets or by pumping. Rubidium has
been selected traditionally because of the suitable spectrum of Rb-85
and Rb-87 isotopes, and that these is relatively easy to come by (both
exist in normal rubidium ore) allowing for simple filtering. Modern
filtering and modern lasers allows a much freer selection.

Beam devices has been built for much more than caesium. In the original
conception caesium was fighting with thallium, with thallium actually
being somewhat better, but was judged a bit impractical at the time.
Rubidium was also built as research beam units, but it has higher
sensitivity to magnetic field pulling, which used to be an issue, which
now largely is gone with servo capability.

The fountains is just an evolvement of the beam devices, but the "beam"
falls backwards halfway "through". In the fountain-context rubidium now
has an edge over caesium. What make fountains feasible is the
laser-cooling as it not only allows cooling, but also bouncing around
the ball of atoms.

Indeed. Wasn't NPL in UK early out as well?

In the end, the actual atom used for a particular device is a result of
what makes practical engineering at the time and achieving the best
performance.

The optical clocks stretches the imagination even more than ion-traps
ever did, but affordable ion-traps is still very interesting.

Cheers,
Magnus

On 05/05/2013 09:28 PM, Poul-Henning Kamp wrote:

Another aspect is that D1 and D2 lines are essentially the same
(121.5674 nm and 121.5668 nm) while for rubidium its 794.760 nm and
780.027 nm which is about 25000 times wider. This makes pumping
state-selection hard for hydrogen but relatively easy with rubidium.

So, for hydrogen you have to do state-selection through magnets, which
is what is being used in hydrogen masers.

There is so many practicalities that goes in to technical decisions.

Cheers,
Magnus