Is the ISS a suitable platform?
I expect getting the experiment package on there would be quite another matter!
DaveB, NZ

-----Original Message-----
From: Tom Van Baak [mailto:tvb@LeapSecond.com]
Sent: Sunday, November 28, 2021 20:09
To: time-nuts@lists.febo.com
Subject: [time-nuts] Re: Project Great

Hi Thomas,

Good to hear the experiment was contagious for you. If you have additional questions let me know.

Your suggestion about Mount Evans and Pikes Peak are excellent. You will enjoy this 2017 paper:

"An Undergraduate Test of Gravitational Time Dilation"
https://arxiv.org/abs/1710.07381
https://arxiv.org/pdf/1710.07381.pdf

As for CSAC, the news is not so good. I've worked with several groups to explore CSAC for gravitational time dilation experiments. Those clocks are so cute and small, it's irresistible; but the numbers just don't add up. Over a day their stability is in the low e-12's vs. a "real" cesium clock like a 5071A in the low e-14's. So when you are doing a relativity experiment trying to detect a frequency shift that's on the order of e-13's you reach for a 5071A instead of a CSAC. The performance is nearly 100 to 1.

One solution is a taller mountain. The best on the planet is Mauna Kea (Big Island, Hawaii) where you can literally drive from sea level to the summit (13,800 ft, 4200 m) in a few hours. The frequency shift up there is 4.5e-13, which is 40 ns per day. But still, to have even the slightest chance of success you'd want your clocks to be good to 1e-13 or better. CSAC aren't even close, and probably neither are telecom Rb.

I'm currently involved with another solution -- a HAB (High Altitude
Balloon) CSAC flight. Getting to 100,000 ft altitude is quite common. Up there, clocks run a whopping 3.3e-12 faster, which is 280 ns/day, or 12 ns/hour. This is a clear case where the amazing low mass and low power of a CSAC is a  critical advantage. However, the numbers still aren't working out and the logistic and environmental conditions are brutal. I won't say it's impossible, but it may take years and a huge bag of tricks before it works or it's proved too impractical.

Jim, I'd be interested in any Cubesat / CSAC results. They don't exactly land in one piece so the typical round-trip clock comparison method wouldn't work. A direct frequency comparison might. In that case the drift and re-trace specs of a CSAC are probably more important than the stability.

/tvb

On 11/27/2021 12:37 PM, Thomas Valerio wrote:

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I would imagine there are already several caesium clocks on board the ISS,
anyway.
Don't forget there is a velocity component in relativistic time shift, as
well as gravitational, so using a moving platform like an aircraft or the
ISS complicated things a lot

Andy
www.g4jnt.com

On Sun, 28 Nov 2021 at 10:03, Dave ZL3FJ 2c39a@silverbears.nz wrote:

Being curious here: has anyone ever taken a Cs in a submarine? E.g. the Marianatrench or so?

As a native from a mountain-deprived country I could not help wondering.

Wilko

On 11/27/21 11:08 PM, Tom Van Baak wrote:

The CHOMPTT folks were trying to do time transfer using optical, but
they also flew a CSAC (maybe even two).   One problem is that "what do
you compare to", as you noted.  One could compare to on board GPS 1pps
or to an onboard OCXO.  Both the CSAC and the OCXO would speed up
relative to surface. But you also have the velocity problem (7 km/s) so
they "apparently" run slow.  I don't know that CSAC vs GPS would
actually be able to do the measurement - the uncertainty in the GPS is
perhaps too high.  Maybe with post processed GPS - GIPSYX/RTGx should
give you position and time to <1ns.

On 11/28/21 2:11 AM, Andy Talbot wrote:

I wouldn't actually think there's a Cs on ISS.  What purpose would it
serve?  We as time-nuts think "of course you'd have a precise source of
time", but really, there's not much need for timing on ISS on a scale
smaller than seconds, if that.  NTP to timestamp files, for instance.

As a practical matter, there's not a lot of "infrastructure" on ISS,
i.e. there's no "house 10 MHz" - the experiments tend to be self
contained.  When I was working on SCaNTestbed, which launched to ISS in
2012, there wasn't even an onboard real-time GNSS time/position feed. We
had a software GPS receiver as part of the testbed.  What you would get
is a "playback" of ground predicts done by GSFC Flight Dynamics for
position and time over MIL-STD-1553 as part of Broadcast Ancillary
Data.  And knowing precisely where you are on ISS is tricky anyway -
it's the size of a football stadium and flexes and moves on the scale of
meters. The BAD data was for some presumed "center of mass", as I recall.

On 11/28/21 9:37 AM, Bernd Neubig wrote:

Right now, for high performance in space, trapped mercury ion clocks
seem to be the ticket.  Deep Space Atomic Clock is working well, and
there's a DSAC2 in the works.

A fundamental problem for this kind of thing is that "infrastructure"
(faster communications, better time) doesn't get a lot of support unless
it enables answering a science question that the community as a whole
deems important. In the case of NASA, it's the decadal studies that
drive a lot (Astrophysics 2020 just came out)- The decadal study says
"it is of great interest to answer question X" and if your technology
helps with that, great, it might get flown.

Good independent time keeping in deep space is an enabling technology
for autonomous navigation and rendezvous, for which there hasn't yet
been a really compelling science need.  Perhaps when we need to do
auto-nav around moons of planets or something like that.

For all that NASA does human exploration, it all is in service of
answering some science question. So NASA doesn't really spend a big
amount on problems like "how do you allow a dozen astronauts on the Moon
to know where they are" - sure, they do studies (I've participated in
some), they keep up on current technology, but they're not going to
invest $100-500M in building a Position, Nav, Timing infrastructure -
that's viewed more as an "operational thing" to "be done by others". 
Everyone sort of assumes that something with GPS-like performance will
be available if needed, one just needs to write the check.

I'm basically a radio and computer guy at heart, so to me, one of the
things which good timekeeping (and PNT in general) enables is large
distributed RF sensors - radio telescopes/interferometers in space.
Large physical extent (with precise knowledge of time and position)
gives you good angular resolution   Precision metrology also lets you do
things like GRACE and GRAIL - measuring the gravitational field of a
body by measuring the distance between paired orbiters - that distance
is measured by, you guessed it, RF and optical links, based on
ultrastable oscillators.  Things like mercury ion clocks have the
potential to replace USOs - and just like in terrestrial timekeeping,
standards that rely on the fundamental physics are desirable over "fine
artisanal craftsmanship" which is what quartz clocks are - you start
with 1000 blanks, pick the best, mount them, pick the best, age them,
pick the best.  And the whole time you pray that you didn't "lose the
recipe".  USOs (and atomic clocks) are invaluable for radio science and
gravity experiments - precisely measuring the orbit of something a long
ways away, or sending phase coherent signals at different frequencies to
a receiver and measuring the relative amplitude and phase for
occultations, or just the interplanetary medium - You want something
that has really good ADEV at tau>1000 seconds, because integration time
is important, for both the radio science and the ranging/gravity science.

This is part of why I got selected to be project manager for SunRISE - a
10km scale interferometer in space.  I know how these kinds of things
work, or, even better, I know when and where to go ask questions, so I
know what I don't know.  What I don't get to do as PM (and is somewhat
frustrating) is design the system that does it. (The phrase from the JPL
Chief Engineer, Rob Manning, is "When you become a manager you give up
your SME card")

Am 28.11.21 um 18:37 schrieb Bernd Neubig:

at first 2012, completely out of touch with the world.

And I can say that the AXTALs simply sat there and worked, a
great exception for this project.

I did a redesign of the analog part of the DMTD system between
the H-Maser and the Cs that was accepted about 7 years ago,
the time stretcher for photon flight time interpolation, a VHDL
triple redundancy library that looked like std_logic/vector
and that had the redundany mostly invisibly under the hood
(we were not given the Xilinx tools), FPGA configuration memory
scrubbing, SEU protection of the CPU, microwave link; constantly
filling the voids by colleges who left.

Adventures of a freelancer.

Cheers,
Gerhard, DK4XP

Hi,

I'm also one of those fascinated by Project Great and this was The project inspired me to start with time related stuf.

Can Cs clock be avoided by prolonged period at high altitudes? Assume you can spend a month at the summit. Is e.g. the Rb drift stable enought to compensate and obtain viable results?

.marek

On 11/28/21 1:06 PM, Marek Doršic wrote:

Sadly, no...  That's what the Allan Deviation tells you.  It says
"here's the best you can do, at this averaging time".  And a lot of
sources may have a low flat spot in the curve, but it eventually trends
up. Except for primary standards like Cs beam.

So if a Rb plateaus at, say, 1E-12, that's the best you're going to do.

Speaking of which, does anyone have a link to a "current state of the
art" graph.

Kind of like this one from Vig, but with specific new technologies like
CSAC etc