[time-nuts] Re: Project Great

[Lux, Jim]

On 11/28/21 9:37 AM, Bernd Neubig wrote:
> -----Jim 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.
>
> You are probaly right about the actual situation. However there is the ESA ACES project idling since years without being launched yet:
> "ACES is an ESA ultra-stable clock experiment, a time and frequency mission to be flown on the Columbus module of the ISS (International Space Station), in support of fundamental physics tests. The mission objectives are both scientific and technological and is of great interest to two main scientific communities:
> • The Time and Frequency (T&F) community; which aims to use ACES as a tool for high precision Time and Frequency metrology
> • The Fundamental Physics community; which will benefit from the use of ACES data for accurate tests of general relativity.
>
> See https://earth.esa.int/web/eoportal/satellite-missions/i/iss-aces
>
> The ACES development was initiated in the 1990s. However, the decision to complete the development of the project has been achieved only at the ESA council at Ministerial Level of November 2008.
> The launch was planned in 2018, but,  as said, the clock ensemble (which BTW includes two AXTAL OCXO 100 MHz) is still sitting on the test bench and waiting and waiting.
>
> B


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")