[time-nuts] The forbidden question

jimlux jimlux at earthlink.net
Wed Jun 5 04:05:37 UTC 2019


On 6/4/19 3:05 PM, Hal Murray wrote:
> 
>> Are we developing these incredible devices just to push boundaries? Or do
>> they have some practical purpose?
> 
> I don't know of any current projects that need a significantly better clock,
> but that's the sort of thing that wouldn't get a lot of publicity - just some
> hand-waving discussions.  "What if we could ..."
> 
> 
> If you want some ideas to BS about...
> 
> You might ask the radio astronomers what they would do with a better clock.

navigation of spacecraft and making better gravity measurements of 
planets and moons.


One of the "pushing the frontiers of clocks" things is that stuff that 
starts in the lab eventually winds up portable. Sure, there's no cesium 
fountain clocks in space (that I'm aware of), but we are doing trapped 
mercury ion clocks (Deep Space Atomic Clock) and chip scale atomic 
clocks (CSAC), both of which fundamentally enable new stuff.

Navigation of spacecraft today is done by precisely measuring the round 
trip time of a signal to the spacecraft and back.  This is great if you 
want to know the position and velocity of the spacecraft *on the ground*.

However, what if the spacecraft needs to know where it is, by itself - 
so it can autonomously navigate. For that, what you need is a good clock 
on the ground, to transmit precisely timed signals (with appropriate 
additional information), and the spacecraft receives them, with a very 
good clock, and figures out where it is.

You might receive these signals at different times, so you need a nice 
stable clock on board to be able to compare time signals received at 
time T1 to signals received at time T2, some hours or days later.

One Way Nav is a *big deal* for enabling spacecraft autonomy, 
particularly with multiple spacecraft constellations.






> 
> What would the radio guys do with better clocks?  I'll bet you can dig deeper
> into the noise when trying to find a spread spectrum signal.
> 
> It would be interesting to look at the history of similar advances.  Cesium
> clocks are now in widespread use.  Rubidium clocks are a spinoff.  Were any of
> the common uses anticipated back when people were struggling to build the
> initial atomic clocks?  How much of the engineering associated with atomic
> clocks was uncovered by the early researchers?
> 


A lot...





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