[time-nuts] Overheard from NASA

Jim Lux jimlux at earthlink.net
Tue May 10 03:45:54 UTC 2011


I wasn't intending to cast aspersions...   I was more giving an example of somewhere that atomic clocks need more work.   And, I'm pleased that this group/list exists..  It's pointed me towards some useful stuff to solve some problems with the KaTS, and, as well, the archives are a great resource to which to point colleagues for help on Allan dev, etc.

FWIW, for flight, the hot ticket is going to be Hg ion, if they can ever get it qualified...the physics package is pretty well there, but the rest is slogging along.

And if someone figures out how to use xray pulsars in a flight qualified way, we'll fall on them with gratitude.

On May 9, 2011, at 18:37, "William H. Fite" <omniryx at gmail.com> wrote:

> Jim, keep in mind that that was not my statement but one made to a small
> group of people, including me, over at the Cape.  The guy is a PhD (I know,
> I know, I am too, and what does it get me?) senior research scientist at
> NASA whose specialty is metrology.  Now, you may be convinced that he is a
> complete idiot but I work with NASA quite often and I can assure you that
> they don't hire idiots as senior research scientists.
> 
> I'm a statistician and in no way qualified even to have an opinion on this
> topic.  Just thought it might interest the group.
> 
> Bill
> 
> 
> 
> On Mon, May 9, 2011 at 8:21 PM, Jim Lux <jimlux at earthlink.net> wrote:
> 
>> On 5/9/11 8:25 AM, William H. Fite wrote:
>> 
>>> Overheard from a senior NASA research metrologist:
>>> 
>>> "The only reason we're doing it is because we *can* (improving clock
>>> accuracy, said in the context of the aluminum clock).  We can already time
>>> so accurately, just as an example, that if we launched a spacecraft today
>>> toward Sirius we could predict its location when the craft arrived many
>>> thousands of years from now, to within a thousand miles or so."
>>> 
>>> That's not a precise quote but it is a close paraphrase.
>>> 
>>> Heck, I thought that was why time nuts did it, anyway.
>>> 
>>> 
>> 
>> When it comes to good clocks on spacecraft, we're a long way away from
>> "better than we need", particularly for small power/mass/volume.
>> 
>> Having a atomic clock on board would let you do things like one-way
>> ranging, particularly techniques such as delta DOR, which can give you
>> "cross range" measurements (i.e. azimuth).
>> 
>> Knowing the position to 1000s of km may not be particularly useful, even at
>> long distances, but as a practical matter, we want to know distances to cm
>> or mm at Jupiter or Saturn distances.
>> 
>> Given that Jupiter is about 600-800E9 meters away (call it a round 1E12
>> meters), that's a precision of 1 part in, say, 1E14.
>> 
>> We use precise measurements of range rate (on the order of mm/s) to
>> determine the gravity field, and from that the internal structure of a
>> planet.  The Juno spacecraft has a coherent transponder that contributes
>> Allan deviation of around 1E-15 or 1E-16 over 1000 seconds, with the rest of
>> the measurement system (transmitter on earth, receiver on earth, propagation
>> uncertainty at 32/34 GHz) contributing roughly comparable amounts.
>> 
>> The transponder (KaTS) receives a signal at 34 GHz from earth at a fairly
>> low SNR and generates a carrier at 32 GHz with a fixed ratio of
>> phase/frequency to transmit back.  The SNR is limited by the power we can
>> transmit on Earth (tens of kW, with BIG antenna gain) and the size of the
>> antenna on Juno.
>> 
>> IF we had a "good" clock on board, we wouldn't need to worry about the
>> "transmitter on earth" and "one way propagation uncertainty" for the
>> outbound path.
>> 
>> A USO (quartz oscillator in a temperature controlled dewar) isn't in this
>> class of performance (and is big and power hungry to boot).
>> 
>> 
>> If you had a good onboard oscillator, you can do VLBI type measurements to
>> measure not only range, but angle to a higher precision than is currently
>> possible.
>> 
>> 
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