[time-nuts] Overheard from NASA

William H. Fite omniryx at gmail.com
Tue May 10 01:37:35 UTC 2011


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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