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 <[email protected]> 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. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
