Why xray pulsars? Millisecond pulsars have shown themselves to be very accurate - wouldn't an ensemble of those be a better choice?
Jim On 10 May 2011 13:45, Jim Lux <jim...@earthlink.net> wrote: > 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" <omni...@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 <jim...@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. > >> > >> > >> _______________________________________________ > >> time-nuts mailing list -- time-nuts@febo.com > >> To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.