[time-nuts] Overheard from NASA

[Jim Palfreyman]

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