[time-nuts] time-nuts Digest, Vol 101, Issue 152

[Jim Lux]

On 12/30/12 8:28 PM, Bob Camp wrote:
> Hi
>
> Indeed, there will always be some EMF into the EFC from some field. You can never really get rid of a loop with some cross section in the EFC circuit. Most of us don't get to worry about 1x10^-16 at 1,000 seconds on our OCXO's….
>

Yes. I think it's not so much the OCXO as the performance of the overall 
system it's in where magnetic fields would be the issue.

I'll have to go look up what the USOs we use have for magnetic field 
performance..  For Cassini, Sami Asmar's 1997 paper says 5E-13/Gauss, 
and the rest of the missions were in the 1E-12/Gauss range..
http://tmo.jpl.nasa.gov/progress_report/42-129/129F.pdf

that USO was built by APL.. And APL's Greg Weaver and colleagues had a 
paper at PTTI in 2004
http://www.pttimeeting.org/archivemeetings/2004papers/paper35.pdf
that describes some of the things (although not magnetic fields).

For superduper accuracy applications like deep space radio science, I 
think the issue today might be more about things like hysteresis and one 
way shifts (aging, radiation, etc.).  In theory, one could calibrate out 
the effect of the rotating magnetic field.  We already have to calibrate 
out the effect of the apparent phase center of the spacecraft antenna 
moving as the spacecraft rotates because the phase center isn't 
perfectly on the axis of rotation, nor is the spacecraft perfectly 
pointed towards earth (at least not at 1E-12 kinds of precision).  The 
magnetic field effect would presumably be another sinusoidal variation 
with the others.

Jensen and Weaver had a paper at PTTI in 2007 talking about the USOs on 
New Horizons, accounting for all the observed changes in frequency. 
Since New Horizons went by Jupiter on its way to Pluto, it was affected 
by the radiation there. I didn't see any mention of magnetic fields, and 
since NH is a spinner, it would get the same rotating magnetic field 
thing we worried about for Juno.

GRACE (which just finished its mission by crashing into the Moon's 
surface) carried a USO on each of the spacecraft.  I wonder if they have 
any data on magnetic fields. I remember it coming up in the design 
reviews, but don't recall if it was considered a big issue, or just one 
of those "this is what we always do, and it seems to work well enough".

APL, of course, is striving for ever better quartz clocks for this kind 
of application.  At JPL, we're looking at flying a trapped Hg ion clock 
(aka DSAC- Deep Space Atomic Clock) as a demo in the next few years to 
give us several orders of magnitude improvement over quartz.

Both are important when you do "one way ranging".. For the more common 
two way ranging (where we send a signal up to the spacecraft and it 
sends it back) the quartz oscillator isn't as important as the 
"turnaround" performance, where the VCXO control is what's important.



http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA485506