[time-nuts] Orbiting crystals

Lux, James P james.p.lux at jpl.nasa.gov
Sun Jun 28 11:33:31 EDT 2009




On 6/28/09 2:59 AM, "iovane at inwind.it" <iovane at inwind.it> wrote:

> Thanks all.
> 
> Let's explain why I would like to look at orbiting crystals.
> 
> Making experiments during eclipses would almost always mean
> that a measuring instrument should be put in the eclipse path.
> This is not always possible, or at least it is quite
> expensive. So, I thought to take advantage of remote crystals
> that could transmit data about themselves while crossing
> eclipses.

Ah.. That's easy.. Pretty much every satellite in orbit derives its transmit
frequency from a crystal oscillator, either a TCXO or a OCXO.  So all you
have to do is pick a LEO satellite that is easy to receive and measure the
transmitted frequency, and that is at a frequency where other effects won't
dominate (ionospheric uncertainties, for instance, probably rule out VHF and
UHF downlinks).   

For instance, QuikScat (which I helped build, so I'm real familiar with it)
has a radar that radiates about 100W through a 40dBi antenna at 13.402 GHz,
derived from an approximately 28 MHz crystal.  It's a huge signal on the
ground, detectable with an open waveguide pointed roughly in the right
direction into a LNA and spectrum analyzer.  It's probably not what you want
(it's chirped and pulsed), but I'm sure there's other satellites around that
would work. In my work (and that of some folks at BYU) we looked at the
received QuikScat signals to try and characterize things like the oscillator
performance, and a back of the envelope analysis showed that you can just
barely detect relativistic effects, after removing all the other known
effects (the chirping, the doppler, etc.).  Look for papers at the 2000
IGARSS conference.


I'd look for something like Iridium or Globalstar and see if there's a
convenient pilot signal that's easily detectable.





More information about the time-nuts mailing list