[time-nuts] Orbiting crystals

Lux, James P james.p.lux at jpl.nasa.gov
Sun Jun 28 20:01:15 EDT 2009




On 6/28/09 3:23 PM, "iovane at inwind.it" <iovane at inwind.it> wrote:

> James Lux wrote:
> 
>> 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). 
> 
> Hi James,
> 
> that's easy for you at Nasa! You have rotary antennas, good receivers and so
> on...
> I thought to GPS sats because there are cheap receivers all around, and hoped
> the 1 PPS would tell something, so freeing me from considering doppler,
> relativity etc.

There are lots of LEO satellites with signals detectable by an omni antenna.
All you need is a downconverter to something easily digitizable (e.g. That
you can feed into a sound card) and your trusty GPSDO to generate another
signal that you digitize at the same time.

As for (cheap) downconverters... That's a bit trickier. Maybe an eval board
from NatSemi or Maxim


> 
> Anyway, please let me know if you find any easily detectable signals that
> doesn't
> require a tracking antenna.

Find one of those databases of LEO satellites and their frequencies, then
look for a convenient band.


> 
> Bye,
> Antonio I8IOV
> 
> 
> 
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