[time-nuts] GPS discipline oscillator vs phase lock
jimlux at earthlink.net
Sun Jun 18 13:36:00 EDT 2017
On 6/18/17 7:10 AM, Attila Kinali wrote:
> On Sat, 17 Jun 2017 06:29:02 -0700
> jimlux <jimlux at earthlink.net> wrote:
>> Well, at JPL we regularly lock two crystal oscillators together that are
>> over a billion km apart with added Allan deviation of less than 1E-15 at
>> 1000 seconds with a radio link at 7.15 GHz. It's how we measure the
>> distance and velocity to spacecraft (a few cm in range and mm/s in
>> velocity) and from that figure out the gravitational fields (among other
> This sounds interesing. What do I have to google for to learn more?
It's just how we do radio science/ranging - you transmit a spectrally
pure signal from earth (typically oscillator locked to a maser), at the
spacecraft you have a very narrow band PLL (traditionally a VCXO) that
locks to the received signal, and you generate the downlink signal from
that same oscillator, transmit it back to earth, and compare.
The transmitted signal is precisely in a specified ratio with the
received signal (880/749 for X-band 7.15 GHz from earth, 8.4 GHz coming
back). For Ka-band, the earth signal goes up at 34 GHz, and comes back
at 32 GHz
A typical spec is that the transponder introduce no more than 4E-16 ADEV
at 1000 sec.
https://descanso.jpl.nasa.gov/ has links to a whole bunch of useful
specifically volume 1 by Thornton and Border talks all about radiometric
The various design and performance series describe the specific
Joe Yuen's "Deep Space Telecommunications Engineering"
Chapter 3 covers receiver design
Chapter 4 covers radio tracking
Then you can look for papers on "deep space transponder" The classic
design papers are in the 90s. IEEE MTT, and the JPL IPN progress
The Cassini Deep Space Transponder is sort of a progenitor of them -
then there's the Small Deep Space Transponder (SDST) designed in the
90s, flying 2000 through now.
Somewhere around 2000, the design started moving away from trying to
lock the oscillator to doing the phase lock and phase/frequency
turnaround in a digital loop, with a fixed oscillator driving DDS or
NCO. At JPL, this would be the "Advanced Deep Space Transponder", but
Thales Alenia Space Italia (TASI) uses a similar approach for their deep
space transponders (look for Juno and BepiColombo)
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