[time-nuts] GPS discipline oscillator vs phase lock
hmurray at megapathdsl.net
Mon Jun 19 00:29:15 EDT 2017
jimlux at earthlink.net said:
>> 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
> 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.
A "spectrally pure signal" gets you a frequency offset for velocity, but it
doesn't get any timing info. How do you get range?
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