[time-nuts] GPS discipline oscillator vs phase lock

[Hal Murray]

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
>> things)

> 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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