Rick Karlquist wrote:
I would like to point out that the environmental sensitivities of the 5071A are unmeasureable, and the measurement threshold is far below 5.8E-14. I would estimate that the 5071A (and ONLY the 5071A among commercial clocks) could get the job done provided that you could compare its frequency to GPS to the stated accuracy. This would be using the 5071 as a secondary standard. You still need to deal with the short term stability of the 5071A, depending on your system needs. JPL uses H masers as flywheels.
We also use the maser as a very low phase noise signal in the 10-1000 second tau range.. We multiply it up and send the (very clean) signal out to the spacecraft, it gets tracked by a loop with a few Hz BW, then sent back to earth where it's compared to the maser again to measure Doppler.
Basically we measure doppler and doppler rate over a few minutes, assuming that the transmitted signal is constant during that time (which the maser is, for all practical purposes). Over a longer time span (e.g. the time between transmit and receive, which could be many hours, implying that we are transmitting from one station and receiving from another) I assume we use atomic standards and/or astronomical sources (pulsars).
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