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The various flavors of GPS satellite constellations use atomic clocks on each satellite. As far as I know, all of the constellations are "shells" of circular MEO orbits in 3 to 6 planes, 20,000 to 25,000 km above Earth's surface, with a few in GEO (Compass). There may be other satellites in GEO that have atomic clocks as well.

How far from earth have atomic clocks been placed and monitored? Could be for any purpose. I'm looking for the farthest distance at which the clock was working and data obtained in some way.

I was thinking about timebases for deep space probes here, but @TildalWave's comment linking to a Von Kármán lecture lead me to read about the Deep Space Atomic Clock project.

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There was Rb-based Ultra Stable Oscillator in outer space, delivered to Titan (VI moon of Saturn) inside the Huygens, brought there by Cassini. (DWE RUSO in http://nssdc.gsfc.nasa.gov/image/spacecraft/huygens_cutaway.jpg). Schemes and paper - RUBIDIUM ULTRA-STABLE OSCILLATORS AT TITAN: THE HUYGENS DOPPLER WIND EXPERIMENT, 1997. THE HUYGENS DOPPLER WIND EXPERIMENT. Titan Winds Derived from Probe Radio Frequency Measurements, 2002

There were some stable non-atomic clocks on several other probes. The Cassini itself was listed to have on-board Ultra Stable Oscillator (quartz-based, for navigation with Doppler effect), similar were used on some Mars missions: https://solarsystem.nasa.gov/docs/Asmar.pdf "ULTRA-STABLE OSCILLATORS FOR PLANETARY ENTRY PROBES", S.W. Asmar et al. NASA JPL (possibly from 2004)

Oscillators on NASA Deep Space Missions

The Voyager 2 USO is still working and the Voyager 1 USO ceased operations nearly 20 years after its procurement, exceeding specifications. ... USO serial #4 was later flown on the Galileo spacecraft. ... several deep space missions have carried USOs for Radio Science experiments (e.g. Mars Global Surveyor, Cassini). These oscillators were single-oven, SC-cut crystals with a typical Allan deviation of 1x10-13 at τ = 100 s. More recently, a flight-qualified USO is being prepared for proximity communications and navigation on the upcoming Mars Reconnaissance Orbiter mission

  1. The Huygens Ultra-Stable Oscillator The ESA Huygens probe to Titan, carried by the Cassini spacecraft, has a Doppler Wind Experiment (DWE), a Radio Science experiment with requirements that were met with a rubidium (Rb) atomic standard 5.

Check also table from the paper - "Table 1: Representative data of parameters characterizing ultra-stable oscillators on deep space missions collected from various sources, in some cases published specifications and, in some cases, in flight data."

I found presentations about the topic and future experiments:

The best overview about USOs is again from Sami Asmar, from 2012:

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    $\begingroup$ This is an incredible answer - thank you for putting together such a broad selection of material. I didn't realise that the use of ultra-stable oscillators on spacecraft was so widespread. The presentations are fascinating. There is much to learn here about the use of precision time measurements from space! $\endgroup$
    – uhoh
    Apr 19, 2016 at 4:13

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