Characteristic Trends of Ultrastable Oscillators for Radio Science Experiments

Telecommunication systems of spacecraft on deep-space missions also function as instruments for radio science experiments. Several missions augmented the radio communication system with an ultrastable oscillator (USO) in order to provide a highly stable reference signal for one-way downlink. Since the first quartz USO was flown on Voyager, the technology has advanced significantly, affording future missions higher sensitivity in reconstructing the temperature–pressure profiles of the atmospheres under study as well as the ability to study other physical phenomena of interest to radio science. The ultrastable class of oscillators has been flown on Voyagers I and II, the Galileo Orbiter, the Galileo Probe, Mars Observer, and Mars Global Surveyor. These have been quartz crystal resonators. The Cassini spacecraft will carry another quartz USO and two rubidium USOs for the Huygens Probe in support of the Doppler Wind Experiment. There are plans to fly USOs on several other future missions. This article surveys the trends in stability and spectral purity performance; design characteristics, including size and mass; and the history of these clocks in space.