Ultra-stable oscillators for planetary entry probes

Ultra-stable oscillators on-board planetary missions were developed for Radio Science instrumentation, functioning as frequency references for the one-way downlink during atmospheric occultations. They have also been flown on planetary entry probes including the Jupiter entry probe, carried by Galileo, and the Huygens Titan entry probe, carried by Cassini, for performing Doppler Wind Experiments. The Jupiter and Titan probes utilized different oscillators, quartz and rubidium, respectively. This paper presents the development of ultrastable oscillators on deep space missions and discusses the tradeoffs encountered when selecting oscillators for planetary entry probes, including factors such as duration of the experiment, the available warm-up time and the Allan deviation and phase noise requirements.