Performance of rubidium and quartz clocks in space

Space programs requiring precision time-keeping and stable frequency generation have been fitted with atomic frequency standards and super-stable quartz oscillators. However, the challenge has been to predict the clock performance in space based on tests conducted on Earth. This paper focuses on the actual performance of rubidium atomic frequency standards and quartz oscillators in space, and demonstrates that the performance in space is predictable from modeling and tests carried out on Earth. Actual test data obtained on Earth as well as data from space are presented. Aging performance and the effects of natural radiation are addressed. Data are presented from a space-based rubidium clock, designed and manufactured by Frequency Electronics, Inc., that is achieving fractional-frequency aging rates of 3 × 10^-14/day and long-term Allan deviation of 1 × 10^-15√t, and similarly, quartz clocks that are realizing aging rates of 1-2 × 10^-12/day and long-term Allan deviation of 1.6 × 10^-14√t. Furthermore, the paper also addresses performance in the presence of solar flares and other space phenomena.