On-orbit performance of Milstar rubidium and quartz frequency standards

Atomic frequency standards play a critical role in many space systems. The most notable examples are the satellite navigation systems (GPS and GLONOSS). The excellent navigational accuracy of these systems is largely due to the outstanding performance of the on-board cesium and rubidium atomic frequency standards. Future navigation systems will have atomic standards with even improved performance than the current ones. A new generation of military communication satellites-(Milstar) carry on-board multiple redundant rubidium atomic frequency standards which have been designed to meet stringent performance and operational requirements. We have over 3 years of on-orbit performance data of a Crystal Master Oscillator (XMO) on-board the 1" Milstar satellite and over 18 months of on-orbit performance data of a Rubidium Master Oscillator (RMO) on-board the 2^nd Milstar satellite. 2 RMOs were also placed on long-term life tests in a simulated, space-like environment. In this paper we present data on the long-term on-orbit performance of an XMO and an RMO on-board the Milstar satellites. In addition to the frequency stability we also present long-term data of 2 parameters which critically influence the RMO performance, light intensity from the Rb resonance lamp and the RMO temperature. These along with many others are telemetered to ground. We also compare the on-orbit data with the ground life test data. Our goal is to get some insight into the physics of longterm aging of the rubidium atomic frequency standards