Performance data on the Milstar rubidium and quartz frequency standards: comparison of ground tests in a simulated space environment to results obtained on orbit

Space programs increasingly use atomic frequency standards for precision timekeeping and stable frequency generation. The excellent navigational performance of GPS satellites is largely due to the on-board atomic frequency standards. A new generation of military communication satellites (Milstar) carries multiple redundant rubidium atomic frequency standards in order to meet various communication and operational requirements. The first Milstar satellite, DFS-1, has on-board multiple redundant quartz crystal oscillators. The second Milstar satellite, DFS-2, was launched late in 1995, and is the first of the Milstar satellite constellation to have on-board rubidium atomic frequency standards. All subsequent satellites are to be launched with Rb standards. The Milstar program embarked upon an extensive ground testing program that included: qualification testing, life testing (two units in a simulated space environment for over 2 years of accumulated operating time), acceptance testing of each flight unit, and a 100 day Flight Integrity Test of each flight unit. To date, this has yielded some of the most extensive long term data of rubidium atomic frequency standards in simulated space environment. In this paper we describe the life test and the flight integrity test portions of the Milstar ground test program. An analysis of the data from these tests is presented In addition, these results are compared to the available on-orbit data from DFS-1 and DFS-2.