Rubidium Frequency Standard for the GPS IIF program and modifications for the RAFSMOD Program

PerkinElmer provides the rubidium frequency standards (RFS) for the Boeing Block IIF GPS navigation satellites. The PerkinElmer RFS-IIF is the latest generation of its high performance rubidium clocks which have performed very successfully on board the block IIR GPS space vehicles for more than 10 years.The RFS-IIF output is the traditional 10.23 MHz rather than the =13.4 MHz "natural frequency" utilized for block IIR. This necessitated the addition of a secondary phase lock loop (PLL) synthesizer to convert the Rb natural frequency to 10.23 MHz. One main design challenge was to add this new section with a minimum impact on size, weight, power, performance, radiation hardness, reliability and design heritage. The second major change in the RFS-IIF was to make a significant improvement in the medium to long term stability by using Xenon lamp buffer gas and a thin-film spectral filter in the physics package to reduce shot noise and improve the signal-to-noise ratio (S/N). This has lowered the white FM noise level to below 1x10-12tau1/2. This paper describes the RFS-IIF design, the changes and improvements made, and the test results obtained. It also discusses the recent redesign activities to eliminate obsolete parts in the RFS-IIF as well as improvements and updates to the PerkinElmer test facility funded by the USAF GPS Wing (GPSW) under the Rubidium Atomic Frequency Standard Modification (RAFSMOD) Program.