GPS block IIR clocks in space: current performance and plans for the future

ITT Industries developed GPS IIR satellite payloads have been on-orbit since July of 1997, providing outstanding signal-in-space performance. Much of the credit for this outstanding performance can be given to the GPS IIR time keeping system (TKS). A key component of the TKS system is Perkin Elmer's rubidium atomic frequency standard (RAFS). We now have over 40 years of on-orbit experience with the GPS IIR TKS and RAFS. In this paper, we present the characteristics of the twelve on-orbit operating frequency standards, eight of which are in family and are the best performers in the GPS constellation, although four of which exhibit detectable features. This paper provides insight into the unique characteristics of each on-orbit operational RAFS