Long-Term Clock Behavior of GPS IIR Satellites

Abstract : The ITT-developed GPS IIR satellite payloads have been on orbit since 1997, and have proven to be the best family of clocks in the GPS constellation. At this time, there is a substantial recorded history of clock behavior, including over 60 clock years of space operation. The age of the oldest clock is over 9 years. A review of the record shows a number of significant characteristics that were not apparent in shorter clock tests. Rubidium clocks, as opposed to cesium clocks, have significant long-term drift. The current literature describes an initial model of drift aging for rubidium atomic clocks followed by a long-term characteristic. A review of the IIR clocks shows what appears to be another significant break point in the long-term drift characteristics. The usual assumption is that the drift tends toward zero drift in the long term. The data indicate that the long-term drift will always remain negative and stay substantially away from zero. It is commonly known that some rubidium clocks generate frequency steps and the frequency steps tend to decrease in size and rate of occurrence over time. We have seen a number of cases of this behavior. Most frequency steps tend to be frequency steps that tend to persist in time, but we also have seen triangular frequency patterns where a sudden jump in frequency is followed by a rapid decay to the long-term drift pattern. Also, we have cases in which a frequency step pattern that has persisted for a significant time suddenly disappears. Although there are cases where the frequency step intensity initially grows after turn-on, the overall pattern is for the frequency step intensity to decay over the long term. Future observations of these clocks will allow us to determine if this decay in frequency step intensity is permanent.