Over the past several years, evidence has grown indicating that rubidium (Rb) vapor-cell frequency standards exhibit a long "frequency equilibration" period following activation. Though frequency equilibration can have important implications for diverse timekeeping systems, the mechanism driving the behavior is not well understood and has been the subject of debate. Here, we investigate this phenomenon, as well as frequency aging, for a number of Block IIR GPS Rb clocks. Consistent with previous work, we find that all six Rb clocks in the study showed a frequency equilibration phenomenon with time constants ranging from 98 to 584 days. Further, all clocks displayed a strong correlation between clock frequency and light intensity during equilibration. Significantly, five of the GPS clocks displayed small light-intensity jumps (~ 0.3%) that could be correlated with small jumps in clock fractional frequency (~ 3x10 -13 ); these were used to estimate the various clocks' light-shift coefficients. The broad range of equilibration time constants and the light-shift coefficient determinations cast doubt on the two mechanisms most often cited to explain frequency equilibration: helium permeation through the resonance cell's glass envelope, and the intensity-dependent light-shift effect. Additionally, the light-shift coefficient determinations cast doubt on the intensity-dependent light-shift effect as the driver of frequency aging.
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