ABSTRACT Frequency aging in the rubidium (Rb) vapor-cell atomic clock plays a significant role in the device's timekeeping ability. Though many researchers have speculated on the physical mechanism(s) driving the linear, deterministic frequency change (i.e., Δ f(t)/fo = At), there is little unambiguous experimental data regarding the phenomenon. Here, long-term data were used from on-orbit global positioning system (GPS) Rb clocks to examine one postulated mechanism for frequency aging (i.e., the light-shift effect). Defining the light shift of the clock's fractional frequency as α I/Io, where α is the light-shift coefficient, we find that temporal variations of the relative light intensity, I/Io, cannot account for frequency aging. However, for the population of clocks considered here, we obtain the intriguing result that α/A = 1.7± 1.5. Thus, it may be that frequency aging is driven by the light-shift effect through temporal variations of the light-shift coefficient.
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