Does the light shift drive frequency aging in the rubidium atomic clock?

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., /spl Delta/f(t)/f/sub o/ = 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 /spl alpha/I/I/sub o/, where /spl alpha/ is the light-shift coefficient, we find that temporal variations of the relative light intensity, I/I/sub o/ cannot account for frequency aging. However, for the population of clocks considered here, we obtain the intriguing result that /spl lambda//A = 1.7 /spl plusmn/ 1.5. Thus, it may be that frequency aging is driven by the light-shift effect through temporal variations of the light-shift coefficient.