Characterization & consequences of GPS Rb clock lamplight variations
In the rubidium atomic frequency standard (RAFS), an rf-discharge lamp produces the device's atomic signal. As a consequence of the light-shift effect, variations in the lamplight's intensity result in variations in the RAFS' output frequency. While the basic physics of the light-shift is reasonably well understood, its operational implications for GNSS performance are only beginning to be fully appreciated. Here, we summarize results examining decade-long histories of on-orbit GPS RAFS lamplight variations. In addition to a deterministic variation that is well modeled by the sum of an exponential and linear trend, the stochastic variations appear to be of three types: lamplight jumps, repeating lamplight ramps, and non-stationary lamplight noise. Employing an estimated GPS RAFS light-shift coefficient, these variations can be used to infer lamplight-induced RAFS frequency variations.
Rubidium clock lamplight variations and long-term frequency instability: First analyses of multiyear GPS data
In the rubidium atomic frequency standard (RAFS), an rf-discharge lamp produces the device's atomic signal. As a consequence of the light-shift effect, variations in the lamplight's intensity result in variations in the RAFS' output frequency. While the basic physics of the light-shift is reasonably well understood, its operational implications for global navigation satellite system (GNSS) performance is only beginning to be fully appreciated. Here, we describe first results examining decade-long histories of on-orbit GPS RAFS lamplight variations and GPS RAFS frequency variations. Our preliminary analyses have focused on one space vehicle's RAFS, and our conclusions are tempered by that present limitation. Nevertheless, our analyses suggest that a RAFS' long-term frequency stability (i.e., τ 106 sec) is likely lower-bounded by the lamp's intensity fluctuations. Moreover, considering the light-shift coefficient for this one particular RAFS over 12 years, we find that the data do not support Camparo's hypothesis regarding RAFS frequency aging and a time-varying light-shift coefficient.
Analysis of the effect of foreign gases in the filtering action of a 85Rb cell
The variation in the 87Rb D1 (λ = 794.8 nm) pumping light profiles due to 85Rb filter cell temperatures (< 80 °C) has been explained by considering both the shift and broadening of the 85Rb absorption spectrum caused by low-pressure foreign gases (< 200 Torr). The coefficients of the pressure shift and broadening by N2 gas were obtained as −8.0 ± 0.5 and 11 ± 1 MHz/Torr, respectively. The intensity ratio and difference of two peaks in the profiles were calculated in order to discuss the variation in the profiles quantitatively. The foreign gas He was examined for comparison. The absorption coefficient of Rb was also discussed.