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.
Light-shift coefficient in GPS rubidium clocks: Estimation methods using lamplight/frequency correlations
The frequency of the Rubidium Atomic Frequency Standard (RAFS) used in Global Navigation Satellite Systems (GNSS) is affected by the light-shift effect, due to the optical-pumping light from the RAFS' rf-discharge lamp. As a consequence, lamplight intensity variations can induce RAFS output frequency variations, with lamplight stability setting a lower bound to RAFS frequency stability (i.e., to the eventual navigation performance of the GNSS). We study this effect by estimating the light-shift coefficient of an on-orbit RAFS and its possible variation in time using two different methods: the first makes use of large observed frequency jumps that are induced by lamplight jumps; the second uses the linear correlation between deterministic frequency and lamplight variations. We validate the methodology using GPS Block IIR RAFS data and present some preliminary results.
Anomalous Annealing Behavior of Arsenic Implanted Silicon as a Function of Dose and Energy
Anomalous annealing behavior of damage in As implanted Si has been investigated by channeling effect measurements. Room temperature As implants with doses from 1014 to 2 × 1016cm-2 at energies ranging from 50 to 250 keV into Si were annealed over the range of 600°C to 1100°C. Anneal temperatures ≥ 950°C reduce the disorder for ion doses of 0.5 to 1 × 1016 cm-2 at energies between 50 and 250 keV. Anomalously high residual disorder was found for the intermediate doses of 2 × 1014 to 2 × 10/cm2 implanted above 100 keV. For this intermediate dose, annealing of implants performed at liquid nitrogen temperatures did not show significant reduction in this damage. However implants at 200° to 300°C indicate a lower amount of damage compared to the room temperature implants.