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 opticalpumping 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.