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.
Management of Phase and Frequency for GPS IIR Satellites
Abstract : The ITT-Industries- developed GPS IIR satellite payloads have been on orbit since 1997, providing outstanding signal-in-space performance. The GPS IIR satellites have the capability of adjusting the phase, frequency, and frequency drift of their broadcast clocks. This paper discusses the intricacies of phase and frequency management in the GPS IIR Time Keeping System. During the initial years of the GPS IIR operation, the only clock correction technique used was the traditional technique of phase adiustment. In year 2000, the GPS IIR clock management operation started to use the full GPS IIR capability of adjusting phase, frequency, and frequency drift. Two approaches were employed. The first approach was applied to GPS satellites SVN43 and SVN46, where the phase, frequency, and frequency drift errors were zeroed during off-line maintenance. The phase and frequency of the other IIR satellites were managed by the second approach using on-line frequency drift adjustments to correct for phase, frequency, and frequency drift errors; and does not require the satellite to be taken out of service. The operation of the two phase and frequency management approaches is compared in this paper. We have the results of over a year of operation of the new phase and frequency management techniques. As a result of these management approaches, the phase and frequency errors due to the GPS IIR rubidium frequency drift are now much smaller, with minimal load on the operation of the GPS system. This improvement is described in this paper by plots of the clock performance before and after the use of the phase and frequency management techniques. The cost of the improvement is minimal as illustrated by a table of the ground control operations used to obtain these results.