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 Frequency Standard for the GPS IIF program and modifications for the RAFSMOD Program
PerkinElmer provides the rubidium frequency standards (RFS) for the Boeing Block IIF GPS navigation satellites. The PerkinElmer RFS-IIF is the latest generation of its high performance rubidium clocks which have performed very successfully on board the block IIR GPS space vehicles for more than 10 years.The RFS-IIF output is the traditional 10.23 MHz rather than the =13.4 MHz "natural frequency" utilized for block IIR. This necessitated the addition of a secondary phase lock loop (PLL) synthesizer to convert the Rb natural frequency to 10.23 MHz. One main design challenge was to add this new section with a minimum impact on size, weight, power, performance, radiation hardness, reliability and design heritage. The second major change in the RFS-IIF was to make a significant improvement in the medium to long term stability by using Xenon lamp buffer gas and a thin-film spectral filter in the physics package to reduce shot noise and improve the signal-to-noise ratio (S/N). This has lowered the white FM noise level to below 1x10-12tau1/2. This paper describes the RFS-IIF design, the changes and improvements made, and the test results obtained. It also discusses the recent redesign activities to eliminate obsolete parts in the RFS-IIF as well as improvements and updates to the PerkinElmer test facility funded by the USAF GPS Wing (GPSW) under the Rubidium Atomic Frequency Standard Modification (RAFSMOD) Program.
Improvement of RHIC warm beam vacuum for high intensity operation
With increasing ion beam intensity during recent RHIC operations, pressure rises of several decades were observed at a few warm vacuum sections. Improvement of the warm sections has been carried out in last year's shutdown. Extensive in-situ bakes, additional UHV pumping, electron detectors and beam tube solenoids have been implemented. Vacuum monitoring and interlock were enhanced to reduce premature beam aborts. The effectiveness of these measures in reducing the beam induced pressure bumps and in increasing the vacuum system reliability are discussed and summarized.