Optically Pumped Cesium-beam Frequency Standard for Gps-i11

We describe an optically pumped cesium-beam frequency standard under development for deployment on the GPS-III satellite constellation. The objective of the project is to demonstrate the feasibility of a space quulifikd optically pumped cesium-beam frequency standard exhibiting a short-term stability of cry (r < 6 x f-ln with lifetime and reliability comparable to that of the current GPS-IIF cesium standards. I n this paper, we report on the architecture and development progress of a prototype instrument that meets these goals with a minimal introduction of unproven technology. I n particular, we discuss the design choices that have been made with particular emphasis on optical pumping and interrogation techniques, laser and optical technologies, and the realization of the prototype frequency standard. Preliminary performance duta are presented. INTRODUCTION To date, cesium-beam frequency standards (CFS) deployed on the GPS satellites have met and exceeded the stability requirements necessary to achieve the timing and position accuracy of the GPS system. The user range error (URE) and timing error objectives for GPS-III have been laid out in the 1999 GPS System Operation Requirements Document (SORD). It has been shown that the technology currently being deployed for the GPS-IIF satellites meets the “Threshold” criterion established by the 1999 SORD, but not its “Objective” goal [l]. It is desirable to improve the stability of the CFS by a factor of two in order to accommodate the immediate and future needs of the GPS program. The cesium frequency standards (CFS) currently being produced for use in GPS employ an atomic beam interrogation technique that has been in use for more than 30 years. Inhomogeneous magnetic fields are used for state selection and analysis and a hot-wire ionizer and electron multiplier are employed for signal detection. This technology is capable of supporting a frequency stability of order o,, = 2~10-”2-”* with an operating lifetime of 10 years. Since the principal noise mechanism in cesium-beam interrogation is the shot noise of the beam, achieving the desired twofold stability improvement would incur an unacceptable fourfold reduction in life for the cesium-beam tube due to cesium exhaustion and related lifelimiting effects. The introduction of optical pumping and detection techniques to the CFS permits more efficient utilization of the atomic beam and, thus, improved stability without reduction of lifetime. The stability of any passively interrogated atomic frequency standard is given by FWHM 1 by = XWWJSIT. vo where FWHM is the Full-Width at Half-Maximum of the resonance line, (S / N)G is the SignaVNoise of the detected signal measured in a 1 Hz bandwidth,