The life test of two Perkin-Elmer rubidium atomic frequency standards (RAFS) at the US Naval Research Laboratory (NRL) in a simulated space environment and flight configuration began 31 March 1997. These clocks were production clocks from the Global Positioning System (GPS) Block IIR build and especially provided for this test by the GPS Joint Program Office and the Block IIR satellite contractor team led by Lockheed Martin. This continuously running test was designed to provide information on long term performance, operation, and potential unique characteristics of these clocks. This included potential failure mechanisms, and day-to-day characteristics of the clocks and their internal monitors. GPS Block IIR was the first on-orbit use of this design and there was initially significant concern over their lack of actual space experience. This paper summarizes the test results to date and compares those results with the observed on-orbit performance of the Perkin-Elmer rubidium clocks in the GPS constellation. The utility of life testing in supporting operation and new space clock introduction into the system is highlighted.

The neural network control of a quartz oscillator has been demonstrated. We have shown that a single neural network can correct an oscillator's output frequency while several environmental sources of frequency shift act on the oscillator. The advantage that a neural network offers over microprocessor controlled oscillators or Kalman filter control techniques is that a neural network does not need an a priori model of the physical system to arrive at the correct control algorithm. The results of this demonstration indicate that a neural network can have beneficial applications in a variety of frequency standard devices. We believe that the neural controller will work best as a control system supervisor, rather than as the main controller of the frequency standard system. Specifically, the neural network would learn the non-linear effects associated with the several servos that control the main physical parameters of a frequency standard (temperature, magnetic field, etc.), and correct the oscillator's frequency based on the state of these servos and any additional sensed environmental parameters. The dominant remaining technical issue is the training of the neural network. During this demonstration program, maintaining a stable, reproducible environment that could be varied quickly and randomly over the whole training parameter space proved to be a significant technological challenge. Future efforts will focus on methods to more efficiently train the neural networks and the identification of specific devices

The proposed design of a new 87Rb maser frequency standard is presented. This device is expected to perform similarly to a hydrogen maser at short and moderate averaging times. An operating maser of this design would much be more compact than a full size hydrogen maser (the cavity need only be ~6 cm in length and diameter). It should also be much less expensive to build. Optical pumping techniques are used to produce a nearly complete population inversion in an evacuated wall coated cell. The population inversion is produced in a cell separated from the microwave interaction region to eliminate problems with light shifts. Since no buffer gas is used the atoms are free to travel through an exchange tube from the optical pumping region to the maser interaction region where oscillation can take place. The system will be completely closed-cycle since atoms will return from the maser region to the optical pumping region where they will be repumped. This eliminates the need for a vacuum system, greatly simplifying operation. The principles of operation and the basis of the stability estimates are discussed

This paper presents some interim results from the environmentul testing program to evaluate the Engi- neering Design Model (EDM) of the EGLG Spuceborne Rubidium Clock. This eflort is in support of the GPS BLOCK IIR program and is intended to characterize the performance of EG&G design Jor BLOCK IIR satellite applications. Iko EG&G EDM units are currerttly under &.st at NRLk Clack Test Facility to measure the long-termSreyuency stability, drqt, und frequency versus temperature characteristics.

A special clock test facility is described to evaluate atomic clocks developed for space and ground application. The facility itself is described and the methodologies involved in long-term performance testing, initial qualification testing of spacecraft clocks, and post-acceptance testing of candidate spaceflight clocks to be evaluated in orbit. The objective of long-term testing is to build a performance and reliability database on newly developed clocks to support eventual operational system acceptance. Initial qualification testing for developing clocks is used to guide development under contract and to support in-house design of components, subsystems and experimental atomic clocks. Typical testing procedures and methodologies are discussed

This paper presents the results of further long-term stability tests on two prototype GPS rubidium frequency standards. These testa, currently underway at the U.S. Naval Research Laboratory, have resulted in the highest stabilities yet reported for such devices. Both units have smooth, highly modelable drift under 2 x 10-14/day and a stability of about 1 x 10-l4 at lo5 to lo6 seconds.

: The NRL TIMATION III satellite, redesignated as GPS Navigation Technology Satellite I (NTS-1) was successfully placed into a medium altitude orbit, 7,300 mi, on July 14, 1974. One of the major experiments performed with the satellite is the investigation of the space-environment performance of two rubidium controlled frequency standards and a specially developed quartz oscillator. System design, ground testing, and flight qualification modifications were performed at NRL. This included modifying commercial-quality rubidium standards to levels acceptable to the flight environment; the design and construction of power control, RF switching, and remote digital tuning circuits; and ground testing the candidate frequency standards in terms of short-term stability (Allen Variance), aging, warmup, tuning characteristics, DC power consumption and environmental effects due to vacuum, radiation, vibration, and temperature.