T. G. Akin, Bryan Hemingway, Steven Peil, Precise Time Department, US Naval Observatory; J. D. Whalen, Computational Physics Inc.

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The U.S. Naval Observatory (USNO) is continually working to improve the technology used to generate its master clock. The USNO rubidium fountains, the first cold-atom clocks to report to the BIPM as continuously running clocks, have been in operation for over a decade and have received the maximum weight in TAI continuously for almost all of that time. New laser technology has improved fountain robustness, and recent demonstrations using an optical local oscillator for microwave generation show improved short-term performance below 10?13 at 1s. Optical-clock development for future master-clock enhancement includes a program to build calcium-beam clocks intended to run continuously as well as an effort to develop a strontium optical lattice. The lattice clock will serve as a gold-standard frequency reference, available for intermittent measurement campaigns.