Radio Propagation Laboratory at the National Bureau of Standards (NBS), now the National Institute of Standards and Technology (NIST). The Radio Propagation Laboratory developed the world’s first atomic clock in 1948. This clock was based on the measurement of a spectroscopic absorption line in ammonia. Because its stability was no better than that of high-quality quartz oscillators, the ammonia system was quickly abandoned for the greater potential accuracy of the cesium atomic beam device. At the heart of this device, brought into operation at the NBS in 1951, was a microwave cavity design developed in 1948 by Norman Ramsey of Harvard University, with funding from the ONR nuclear physics branch. This cavity design and the interrogation method developed with it have proven so essential to high-accuracy atomic clocks that they remain part of all advanced clocks today. (Ramsey received the Nobel Prize for this work in 1989.)

The National Physical Laboratory of Britain (NPL) had developed this cesium beam standard into an operable clock by 1955. NPL then teamed with William Markowitz of the USNO to measure the frequency of the cesium transition relative to Ephemeris Time. The result of this measurement now defines the fundamental unit of time, the second. In a remarkable effort led by Jerrold Zacharias of the Massachusetts Institute of Technology and partially funded by ONR, in 1955 the technology of the cesium atomic beam clock was transformed into a commercial product, the Atomichron, at the National Company. The NRL took delivery of the first unit produced.

In the late 1950s the rubidium cell clock was developed at NBS. It was tested in collaboration with NRL, using NRL’s Atomichron and a classified NRL microwave synthesizer that NBS researchers were not allowed to examine. This rubidium clock technology is the workhorse of our space-based clocks today. In 1960, with ONR funding, Ramsey developed the hydrogen maser. Subsequently, with funding and technical support from NRL, the hydrogen maser clock was brought into semicommercial production. Forty years later, this clock technology still produces the best short- to medium-term clock stability commercially available. These three clock types—the cesium atomic beam, the rubidium gas cell, and the hydrogen maser—make up the totality of our commercially available atomic clock technology.

Navy involvement has also been vital to the development of the most advanced laboratory atomic standards and the technology that will produce our next generation of high-performance atomic clocks. NRL funded the development of the buffer-gas-cooled mercury ion frequency standard at Hewlett-Packard. Several units were produced and delivered to the USNO. ONR funded basic research in the laser cooling of ions and atoms, which has led to our most accurate laboratory standards today—the cesium atomic fountain and the emerging optical clocks. (This work also led to the 1997 Nobel Prize in physics for Bill Phillips of NIST.) ONR-funded work in Bose-Einstein condensates (for which the 2001 Nobel Prize in physics was awarded to Eric Cornell of NIST, Carl Wieman of the University of Colorado, and Wolfgang Ketterle of the Massachusetts Institute of Technology) and the area of quantum entanglement is expected to find application in future, still higher performance atomic clocks.

Space Clocks, Timation, and GPS

Immediately following the launch of the first artificial Earth-orbiting satellite, Sputnik, by the Soviet Union in 1957, the Navy set up the Naval Space Surveillance System (NAVSPASUR) to track satellites, and shortly afterward a group at Johns Hopkins University’s Applied Physics Laboratory (JHU/APL) began to track satellites by Doppler shift. Operated in reverse, this technique allowed simple two-dimensional navigation. The concept led APL to develop the first satellite navigation system, Transit, in the early 1960s, with Navy and Advanced Research Projects Agency (ARPA) funding. In 1964, Roger Easton of the NRL put forward a concept for an improved system that would



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