Rubidium Frequency Standard, Model FRK

This compact rubidium frequency standard is the commercial Model FRK, first made by Efratom Elektronik, Munich, Germany, and later by Efratom California in Irvine, Ca. Gerhard Hübner and Ernst Jechart established the firm in 1971 and a year later supplied examples of the clock to the Naval Research Laboratory (NRL), Washington, D.C., for inclusion on NTS-1, the first of the Navigation Technology Satellites (NTS) launched in 1974 to validate the key concepts and hardware for the Global Positioning System (GPS).
Relatively large rubidium frequency standards had been developed in the 1950s, but the FRK—weighing roughly three pounds and measuring about four inches on a side—was the smallest frequency standard of any type available.
Efratom established a branch in Irvine, California, in 1973 and manufactured compact rubidium frequency standards there for a variety of customers. The firm became a division of Ball Aerospace in 1982 and then part of Datum in 1995. Symmetricom acquired Datum in 2002.
Carroll O. Alley et al., “Performance of the New Efratom Optically Pumped Rubidium Frequency Standards and Their Possible Application in Space Relativity Experiments,” Proc. of the 4th Ann. Precise Time and Time Interval (PTTI) Appl. and Planning Meeting, 1972, 29-40.
Brief description of an atomic clock
Electromagnetic waves of very specific and consistent frequencies can induce atoms to fluctuate between two energy states, and by measuring that frequency we can determine the “tick” of an atomic clock. A second in a cesium clock, for example, is defined as 9,192,631,770.0 cycles of the frequency that causes the cesium atom to jump between those states. Different atoms “tick” at different rates – strontium atoms tick about 10,000 times faster than cesium atoms – but all atoms of a given element tick at the same rate, making atomic clocks much more consistent than clocks based on macroscopic objects such as pendulums or quartz crystals.
Steven Jefferts, physicist, National Institute of Standards and Technology.
For additional background information go to:
Object Name
Efratom rubidium-vapor frequency standard
Efratom California, Inc.
Physical Description
steel (overall material)
glass (overall material)
overall: 4 1/8 in x 3 7/8 in x 4 11/16 in; 10.4775 cm x 9.8425 cm x 11.90625 cm
ID Number
accession number
catalog number
Measuring & Mapping
Science & Mathematics
Time and Navigation
Modern Physics
Science & Scientific Instruments
See more items in
Medicine and Science: Modern Physics
Time and Navigation
Time and Navigation, National Air and Space Museum
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Efratom California, Inc.
Additional Media

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