Rubidium Frequency Standard, Model FRK

This object, marked “F. Nr. E250/116,” is an unflown back-up for two rubidium frequency standards installed on NTS-1, the first of the Navigation Technology Satellites (NTS) launched to validate the key concepts and hardware for the Global Positioning System (GPS). NTS-1 (originally named Timation III) was built at the Naval Research Laboratory (NRL), Washington, D.C. It was launched in July 1974 and carried the first atomic clocks into space.
The compact rubidium frequency standard for NTS-1 is the Model FRK made by Efratom Elektronik, Munich, Germany. Gerhard Hübner and Ernst Jechart established the firm in 1971 and a year later supplied examples of the clock to NRL. Researchers had constructed relatively large laboratory rubidium frequency standards in the 1950s, and portable commercial versions were available by the mid1960s. But the FRK—weighing roughly three pounds and measuring about four inches on a side—was the smallest frequency standard of any type available in the early 1970s and attractive for space applications. NRL modified the commercial model for use in space, specifically to permit controlling its rate and frequency output from the ground.
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.
1. 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.
2. Stephen A. Nichols, Joseph D. White and Danzy, Frederick. Design and Ground Test of the NTS-1 Frequency Standard System. Naval Research Laboratory Report 7904. Washington, D.C., September 5, 1975.
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
Physical Description
steel (overall material)
glass (overall material)
overall: 3 7/8 in x 3 7/8 in x 5 5/8 in; 9.8425 cm x 9.8425 cm x 14.2875 cm
place made
Bundesrepublik Deutschland
ID Number
catalog number
accession number
Measuring & Mapping
Time and Navigation
Science & Mathematics
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
Naval Research Laboratory
Additional Media

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