Rubidium standard

A rubidium standard or rubidium atomic clock is a frequency standard in which a specified hyperfine transition of electrons in rubidium-87 atoms is used to control the output frequency. It is the most inexpensive, compact, and widely used type of atomic clock, used to control the frequency of television stations, cell phone base stations, in test equipment, and global navigation satellite systems like GPS. Commercial rubidium clocks are less accurate than cesium atomic clocks, which serve as primary frequency standards, so the rubidium clock is a secondary frequency standard. However, rubidium fountains are currently being developed that are even more stable than caesium fountain clocks.