My First Atomic Clock – Rubidium Frequency Standard

http://en.wikipedia.org/wiki/Rubidium_standard

“A rubidium standard or rubidium atomic clock is a frequency standard in which a specified hyperfine transition of electrons inrubidium-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.”