Atomic Clocks for GNSS

This chapter focuses on the atomic frequency references (AFRs) that are used in global navigation satellite system (GNSS), providing some introductory context, the basic concept of an atomic clock and related terminology, and more specific descriptions of the three types of atomic clocks that are used in GNSS space systems. Three types of AFRs are currently used in GNSS satellites: rubidium (Rb) vapor cells, cesium (Cs) atomic beams, and hydrogen (H) masers. All of these clocks are based on quantum transitions at microwave frequencies between hyperfine levels in the ground state of the atoms. Lasers are obviously necessary for both Cs and Rb atomic fountain clocks based on cold atoms. Research and development of advanced clocks today are dominated by laser‐driven systems. Worldwide, several groups are working on new advanced atomic clocks for future science missions in space.