Atomic clock

Geography glossary

Also called: Rubidium Atomic Frequency Standard, RAFS · Topic: Remote Sensing, GIS and GNSS · NCERT: Beyond NCERT

Meaning

An atomic clock is a very precise clock that keeps time by counting how atoms, such as rubidium atoms, absorb and give off energy at one steady rate that never changes. That rate is so regular that the clock barely drifts. Navigation satellites need such clocks. Each satellite sends out a time signal. A receiver then works out its distance from the time the signal took to reach it. If a clock is wrong by even a tiny fraction of a second, the position comes out wrong by a long way. In India, the Rubidium Atomic Frequency Standard (RAFS) is the atomic clock built for NavIC satellites.

Example

Clock failures were a weak point for India's NavIC system. All three atomic clocks on IRNSS-1A had stopped working by 2016. The imported clock on IRNSS-1F stopped in March 2026. To reduce this dependence, the Space Applications Centre (SAC) in Ahmedabad built an indigenous RAFS. It first flew on NVS-01, which was launched from Sriharikota in May 2023.

Don't confuse with

  • Receiver clock: The clock in a phone or GPS receiver is cheap and much less precise. A receiver needs signals from at least four satellites. Three fix its position, and the fourth corrects the error in its own clock.
  • Atomic Clock Monitoring Unit (ACMU): This unit, built by SAC, is not a clock. It monitors the atomic clocks on board and produces the satellite's 10.23 MHz master timing signal from them.

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