Atomic Clocks

An atomic clock is a clock that keeps time by counting the natural "ticks" of atoms. Every atom of a given element, like caesium or rubidium, switches between two energy states at one fixed, unchanging rate. By counting these switches, the clock keeps time so accurately that it may drift by only about one second in a million years or more. Every navigation satellite, whether GPS or NavIC, carries atomic clocks, because the whole idea of satellite navigation depends on knowing the time almost perfectly.

Why does navigation need such exact time?

A navigation satellite keeps sending out a message that says, in effect, "I am here, and the time now is this." Your phone notes when the message arrives. The gap between sending and arriving tells it how far away the satellite is, because the signal travels at the speed of light. Light covers about 300,000 km every second.

So a clock error of just one millionth of a second (one microsecond) makes the distance wrong by about 300 metres. An ordinary quartz watch would make your map point to the wrong town. Only atomic clocks are steady enough.

How does a receiver find its position?

Your phone measures its distance from several satellites at once. With distances from three satellites, it can work out where it is. A fourth satellite is needed because the cheap clock inside your phone is not exact, and the fourth signal lets the phone correct its own clock. This is why NavIC needs at least four working satellites for a standalone position fix.

Where did atomic clocks come from?

For most of history, time was based on the Earth's spin: one day, divided into hours, minutes and seconds. But the Earth's spin is slightly uneven. In 1955, Louis Essen built the first practical caesium atomic clock at the National Physical Laboratory in the United Kingdom. In 1967, the 13th General Conference on Weights and Measures (CGPM) redefined the second using caesium atoms.

Since then, one second is the time taken for 9,192,631,770 cycles of a particular radiation from the caesium-133 atom. The wording was updated in 2019, but the number stayed the same.

How does an atomic clock work?

You can think of it like a music teacher tuning a guitar to a perfect note.

1. The clock has an ordinary electronic oscillator (a crystal that vibrates, like in a quartz watch). On its own it slowly drifts.
2. It sends microwaves at roughly the right frequency into a small chamber of atoms (caesium, rubidium or hydrogen).
3. The atoms react strongly only when the microwaves are at their exact natural frequency.
4. A detector checks how strongly the atoms react, and keeps nudging the oscillator until the reaction is strongest.
5. The oscillator is now "locked" to the atoms. The clock counts its cycles to measure time.

So the atoms do not run the clock; they act as a perfect tuning fork that keeps correcting it.

The main types of atomic clocks

• Caesium clocks: The basis of the official definition of the second. Large ones in national laboratories are "primary standards", the reference clocks for a country's time.
• Rubidium clocks: Smaller, lighter and cheaper, but they drift a little more. They are the workhorse of navigation satellites, including GPS and NavIC.
• Hydrogen masers: Very stable over short periods. Europe's Galileo satellites each carry two rubidium clocks and two passive hydrogen maser clocks. ESA figures put the maser's accuracy at about one second in three million years, and the rubidium clock's at about three seconds in a million years.
• Optical clocks: The newest kind, using light instead of microwaves. They are even more accurate and may one day lead to a new definition of the second.

Why satellites carry more than one clock

A satellite in space cannot be repaired. So each navigation satellite carries backup clocks. Each first-generation IRNSS satellite carried three rubidium clocks: one in use and two spares. Ground stations constantly compare the satellite clocks with very precise clocks on Earth and send corrections. For NavIC, this ground timing work is done from ISRO's navigation centre at Byalalu, near Bengaluru. Satellite clocks also need a small correction for Einstein's relativity, because clocks in orbit run at a slightly different rate than clocks on the ground.

India's experience: the clock crisis

The first-generation IRNSS satellites used rubidium clocks imported from a Swiss firm (most reports name Spectratime). From 2016, these clocks began failing. By 2017, all three clocks on IRNSS-1A had stopped, so the satellite could no longer give navigation signals. Similar clock failures were seen on Europe's Galileo satellites around the same time. ISRO sent up a replacement, IRNSS-1H, in 2017, but its heat shield did not separate and the mission failed.

The next replacement, IRNSS-1I, was launched in April 2018. As older satellites lost clocks or reached the end of their life, NavIC kept shrinking. IRNSS-1F completed its 10-year design life on 10 March 2026, and its last working clock stopped on 13 March 2026. It now gives only one-way messaging services, not positions. With that, the system fell to three positioning satellites.

The indigenous answer

To end this dependence, the Space Applications Centre (SAC), Ahmedabad, developed India's own rubidium atomic clock. It flew for the first time on NVS-01 on 29 May 2023. ISRO says the clock has been working in orbit since July 2023. All later NVS satellites carry Indian-made clocks. NVS-03 (GSLV-F18 mission, October 2026) carries an Indian-made atomic clock together with procured (imported) clocks. This mix of sources means one supplier's fault is less likely to stop every clock on board.

Atomic clocks and Indian Standard Time

Atomic clocks are not only for satellites. India's official time, Indian Standard Time (IST), is kept by the CSIR-National Physical Laboratory (CSIR-NPL), New Delhi, using a group of atomic clocks. IST is set at UTC + 5 hours 30 minutes. In January 2025, the Department of Consumer Affairs released the draft Legal Metrology (Indian Standard Time) Rules, 2025.

The idea, called "One Nation, One Time", is to make IST from NPL the single legal time reference across sectors, shared with high accuracy (including through NavIC), instead of relying on foreign sources such as GPS time.

Commonly confused concepts

• Atomic clock vs nuclear or radioactive material: An atomic clock has nothing to do with radioactivity or nuclear power. It only uses the natural, safe energy switches of ordinary atoms.
• Caesium vs rubidium clock: Caesium defines the second and is used in national reference labs. Rubidium is smaller and is the usual choice inside satellites.
• Atomic clock vs quartz clock: A quartz watch uses a vibrating crystal that drifts with heat and age. An atomic clock uses a crystal too, but keeps correcting it against atoms, so it barely drifts.
• UTC vs GPS time vs IST: UTC (Coordinated Universal Time) is the world's reference time, built from many atomic clocks. GPS time is the internal time of the GPS system and does not add leap seconds, so it runs a few seconds ahead of UTC. IST is UTC plus 5 hours 30 minutes.

Issues, criticism and the way forward

• Import dependence: India's early reliance on foreign clocks left NavIC exposed when they failed. Indigenous clocks from SAC are the main fix.
• Single points of failure: Even with three clocks, a common design fault can kill all of them, as IRNSS-1A showed. Experts suggest mixing clock types (for example rubidium plus hydrogen maser, as Galileo does).
• Slow replacement: Launch failures and delays (IRNSS-1H in 2017, NVS-02 in 2025) let the constellation shrink. A steady pipeline of spare satellites is the way forward.
• Ground timing strength: A strong national time network, with NPL's clocks linked to NavIC and to telecom, power and banking systems, makes the country less dependent on any single satellite.

Concepts to Know

• Frequency: How many times something repeats in one second. It is measured in hertz (Hz). 1 Hz means once per second.
• Oscillator: A device that vibrates or swings back and forth at a steady rate, like a pendulum or a quartz crystal. Clocks count these swings.
• Constellation (satellite): A group of satellites that work together as one system.
• Leap second: An extra second sometimes added to UTC so that clock time stays in step with the Earth's slightly uneven spin.
• Microsecond / nanosecond: A microsecond is one millionth of a second. A nanosecond is one billionth of a second.