Your receiver works out distance from how long each satellite's signal took to arrive, so the satellites must keep time to billionths of a second — only atomic clocks can.
A navigation satellite is really a flying clock with a radio. It broadcasts the exact time its signal left. Your phone notes when the signal arrived. The difference, multiplied by the speed of light, is the distance to the satellite.
Light covers about 300 km in a thousandth of a second. So the clocks must be extraordinarily steady. That is why every NavIC satellite carries an atomic-clock.
An atomic clock keeps time by counting the natural vibrations of atoms. The second itself is defined this way: NIST explains that atomic clocks count 9,192,631,770 oscillations of the microwaves absorbed and emitted by caesium atoms, and call that one second.
NASA notes that ordinary quartz clocks drift too much: after six weeks one may be off by a full millisecond, which for navigation means an error of about 300 km.
Worked example: light travels about 30 cm in a billionth of a second (a nanosecond). NavIC's standard service promises timing better than 40 ns, which corresponds to about 12 m of range.
The first-generation IRNSS satellites each carried three rubidium clocks, one in use and two spares. By January 2017 all three clocks on IRNSS-1A had failed, and ISRO's then chairman A. S. Kiran Kumar said a replacement would be launched in the second half of 2017. That replacement, IRNSS-1H, was lost in a launch failure in August 2017. An eighth satellite, launched in 2018, finally replaced IRNSS-1A.
The problem did not end there. ISRO says IRNSS-1F completed its 10-year design life on 10 March 2026, and its onboard atomic clock stopped three days later. The satellite stays in orbit for one-way messaging.
NVS-01, launched on 29 May 2023, was the first Indian navigation satellite to fly an indigenous rubidium atomic clock, built by the Space Applications Centre in Ahmedabad. NVS-02 carries a mix of indigenous and imported atomic clocks.
Read the status note beside each IRNSS satellite in the list — several say 'Clock failure — messaging only'. Open the NavIC tracker
The range to a satellite is
ρ = c × (t_receive − t_transmit)
with c ≈ 3 × 10⁸ m/s. A timing error Δt therefore gives a range error
Δρ = c × Δt
A phone's receiver has only a cheap quartz clock. It handles that by treating its own clock error b as a fourth unknown alongside its position (x, y, z):
ρᵢ = √[(x − xᵢ)² + (y − yᵢ)² + (z − zᵢ)²] + c × b, i = 1…4
NASA's guidance chapter puts it simply: a receiver needs four satellites in view — three for position and one for the clock bias. The satellites' clocks, by contrast, must be right in an absolute sense, because each one's error goes straight into its range.
Stability matters more than perfect accuracy. NASA notes that even the atomic clocks on GPS satellites drift and are corrected from larger ground clocks twice a day. A clock that drifts smoothly can be corrected; one that stops cannot. ISRO's NVS-01 brochure calls the onboard clock a Rubidium Atomic Frequency Standard (RAFS) and notes it acts as the stable frequency reference for the navigation signals.
Redundancy helps only if failures are independent. IRNSS-1A carried three clocks and lost all three, which suggests a shared cause rather than bad luck.
Position comes from signal travel time; a timing error of one millisecond means about 300 km of error. Only atomic clocks are stable enough to keep errors to nanoseconds.
All three of its rubidium atomic clocks — one primary and two backups — failed, which was confirmed in January 2017. A replacement satellite was later launched.
Launched on 29 May 2023, it was the first NavIC satellite to carry an indigenous rubidium atomic clock, developed by the Space Applications Centre, Ahmedabad.
c × Δt = 3 × 10⁸ × 10 × 10⁻⁹ = 3 m.