The very stable clock at the heart of each NavIC satellite. Clock failures on IRNSS-1A led India to develop its own rubidium clock, first flown on NVS-01 in 2023.
Satellite navigation is really timekeeping. Radio signals travel about 30 cm in a billionth of a second, so a small clock error becomes a large position error. Each NavIC satellite therefore carries rubidium atomic clocks, which keep time far more steadily than ordinary clocks. The clock is the stable frequency reference for the whole navigation payload.
The first-generation IRNSS satellites each carried three imported rubidium clocks, supplied by one vendor. In 2016 ISRO announced that all three clocks on IRNSS-1A had failed. A replacement, IRNSS-1H, was lost at launch in 2017; IRNSS-1I followed in April 2018.
ISRO's Space Applications Centre in Ahmedabad then developed an Indian rubidium clock. NVS-01, launched on 29 May 2023, was the first NavIC satellite to fly it. NVS-02 and NVS-03 carry a mix of Indian and imported clocks.