Three Atomic Clocks Have Failed Onboard India's 'Regional GPS' Constellation

A replacement satellite will be launched in the second half of 2017, according to ISRO chairman A.S. Kiran Kumar. The cause of the problem remains unclear.

Three atomic clocks onboard one of the satellites of the Indian Regional Navigation Satellite System (IRNSS) have failed, as ISRO chairman A.S. Kiran Kumar told The Hindu. The IRNSS is India’s version of the US’s globally used GPS system, and covers a wide swath of Asia and the Indian Ocean. It is a constellation of seven satellites in a geosynchronous orbit around Earth, and they coordinate to provide centimetre-level tracking precision to the Indian armed forces. The three atomic clocks used onboard each satellite of the IRNSS are used for precise coordination and to well-account for the effects of general relativity.

With the failure of all three clocks – one primary and two backups – onboard IRNSS 1A, Kumar has said a replacement satellite, 1H, will be launched in the second half of 2017. It is unclear what caused the failure, although the issue is neither new nor likely to be unique. Earlier this month, the European Space Agency (ESA) reported that three rubidium atomic clocks and six hydrogen maser clocks onboard its counterpart of the IRNSS, called Galileo, had failed. Neither ESA nor ISRO have declared their respective constellations ineffective as a result. The IRNSS should be back to normal with the launch of 1H while the Galileo is still not at full strength: only 18 of 30 satellites are currently online. Moreover, a seventh hydrogen maser clock that failed has managed to restart itself.

In the same vein, ISRO is also attempting to restart its clocks. Until such time, Kumar said the constellation will continue to work normally while the 1A provides fuzzy, imprecise measurements.

All the rubidium atomic clocks onboard IRNSS and Galileo – as well as China’s Compass navigational constellation – were manufactured by Spectracom, purported to be a market leader; ISRO integrated them with the IRNSS using technology developed by the National Physical Laboratory (NPL), Delhi. While China’s space programme has been typically closed off to foreign media, ESA and ISRO have yet to clarify whether the fault lies with Spectracom’s design or with something about current conditions in space. It is pertinent at this point to note that two atomic clocks on two Galileo satellites had already begun to fail in March and July, 2016.

Rubidium atomic clocks, though less accurate at measuring the passage of time than are hydrogen maser clocks, are still quite accurate, relatively cheap and widely used. They neither gain nor lose a second in tens of millions of years. They measure time according to the frequency of microwave emissions by electrons in a cooled rubidium atom, called the rubidium standard. When the atoms are of the element caesium instead, the resulting clock is also referred to as a primary frequency standard given its use in defining the standard times of many countries. Our own Indian Standard Time is ‘kept’ by caesium atomic clocks in the NPL.

Note: This article was edited on February 4, 2017, to remove the reference to Spectracom being a Swiss company, and to state that the clocks were installed onboard IRNSS using help from NPL.