India has notified a no-fly zone, indicative of a likely missile test, in a large part of the Indian Ocean for 15 and 16 December.

The notification suggests that the missile likely to be tested will fly to a maximum distance of 5,400 kilometers.

Experts believe India is preparing to test its K-4 submarine-launched ballistic missile (SLBM) or land-based Agni-V missile. While the latter has a stated range of 5,000 km, the former is believed to be capable of striking targets up to a range of 3,500-4,000 km.

Earlier this month, India cancelled a notification for a no-fly zone after two Chinese intelligence-gathering vessels entered the Indian Ocean. Equipped with advanced sensors, these Chinese ships can track ballistic missiles and satellites and gather signal intelligence.

A new notification for 21 and 22 November was issued later. The Strategic Forces Command, responsible for managing India's nuclear weapons, carried out a training launch of Agni-III missile on the first of those days.

Why is the K-4 test important?

India's only operational ballistic missile submarine (SSBN), INS Arihant, currently carries K-12 SLBMs. With a range of just 750 km, a K-12 fired from an SSBN in the Bay of Bengal can't reach China's industrial or population centres. However, with a range of over 3,500 kilometres, the K-4 missile will give India's SSBNs the capability to strike targets in a large part of China, including Beijing, from the northern region of the Bay of Bengal.

The addition of the K-4 missile to India's SLBM arsenal will improve the credibility of the sea leg of India's nuclear triad, which is the ability to launch a nuclear strike from land, air, and sea. 

Given its 'no first use' nuclear posture, India needed to demonstrate the capability to launch a second strike or the ability to respond to a nuclear attack with one of its own. An assured second-strike capability is an essential element for effective strategic deterrence.

SSBNs are believed to be the most survivable of the three legs of the nuclear triad. While land-based missiles and aircraft meant for nuclear weapons delivery can be targeted by the enemy in a first, counterforce strike, taking out SSBNs at sea is relatively more difficult. These boats can remain underwater for long durations of time to avoid detection.

In case of a first strike that destroys nuclear weapons and land-based nuclear delivery systems, India can launch a retaliatory strike with its surviving SSBNs. For this, India will need a continuous at-sea deterrent, and the commissioning of the three SSBNs currently in the pipeline — Arighat, S4 and S4* — will give India this capability.