With cruise missile and air strikes being India's most likely response to a hypothetical Pakistani terrorist outrage in the future, retaliation from the Pakistan air force is inevitable.
That is where the S-400 will come into play, says Ajai Shukla.
Addressing a parade at Hindon to mark the 86th anniversary of the Indian Air Force, Air Chief Marshal Birender Singh Dhanoa welcomed the $5.43 billion (about Rs 38,000 crore/Rs 2380 billion) purchase of five regiments of S-400 surface-to-air missiles from the Russian air defence specialist, Almaz-Antey.
When the long-range S-400 enters service in 2021, it will greatly enhance the IAF's capability for national air defence -- its primary responsibility to detect, intercept and shoot down enemy aircraft attempting to penetrate Indian airspace.
Given the IAF's depleted numbers -- it has just 31 fighter squadrons against an authorised 42 -- the S-400's capabilities are essential.
With cruise missile and air strikes being India's most likely response to a hypothetical Pakistani terrorist outrage in the future, retaliation from the Pakistan air force is inevitable. That is where the S-400 will come into play.
Experts regard India's current air defence set-up as weak, with numerous gaps that a skilled adversary would exploit.
Besides the shortage of fighter aircraft, India's radar network -- which should ideally detect PAF fighters as soon as they take off from their bases -- has insufficient range and gaps in coverage.
The IAF's Soviet-era and Russian-origin SAMs, such as the Pechora SAM-3 and the OSA-AK SAM-8, have inadequate ranges of under 35 kilometres.
By 2021, when the S-400 enters service, India's air defence will be improving.
The IAF will by then have its full compliment of 272 Sukhoi-30MKIs, and the first Rafale squadron and two Tejas squadrons would have entered service.
Simultaneously, the capable Indo-Israeli medium range SAM (MR-SAM) -- with a detection range of 150 kilometres, a strike range of 70 kilometres and a far higher hit probability than current missiles -- would be getting inducted in significant numbers.
The IAF, which funded 90 per cent of the MR-SAM's development cost of Rs 100.75 billion, has ordered 18 units.
Meanwhile the Akash SAM, developed by the Defence R&D Organisation and built by Bharat Electronics Ltd, is also being inducted into service in numbers.
The Akash has a range of just 25 kilometres, but there is a project to upgrade that.
National air defence includes multiple layers of surveillance sensors and strike capabilities -- both defensive and offensive.
The most offensive air defence option, and one to which the IAF would allocate most aircraft at the start of a campaign, is to knock out enemy fighters on the ground.
This requires IAF strike aircraft to penetrate deep into enemy territory after jamming enemy radars, drop cluster bombs to destroy enemy aircraft and destroy runways with deep penetration bombs.
The Israeli air force knocked out almost the entire Egyptian, Syrian and Jordanian air forces on day one of the 1967 Six Day War, but this is unlikely in the India-Pakistan context.
Therefore, the IAF's air defence plan must also cater for retaliatory strikes by Pakistan air force fighters.
Multiple layers of sensors detect incoming fighter strikes.
Amongst the most reliable, surprisingly, is a chain of 'mobile observation posts' (MOPs) all along the border -- each one a single human with a radio set, trained to identify and report enemy aircraft flying across the border.
Behind the MOPs, a chain of surveillance radars looks into enemy airspace to detect aircraft activity.
Looking even deeper are 'Airborne Early Warning Command and Control (AEWC&C) systems, like the IAF's three Phalcon systems, mounted on Russian IL-76 transport aircraft.
From on high, where the earth's curvature does not obscure visibility, they detect even low-flying aircraft at ranges of 400 kilometres and direct IAF fighters precisely onto them.
All these air defence elements are networked through data and voice communication channels to an autonomous 'integrated air command and control system' (IACCS), which also links with civilian air traffic control radars.
The S-400 is designed as a stand-alone system, with its own radar, missiles, logistics and data networks.
Therefore, each S-400 unit will provide an extra layer of protection to pre-designated high-value targets -- the national capital or the Bhabha Atomic Research Centre, Mumbai.
However, the S-400 units will not operate autonomously, but will be quickly plugged into the IACCS. This will enhance surveillance and strike capabilities and also cross-verification of inputs across the system.
"We are old hands at networking systems from diverse sources, enabling them to inter-operate. We have done it with our AEWC&C system. On board our warships, weapons and sensors from many different countries operate seamlessly. The S-400 will fit smoothly into the IACCS," says a senior air marshal.
Each S-400 unit consists of a 91N6E Big Bird acquisition radar that detects targets, and a 55K6E command post that autonomously makes decisions relating to engagement.
It would allocate each target to one of six 98Zh6E fire units, each having 12 'transporter-erector-launchers' (TELs), with four missiles on each TEL.
Once a missile is launched, the fire unit's 92N6E Grave Stone multimode engagement radar guides it to the target.
Finally, each S-400 fire unit has a 30TS6E logistical support system with missile storage, test and maintenance equipment.
The entire S-400 unit is highly mobile, being carried by all-terrain, wheeled vehicles with autonomous power supply, navigation and geo-location systems, communications and life support equipment.
Given that, it is ideally suited for safeguarding the mechanised units in army strike corps from enemy air attacks.
With the United States trying to wean India's military away from Russian equipment, media reports have speculated that Washington offered India its Theatre High Altitude Air Defence System (THAADS) as an alternative to the S-400.
However, the THAADS is primarily an anti-ballistic missile system, whereas the S-400 is optimised as an anti-aircraft system.