Система противовоздушной обороны S-400 важна для Индии, поскольку она заполняет критический пробел в концепции Индии о надежной многоуровневой противовоздушной обороне, которая была усилена в качестве средства для борьбы с возникающими угрозами в виде бомбардировщиков, стратегических воздушных самолетов, истребителей, Крылатые ракеты и многое другое. «Спектр военных действий 21-го века» потребовал использования наземных систем противовоздушной обороны (GBADWS), поскольку воздушные угрозы могут восприниматься и использоваться в различных конвертах угроз в зависимости от их характера и высоты полета.
India is trying to develop a GBADWS by inducting a mix of Short Range Surface to Air Missiles (SRSAM) , Medium Range Surface to Air Missiles ( MRSAMs ) , Long Range Surface to Air Missiles ( LRSAMs ) and finally Very Short Range Air Defence (VSHORAD) Missiles. India is reinforcing its MRSAM and LRSAM by inducting Aakash and Barak-8 missiles en masse simultaneously retiring its older air defence systems. However the one critical gap where India was lacking was need for a Long Range Air Defence System which can engage Strategic targets at a much longer range.
At this point , S-400 points out a game changing attitude as this will allow India to reinforce long range air surveillance and knock out aerial threats out off the sky based upon their threat perception. S-400 relies on four different type of missiles to reinforce its threat envelope though India is buying only two of these missiles : very-long-range 40N6 (400 km) and the long-range 48N6 (250 km) surface-to-air missiles from Russia.
Keep in mind that possessing a missile with 400 km range do not give India the capability to simply knock anything out of the sky. 40N6 Missile will be used only against low maneuverable targets like Strategic Airlifters , Airborne Early-warning radars , Electronic Warfare Aircrafts and Reconnaissance Aircrafts. It can also knock out Low Earth orbit (LEO) satellites and Intermediate Ranged Ballistic Missile Warheads in terminal phase.
48N6 Missile , though possessing a lower range , gives the option to take on more ‘intelligent’ targets like Fighter Aircrafts , HALE/MALE Drones , Cruise Missiles and more. S-400 also comprises of a 55K6E command and control system based along with 92N6E Multimode Engagement Radar which can track 100 targets in Track While Scan mode , 96L6 Cheese Board 3D Radar which can detect threats upto 300 kms and finally the 91N6E Big Bird Battlefield Radar which can detect aerial threats ( including aircrafts , drones and missiles ) within the distance of 600 km tracking upto 300 targets.
S-400 also reinforces the Air Superiority concept of Indian Air Force in the light of its falling Squadron Strength which has sent jitters in the establishment with the situations further graving up in light of corruption allegations in recent deals. This is what made S-400 a Tier 1 priority for Indian Defence Acquisition aimed at giving a shot to the ageing Air Defence Capabilities of Indian Air Force.
Though there have been a lot of apprehensions that the deal would invoke the Section 231 of CAATSA and would bring sanctions on India ( nothing new for us , after all ) , US just gave a casual statement that it does not intend to damage its ally defence capabilities thus giving an approval to the deal in a low tone.
Since 5 S-400 Systems are being inducted , 3 of them will most likely be placed on the Western Border while 2 of them in Eastern Border to cover the whole Opposing Force Air Bases in case of a conflict. On the Western Side , the 3 S-400 Systems can place an air superiority against majority of Pakistani Airbases in case of a conflict which includes strategically most viable airbases at Masroor , Karachi , Sargodha and Minhas Air Bases.
In the East , S-400 would be a game changer against PLAAF in Tibet Autonomous Region especially when PLAAF is fielding more combat capable hardware in the region. PLAAF has some 14 ( or more ) major airbases from where it can operate with 20 minor airstrips as well which are strategically placed at Lhasa , Nqari , Xigase with majority of its Flanker and their derivatives along with J-10 and J-20 now being deployed there. S-400 can give India a strategic deterrent to counter aerial threats more proactively conforming to India’s strategic autonomy.
Hope my answer serves the purpose =)