No system is fool proof including SAM's and many others.Any surface to surface missile with range more than 500km is called a ballistic missile with different name.
Yes S-400 should be imported there is no other option at present.
PDV and ashwin are not SAM's. Barak-8 with a total weight of approx 300kg and warhead of 60kg can it perform ABM role...
Why do we need protection against cruise missiles and aircraft...?Because their target is the cities and major installations.
Never heard of the word "Multi-layered" defense.No SAM missile can guarantee that it can stop aircraft.Never heard of word hit probability.
It's SAM's only job.
What ABM protection for bases..........
Never heard of surface to surface missiles..they are cheaper and can be fired in salvos.So you need multi layered defense
Behind mountains....?By the way once you hit the entrance of tunnel it will be end of it.
SAMs are generally fully accurate for aircrafts and subsonic missile. Hit probability of Akash is 88% per missile which means 2 missile and it becomes 98.5%. One can even fire 3 missiles if necessary. Akash SAMs are cheaper and can be used in larger numbers while aircrafts and long range cruise missiles are expensive.
Next,layered defense is something I understand but I don't see the need to waste unnecessarily high resources on inner layers. A minimal defense of 1-2 SAM system per base my be needed.
Ballistic missile, however can only be intercepted at reentry phase as during mid flight, it will be pretty high and intercepting it will be like intercepting satellite. So, ABM have to be a point defence system than can defend only a certain area and not entire border.
Ps:
Ballistic missiles are a different ball game. I am not even considering it now. I will come to it later. But, I want to clarify that all surface to surface missiles of 500km range are not ballistic missiles.
Ballistic Missiles are missiles that work with initial stage powered by rocket and then later guided by gravity. Cruise missiles are jet powered throughout the flight and are not guided by gravity but by rocket motor till it hits the target. It has some Maneuverability during flight. You can consider cruise missiles to be like UAV with less maneuverability while ballistic missiles are like big bullets flying after being fired from cannon.
Now, the cruise missiles and aircrafts are slower than ballistic missiles as they follow a controlled flight by their motors while ballistic missiles fly in exo atmosphere zone and travel at 6Mach + speed for MRBM (Medium range ballistic missile with range 500+ km to 2500km). Ballistic Missiles face little air resistance as they travel in exo atmosphere and hence can maintain the speed. Cruise missile have to have lower speed due to air resistance and motor limitations. Cruise missiles have much better accuracy than ballistic missiles while they are easier to intercept. Ballistic missile on the other hand are much less accurate and can't maneuver to hit behind hillside, tunnels. They are generally good for hitting civilian open areas and not military assets