What impact will S Korea's expanded missile defence system have?

he verbal threats from the North Korean leadership - and its
recent nuclear and missile tests - have prompted a
fundamental rethink in its southern neighbour.
The long-standing hopes that Pyongyang might eventually be
induced to give up its nuclear weapons programme have
proved illusory.
South Korea is accordingly reassessing its security needs and
it is clear that an expanded missile defence system is going to
be a key part of its response to the North's more aggressive
behaviour.
Even before the latest threats from Kim Jong-un , the US and
South Koreans had begun urgent consultations to explore the
feasibility of deploying a system known as Terminal High
Altitude Area Defense (Thaad) to the Korean Peninsula at the
earliest possible date.
Will carrots or sticks change North Korea?
Missile defences in the region
It is not yet clear if the missiles would be sold to the South
Koreans. It is possible an interim arrangement might see some
US batteries deployed to give an initial capability.
South Korea already operates a variant of the US Patriot anti-
missile system and further Patriot batteries are deployed in
South Korea by US forces based there. But these are intended
to hit incoming missiles at relatively low altitudes.
Thaad is a much more capable and longer-range system. It
destroys incoming missiles at a much higher altitude, beyond
the Earth's atmosphere.
This makes it especially useful in countering missiles that
might carry a nuclear warhead. (It should be noted in passing
that there is no evidence yet to suggest that North Korea has
sufficiently miniaturised a nuclear weapon to enable it to be
mounted on a ballistic missile).
The Thaad interceptor is produced by the US company
Lockheed Martin. It is an extremely fast missile with a
maximum speed of 2,800 metres per second (10,080km/h). It
is capable of making interceptions at an altitude of 150km i.e.
beyond the atmosphere.
The Thaad system is made up of six truck-mounted launchers
carrying some 50 interceptor missiles, and a fire control and
communications unit, all linked to a powerful X-band radar
system - manufactured by Raytheon - capable of detecting
targets at very long range.