One of my college professors was an outspoken opponent of missile defense systems (and the Patriot system specifically) that worked during the missile's reentry/descent phase.
His objection is that it's too easy for an opponent to defeat the system either by overwhelming it (MIRVs for instance) or by designing the reentry vehicle to make random movements which would make it really difficult for a intercepter to track. Even if the interceptor can hit it, it's more likely to knock the warhead off course instead of destroying it. If a nuclear missile is aimed at NY and an interceptor hits it so that it falls to Philly instead, that's still a net loss.
Saddam inadvertently hit upon the both methods - his engineers tried to improve on the Soviet scud design to give them more range (which they were successful at) but their improvements made the missile more likely to break up on reentry (which presented more targets than the tracking radar anticipated) and the lack of aerodynamics of the resulting pieces (including the warhead) the missiles fall in unpredictable ways which caused tracking problems. An opponent that actually tried to game the system could make his missiles more difficult to hit.
The professor is advocating for boost-phase missile defense since the missile movements are much more predicable.
But it's not like boost-phase intercept is a magic bullet (ha!). Well, really, it generally takes a magic bullet. Launch sites are typically far away. Unless you have a interceptor several times faster (or interceptor site much closer to the launcher than the target), then it's very hard to actually reach the missile while it's in boost phase.
That airborne laser that's been in development for seemingly forever was pretty much determined to be the best way to intercept. You'll notice that it combines BOTH elements. That 747 is flying within LOS of the missile path while in boost phase, and it's also using the fastest projectile possible.
You'll also note that the Patriot's role is medium tactical air defense as well as theater anti-ballistic missile defense. The second role was basically tacked on, and then later massively expanded (once it became obvious that there aren't many air forces in the world that can actually fight the US).
In the end, I'm sure every general and admiral actually out to improve their warfighting abilities would want both systems. It's all about defense in depth. It's the reason why warships have three different sets of anti-air missiles, while we still have stingers when we have patriot missiles, and why US fighter aircraft still carrying short range missiles and guns.
If you catch something on the launchpad that's a relatively low expense, to catch it i in flight you need to actively defend each and every possible target and a lot of sexy hardware to do so.
yeah, I think its very hard to defend against missiles.
I've always thought they should prove that it will work in a realistic physics-based simulation before spending billions on these systems. But they seem to have some value against relatively crude missiles. Plus their value as a bargaining chip, although that depends on how smart the adversary is.
His objection is that it's too easy for an opponent to defeat the system either by overwhelming it (MIRVs for instance) or by designing the reentry vehicle to make random movements which would make it really difficult for a intercepter to track. Even if the interceptor can hit it, it's more likely to knock the warhead off course instead of destroying it. If a nuclear missile is aimed at NY and an interceptor hits it so that it falls to Philly instead, that's still a net loss.
Saddam inadvertently hit upon the both methods - his engineers tried to improve on the Soviet scud design to give them more range (which they were successful at) but their improvements made the missile more likely to break up on reentry (which presented more targets than the tracking radar anticipated) and the lack of aerodynamics of the resulting pieces (including the warhead) the missiles fall in unpredictable ways which caused tracking problems. An opponent that actually tried to game the system could make his missiles more difficult to hit.
The professor is advocating for boost-phase missile defense since the missile movements are much more predicable.