For instance, defenses that take place during the terminal phase are often the simplest to build in technical terms as they require only short-range missiles and radars.
However, terminal defenses also face the most difficult targets, the multiple warheads and decoys released during the post-boost phase.
In contrast, boost-phase defenses are difficult to build because they have to be located close to the target, often in space, but every success destroys all of the warheads and decoys.
The boost phase is the portion of the flight of a ballistic missile or space vehicle during which the booster and sustainer engines operate until it reaches peak velocity.
Such defenses have the advantage of being able to easily track their targets through the infrared signature of the rocket exhaust, and that boosters are generally much less robust than the warheads or bus.
Boost-phase intercepts are also generally the most difficult to arrange, as they require the interceptor to be within attack range within the few minutes while the missile engines are firing.
This used an x-ray laser stationed on a submarine off the coast of the Soviet Union that would "pop-up" a weapon when a launch was detected.
It has the added difficulty of having to use much more sensitive tracking systems as the rocket engine on the bus is far less powerful and is likely very "cold" in relation to the booster.
During this phase the payload follows a ballistic trajectory, with warheads, decoys and radar reflectors mixed together in an extended formation known as the target cloud.