The problem with ICMB interception isn't really target aquisition or tracking. It's pure and simple speed. A typical short range missile might be moving at mach 2 or 3 at best. An ICBM re-entry vehicle is moving at a balmy mach 14 or faster depending on trajectory. A timing error of 1 millisecond at mach 3 is about 1 m of position error. At Mach 14 that's closer to 5m. The difference between getting within blast radius and a mis. And every second you spend trying to get a good targeting solution means the target has moved nearly 5 km closer!