But not the reverse. If you are guiding a missile externally at a slow moving target like a ship, and just switch on the active radar targetting system a minute before impact, the shipboard radar crew don't have time for countermeasures. If their missile was like an Exocet, the targeting system computes the ships side profile and locks on to a predetermined locationThree minutes is an eternity when you are facing a missile travelling around Mach 1. Three seconds is somewhere around half a mile.