A Patriot missile battery operates slightly differently depending on whether it is firing PAC-2 or PAC-3 missiles. We will look at the operation of the PAC-2 missile first.
The radar antenna scans the sky looking for incoming targets. Once it finds a target, it scans it more intensely and communicates with the ECS. The goal of the scan is to determine the speed and heading of the target and also to identify it as a friend or a foe. When the operator or computer decides that it has an incoming foe, the ECS calculates an initial heading for the Patriot missile. It chooses the Patriot missile it will launch, downloads the initial guidance information to that missile and launches it.
Within three seconds the missile is traveling at Mach 5 and is headed in the general direction of the target. The radar antenna on the ground has three roles at this point:
- It continues to track the incoming missile.
- It acquires and tracks the outbound Patriot missile to provide the ECS with information on its heading and speed.
- It illuminates the incoming target.
The illumination signal reflects off the target and is received by an antenna in the nose of the PAC-2 missile that is heading its way. The PAC-2 missile then relays this signal back to the ECS. The ECS uses the illumination signal information along with the radar's information on the track of the incoming target and outbound Patriot to steer the Patriot missile. The ECS sends guidance commands to the Patriot missile to adjust its course. When the Patriot missile is at the point of closest approach to the target, its fragmentation bomb explodes.
Unlike the PAC-2, the PAC-3 missile contains its own radar transmitter and computer, allowing it to guide itself. Once launched, it turns on its radar, finds the target and aims for a direct hit. This has been compared to hitting a bullet with a bullet. The difference is that both the incoming target missile and the outbound Patriot missile are traveling up to five times faster than a typical bullet and are closing in on one another at up to Mach 10, or two miles per second. At that speed there is no room for error -- if the missile miscalculates by even 1/100th of a second, it will be off by more than 100 feet (30.5 meters).
For more information on the Patriot missile system and related topics, check out the links on the next page.