The Patriot missile system has a remarkable goal: It is designed to detect, target and then hit an incoming missile that may be no more than 10 to 20 feet (3 to 6 meters) long and is typically flying at three to five times the speed of sound. The upgraded Patriot system can also destroy incoming aircraft and cruise missiles.
The Patriot missile system has been in the news recently for two different reasons:
In this edition of How Stuff Works we will look at the technology that allows a Patriot missile to accomplish its goal.
- It is able to shoot down enemy missiles like Iraqi Scud missiles and protect soldiers and civilians from a missile attack. Patriot missile batteries have been activated several times in the current war, and were used extensively in the 1991 Gulf war.
- It has shot down one British aircraft and targeted an American F-16. The F-16 fired a missile that destroyed the Patriot missile radar.
Like the Stinger missile and the Sidewinder missile, the Patriot is a guided missile. However, the Patriot is somewhat more sophisticated. In both the Stinger and Sidewinder missiles, the infrared seeker is sensitive to engine heat. A human being is responsible for finding and identifying the target, appropriately aiming the missile so that the its heat-seeking eye can lock onto the target, and then firing the missile.
A Patriot missile, instead, depends on radar. The Patriot missile system uses its ground-based radar to find, identify and track the targets. An incoming missile could be 50 miles (80.5 kilometers) away when the Patriot's radar locks onto it. At that distance, the incoming missile would not even be visible to a human being, much less identifiable. It is even possible for the Patriot missile system to operate in a completely automatic mode with no human intervention at all (this may be what happened recently when a patriot missile system shot down a British aircraft). An incoming missile flying at Mach 5 is traveling approximately one mile every second. There just isn't a lot of time to react and respond once the missile is detected, making automatic detection and launching an important feature.
While the Stinger is a shoulder-launched weapon and the Sidewinder launches from aircraft, Patriot missiles are launched from Patriot missile batteries based on the ground. A typical battery has five components:
- The missiles themselves (MIM-104)
- The missile launcher, which holds, transports, aims and launches the missiles (M-901). This part is necessary because each missile weighs almost one ton.
- A radar antenna (MPQ-53 or MPQ-65) to detect incoming missiles.
- An equipment van known as the Engagement Control Station (ECS) houses computers and consoles to control the battery. (MSQ-104)
- A power plant truck equipped with two 150-kilowatt generators that provide power for the radar antenna and the ECS.
Since a Patriot missile battery can have up to 16 launchers, and there are also spare missiles to re-supply the launchers as missiles are fired, you can see that deploying a Patriot missile battery is not a small endeavor. Each launcher is roughly the size of a tractor-trailer rig, as is the ECS and the power supply truck. There are also operating personnel, technicians, support personnel, fuel for the generators, security forces to protect the battery, etc. This article describes a "convoy of about 300 vehicles, which included infantry forces, tanks and Marines" to move a Patriot missile battery to the front lines and make it operational. The deployment of Patriot missiles is not a decision made lightly.
In the following sections we will look at each of the different components, and then how the system operates as a whole.
A Patriot missile is a single-stage solid rocket that currently comes in two forms. There is the older PAC-2 missile, which is larger and not as effective as the newer PAC-3 missile deployed in 2002.
The PAC-2 missile:
Four PAC-2 missiles fit on a launcher.
- is 17 feet long (5.2 meters)
- is 16 inches (41 cm) in diameter
- has fins that extend out another 16 inches (41 cm).
- weighs almost 2,000 pounds (900 kg)
- carries a 200-pound (90 kg) fragmentation bomb with a proximity fuse.
- flies at Mach 5 and is supersonic within a second after launch.
The idea is for the PAC-2 to fly straight toward the incoming missile and then explode at the point of nearest approach. The explosion will either destroy the incoming missile with the fragments from the fragmentation bomb, or knock the incoming missile off course so it misses the target.
The PAC-3 missile is the same length as the PAC-2 but weighs only a third as much at 686 pounds (312 kg). It is only 10 inches (25 cm) in diameter. The smaller size means that 16 PAC-3 missiles can fit on a launcher. The fragmentation warhead weighs only 160 pounds (73 kg) in the PAC-3.
The idea behind a PAC-3 is for the missile to actually hit the incoming target and explode so that the incoming missile is completely destroyed. This feature makes it more effective against chemical and biological warheads because they are destroyed well away from the target.
The biggest difference between the PAC-2 and the PAC-3, and the thing that allows the PAC-3 to actually hit its target, is the fact that the PAC-3 has its own built-in radar transmitter and guidance computer. The operational differences between the PAC-2 and PAC-3 are discussed later in the article.
PAC-3 missiles currently cost two to three million dollars each. (See this Web site and this site for details.)
A Patriot missile battery can have up to 16 launchers. All of the launchers in the battery communicate with the single ECS van through either fiber optic cables or radio links. The ECS van sends commands to the launchers to fire the missiles.
Each launcher is about the size of a tractor-trailer rig. A launcher can hold four PAC-2 missiles or 16 PAC-3 missiles. After firing its missiles, a re-supply truck with a crane pulls up next to the launcher to load it with new missiles.
Each launcher has its own power supply to power the electronics and point the missiles, although a Patriot missile does not have to be aimed directly at the target when it launches.
Each Patriot missile battery has one high-power radar antenna that plays a variety of roles. The antenna can:
- Scan the skies looking for incoming targets.
- Detect a potential target.
- Determine the trajectory, speed and heading of the incoming target.
- Provide information to identify the target. Ideally the radar provides enough information to determine whether the target is a friend or a foe, although there have been causalities in the current Iraq war because of mis-identification.
- Track Patriot missiles once they are launched to help aim them at the target.
- Illuminate the target, which is important to the Track-via-Missile guidance system used by the PAC-2 missiles.
The traditional image of a radar antenna is the rotating, parabolic antenna seen on top of airport control towers and aircraft carriers. The Patriot system instead uses a phased array antenna. This antenna contains 5,000 phase-shifting elements that allow the antenna to send out multiple, narrow, precisely-aimed radar beams that scan the sky. With these beams, the Patriot's radar can track up to 100 potential targets as well as up to nine outbound Patriot missiles.
The radar antenna has a 63 mile (100 kilometer) range.
The ECS van is the command center of the Patriot missile battery. The ECS contains stations for three operators as well as the computers that control the battery. The radar antenna and all of the launchers in the battery connect to the ECS, and Patriot missiles in flight also communicate with the ECS.
Inside the van there are two radar consoles. Operators can see the status of all of the targets that the system is currently tracking. Operators can let the system run in fully automatic mode, or they can intervene to select or deselect targets. There is also a communication station that allows the battery to communicate with other batteries or with the command center for the region.
How It Works
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 since it is currently the more common missile deployed.
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.
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.
- 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.
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.
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