The Patriot missile defence system is designed to detect and destroy incoming enemy tactical ballistic missiles, cruise missiles or aircraft.
It comprises a radar, control centre and associated power supply and communications towers, and launchers on trucks.
The manufacturer says it can take 16 launchers, the US Army uses usually five to eight in a battery.
Each launcher holds four missiles - or 16 in the latest "Pac-3" version.
The launchers can be up to a kilometre away from the radar and control hub, linked by microwave signals.
The trailer-mounted radar is a "phased array" type - it scans the sky with a narrow beam which flicks between thousands of locations each second.
If it detects something, up to 62 miles (100 km) away, it focuses on it and relays information about it to the control centre.
It can track up to 100 targets and send guidance data for up to nine missiles.
The "engagement control station" has three operators, with two radar consoles and a radio link.
The system can be automated, although an operator is able to override it.
When the control computer has decided the best time to launch, it tells a launcher to fire a missile or missiles - typically two might be fired at a target in rapid succession because one alone does not guarantee a hit.
The five-metre-long missiles are supersonic within moments of leaving the launcher, having taken their initial course from the control computer, and accelerate to five times the speed of sound.
The latest, smaller version of the missile does have a warhead but is meant to destroy its target primarily by simply hitting it. It has its own "seeker" to fine tune its approach to the target.
The older type weighs almost three times as much and has a range of up to 100 miles (160km). It uses a more complex process called "track via missile".
The Patriot missile homes on an electronic signal "painted" on the target by the radar, and relays its position in relation to the target back to the radar and on to the control centre.
Updated flight instructions are sent back up to the missile to guide it to the target. The computer can adjust the missile's fuse, depending on the target, to decide the best moment to detonate the 198lb (90kg) warhead.
A drawback to this method is it takes at least nine seconds from launch to become effective.
Patriots went into use in 1981 but came to fame during the 1991 Gulf War - when they had limited success against Iraqi Scud missiles.
In one instance - now notorious as an example of what can go wrong with computer software - 28 American soldiers were killed when a Scud hit a barracks in Saudi Arabia, because a software fault in the defending Patriot system meant it could not work properly.
The error meant its tracking system was half a second out - during which time a Scud travels half a kilometre.
Three years ago the US Army set about replacing "hundreds" of Patriots after identifying a problem with some of their components having deteriorated over time.
In March 2003 a Patriot battery in Kuwait mistakenly shot down an RAF Tornado jet returning from a mission over Iraq, killing the two crew.
Patriots are used by the USA, Germany, Netherlands, Japan, Israel, Saudi Arabia, Kuwait, Taiwan, Greece.