It has been 30 years since the missile cruiser Ticonderoga first took the Aegis system to sea. In 1982, Aegis was a vastly expensive anti-aircraft system entering a world in which much simpler ones had not worked entirely well. Critics saw it as several steps beyond what was practical or useful. The U.S. Navy saw it as an especially powerful system that would bind together its more numerous simpler ones in a fight against increasingly numerous and sophisticated missiles, particularly Soviet ones. Thirty years later, Aegis is the standard U.S. Navy air defense system. Nearly all of the simpler, cheaper ones are long gone. Aegis is accepted worldwide as the standard for air defense and five allied navies have adopted it: Australia, Korea, Japan, Norway, and Spain. Two others, Germany and the Netherlands, have adopted variants with other radars. Aegis has extended its reach beyond the atmosphere to deal with ballistic missiles, and it is being deployed ashore to support NATO. The main current Western alternative to Aegis, PAAMS (Principal Anti Air Missile System) incorporates much of the thinking that Aegis introduced.

All of this has happened so gradually that few realize how remarkable it is. Other countries that field naval air defense missiles have found themselves discarding system after system in hopes of developing entirely new successful ones. The U.S. Navy has been alone in pursuing what is now called spiral development: a step by step process in which each new version of a system incorporates a great deal of what has gone before. It might be imagined that the result would be mediocre – but Aegis shows that it is the opposite.

Aegis itself incorporated the Standard Missile already developed for earlier systems. At that stage, the Navy did not yet understand the virtues of spiral development; it just did not have the money to develop both a new missile control system and a new missile. It happened that because the missile was well understood, it was considerably easier to build a new control system for it. In much the same way, when Aegis was extended to deal with ballistic missiles outside the atmosphere, it made a considerable difference that it used an existing missile airframe and motor and booster for the part of its flight inside the atmosphere.

Aegis was conceived to deal with massed missile attacks, which the Soviets were expected to mount mainly using large numbers of missile-firing bombers (ultimately Backfires). The threat was saturation. Any defending missile system can handle only so many targets at the same time – it is said to have only so many channels. Previous systems, such as Tartar, had individual radar directors that were assigned to particular targets. The directors also guided the defending missiles, so it was one director, one channel. The U.S. Standard Missile homed on radar energy produced by the director and reflected off the target.

How many directors a ship could carry depended on the ship’s topside geography. A director had to illuminate a target all the way from launch to explosion – as a safety feature, Standard and other such semi-active missiles destroyed themselves if they did not sense reflected radar energy within a set time after launch (typically 30 seconds). That meant that illuminators had to be on the ship’s centerline. This space had to be shared with other fire control radars, such as those for guns (in some systems these radars had some missile control capability). The absolute limit seems to have been six such radars, and that was rarely reached.

The key perception in Aegis was that an electronically scanned radar could both search the sky and track individual targets precisely enough for fire control. It could do so because the beam, which would normally sweep the sky, could be reformed automatically to focus briefly on a target once the target had been detected. A conventional rotating search radar could not possibly do that, because it could not suddenly stop and reverse to go back to where a target had been seen. By flipping back periodically, the electronically scanned radar could track targets. It could also track missiles the ship fired. The track data would be obtained automatically. Only a computer could control the radar and make sense of the data it produced.

That data in turn would form a picture of the air situation around the ship – what is often called a “tactical picture.“ By the time Aegis was conceived, in the late 1960s, the U.S. Navy already used computer tactical pictures in its Naval Tactical Data System (NTDS). It took a computer to keep up with a rapidly changing situation, displaying the result to decision-makers. However, a tactical picture in a computer’s memory offered further possibilities. Both NTDS and Aegis were conceived as antiaircraft systems, NTDS emphasizing fighter control and Aegis missile control.

In both cases, a key question was whether a particular weapon (fighter or missile) could or should be used against a particular target. That could be answered by a computer program with access to the tactical picture in the same computer’s memory. The more powerful the computer, the more complex the tactical questions it might answer, generally in the form of what-if combinations. The great difference between NTDS and Aegis was the precision of the tactical picture – it took a lot less precision to control fighters, because fighters carried their own radars. Generally, the NTDS picture was formed on the basis of longer-range search radars.