In its first combat test, the Patriot air defense system did precisely what it was supposed to do: It tracked and identified a Scud missile headed for an allied air base in Saudi Arabia Friday morning, sending its own missile to a pinpoint interception that destroyed the intruder.

The pride and joy of the air defense community, the Army's Patriot missile works like no other surface-to-air system.

On the ground, the system's centerpiece is a "phased-array radar," an advanced type whose face consists of about 5,000 individual antenna elements that are designed to detect and track multiple incoming aircraft or missiles.

Once a target is located, computers -- or technicians housed in a mobile "engagement control station" near the radar array -- make the decision when and where to fire the Patriot.

After the Patriot is airborne, flying at more than three times the speed of sound, the ground radar continues to send out pulses.

The reflected signal is received by both the ground station and the missile. The cruising Patriot sends a constant stream of this data to the ground, where computers almost instantaneously compare the signals received in the air and on the ground.

Based on these correlations, the ground installation then transmits flight path adjustments to the missile, where an on-board computer alters the course to help the Patriot intercept its target, up to 50 miles away from the launch point.

Military experts rave about the Patriot as an example of complex technology that seems to work. In 15 test firings against ballistic missiles since 1986, the Patriot intercepted its target in every case. Contractor Raytheon Co. in Lexington, Mass., says the Patriot's computers and radar are twice as reliable as required by the Army.

But despite the initial wartime success in Saudi Arabia, the system has yet to prove itself in combat when faced with multiple approaching missiles or aircraft.

"One at a time is not the real challenge {for} the system," said William Taylor, vice president of political-military studies at the Center for Strategic and International Studies.

Nearly 20 years in development and production, the Patriot was conceived as solely an antiaircraft system.

As new threats emerged in Europe, and the debate over the Strategic Defense Initiative, or"Star Wars," increased concerns about the effectiveness of defense systems, the Patriot was modified to be used against both cruise and tactical ballistic missiles. The latter capability was added in 1986.

The program was dogged in early days by cost overruns and malfunctioning radar, and the first battery of Patriots was deployed behind schedule in 1985 in Europe.

With the onset of the Persian Gulf crisis, Raytheon received a $513 million Army order for the sale of Patriot batteries to Saudi Arabia.

Although the Pentagon supplied Patriot systems to Israel in early January, a Raytheon official said the Israelis had been unable to train technicians quickly enough to operate it. Other sources, however, said the Israelis lacked confidence in the system.