The Patriot air defense system was designed to protect relatively compact strategic installations, not entire cities, and to knock out but not necessarily obliterate incoming missiles.

Officials familiar with the system made these observations yesterday in explaining why an interception of an Iraqi Scud missile by a Patriot could nevertheless result in damage and injuries in densely populated areas.

The system was intended as a shield for isolated airfields or radar or communications sites where debris would do little harm, an official said. In a densely populated area, like Tel Aviv or Dhahran, an incoming missile knocked off course or only partially destroyed would do substantially more damage.

While a direct impact with a Scud's warhead would likely leave only small fragments to drop to the ground, large pieces of debris remain when the Patriot makes a less precise hit or explodes only in the vicinity of the incoming missile, which sometimes occurs.

With just seconds to compute a trajectory and launch, the Patriot system sometimes is able only to knock an incoming target off course, as happened Tuesday when a deflected Scud slammed into Tel Aviv, apparently causing the deaths of three people from heart attacks, and injuring scores more.

Also working against the Patriot is its original design as an antiaircraft system. The large size and relatively slow speed of aircraft make them far easier targets than Scuds.

In the past few years, contractor Raytheon Co. modified the Patriot's software program and conventional explosives warhead to give it anti-ballistic missile capabilities, but the system was never intended for more than "low-density engagements," said Stephen Meyer, a Massachusetts Institute of Technology professor of defense and arms control.

The warhead alterations also were not radical enough to enable the Patriot to pulverize a tactical ballistic missile, an Army official said.

At 17 feet long and one ton, the Patriot is much smaller than the 41-foot, eight-ton Scud. "There's just not enough bird there to take care of the whole Scud," the Army officer said. "It's like David going up against Goliath. We're very accurate with our shots, but you just can't destroy the entire Scud because there's too much of it."

The Scud's high speed, roughly twice that of the Patriot's, also makes a direct hit difficult. The Patriot's radar likely picks up an incoming Scud, traveling at more than a mile a second, when the Scud is roughly 35 to 50 miles away, Meyer and others said. Satellites that detect heat from the Scud's launch would provide additional warning, but in either case, the Patriot's computer has no time to spare in analyzing the Scud's trajectory, calculating a planned intercept point and launching the missile. Once airborne, the window for a direct hit is only a split second.

A Patriot missile fired at an incoming Scud is guided to its target by a sophisticated information network.

1. The Patriot and a ground radar system track the Scud. The Patriot sends its data to the ground radar for computer correlation, and receives course corrections in return.

2. A ground-based computer determines distance and speed of the enemy missile, then programs the Patriot to blow up as closely as possible to the Scud. Ideally, detonation occurs within a few feet.

3. When the Patriot explodes, it sends more than 300 metal fragments -- each about 1 inch long -- into the air. The combination of the explosion and the flying metal chunks, is designed to incapacitate the Scud.

4. While a Patriot may successfully explode in front of a Scud, large chunks of the eight-ton enemy missile can remain intact, falling on population centers and causing extensive damage. Compiled by James Schwartz -- The Washington Post