Libyan ruler Moammar Gadhafi has upgraded his air defenses following the U.S. raid on his country, adding a second SA5 missile site at Benghazi on the Gulf of Sidra. But otherwise he seems unable to rejuvenate his military, U.S. officials said yesterday.

The Soviet Union so far has sent Gadhafi only limited amounts of new weaponry and equipment.

The Soviet emphasis, officials said, has been on improving Gadhafi's antiaircraft radar warning and fire control network, not on massive shipments of missiles.

By contrast, Syrian gunners had so many Soviet missiles piled up during the Dec. 4, 1983, U.S. bombing raid on Lebanon that they downed two Navy planes by firing barrages of missiles.

Defense officials said that the United States, the Soviet Union, Syria and Israel are studying the offense-defense tactics during two U.S. attacks on Libya for keys to how bombers might survive against modern antiaircraft missiles.

Libya fired SA5 missiles at Navy planes on March 25 without any success, and shot down only one of 25 attacking Air Force and Navy bombers in the night raid of April 15.

The U.S. European command, headed by Gen. Bernard W. Rogers, has ordered a "lessons learned" audit of the night bombing raid -- which is standard procedure -- while other assessments apparently are under way in the other countries, officials said.

Israel's prime interest, according to Americans who have talked to their military leaders, is the electronic warfare tactics employed -- tactics that the Israelis might add to their bag of tricks for any future bombing of Syria which, like Libya, has deployed Soviet antiaircraft missiles.

Shortly after the U.S. attacks, Gadhafi seemed to be moving with vigor in questioning his military leaders as to why his air defenses had proved so ineffective, officials said. Intelligence officials expected that Gadhafi would clean house within his high command, officials said. But this apparently has not happened.

Gadhafi, officials said, seems to be wandering listlessly from one summer residence to another rather than rebuilding his tarnished military forces.

The Soviet Union and Syria are gambling heavily on the ability of the SA5 to knock down invading aircraft at distances of more than 50 miles. The Soviets have built 100 SA5 sites in their country with more than 2,000 missiles, according to the International Institute for Strategic Studies, while Syria has deployed at least 48 SA5s along with other Soviet made antiaircraft missiles.

Soviet technicians not only have been attempting to determine why their missiles did not perform better in Libya, but also helping to install more of them and improve their command and control systems, officials said.

The original SA5 site at Surt, which Navy bombers knocked out of action with radar-seeking Harm missiles during the Gulf of Sidra operations, is back in service with all 12 launchers ready to fire, officials said. The second site at Benghazi is partially operational, they said, and is expected to be a duplicate of the one at Surt, providing wide coverage of the Gulf of Sidra.

Syria sent a delegation of air defense experts to Libya after the first Navy attacks in the Gulf of Sidra when SA5 missiles fired from Surt fell harmlessly into the water, officials said.

It is not known whether the Syrians concluded the Libyans or the missiles were at fault, although the Soviet improvements to the command and control equipment suggest that the way they were fired was suspect, officials said.

Syria had sent a small group of fighter pilots to Libya, but none of them was used during the March and April strikes.

The United States intercepted communications of Libyan or Syrian pilots refusing to take off during the April 15 raid, according to military sources.

U.S. pilots were astonished at how unprepared Libyan defenses were during the April 15 night attack.

The Libyans even left runway lights on while airfields were being bombed.

An Israeli air force leader said the U.S. electronic jamming of Libyan fire-control radar during the night raid proved so effective that it indicated the West, although outnumbered in planes and antiaircraft missiles, would be able to penetrate Soviet-made air defenses in Libya, Syria or the Soviet Union with acceptable losses.