Israeli warplanes repeatedly penetrate into the northwest corner of Saudi Arabia, without encountering resistance, near the important Saudi air base and military installations at Tabouk, according to knowledgeable sources here.

The Israeli planes face no Saudi early-warning radar system to detect their violation of Saudi airspace, Saudi and foreign sources said, and thus can be relatively sure they will not be spotted soon enough for Saudi air defenses to oppose them. There has been no known clash because of the Israeli overflights.

A high Saudi intelligence official underlined that such vulnerability with respect to Israel is a major reason for the kingdom's decision to press for purchase of Airborne Warning and Control System radar planes from the United States, although threats from Iran and Afghanistan across the Persian Gulf also are an important motive.

A White House assertion that the Saudi government wants the sophisticated U.S. aircraft mainly for protection against attacks from across the gulf is untrue, he added, because the kingdom is trying to build a surveillance system to warn of threats from any direction, including Israel.

Although the royal family has refrained from comment on the assassination of Egypt's president Anwar Sadat last week, that dramatic reminder of Middle Eastern instability is likely to increase Saudi determination to seek the best security equipment money can buy.

The Saudi source, himself a member of the royal family, recalled that one of the two groups of Israeli F16s and F15s that bombed Iraq's nuclear reactor near Baghdad last June streaked unscathed through Saudi airspace, passing near Tabouk on the way and the way back. The Israeli planes were spotted only by watchmen on the ground too late to send up any resistance, he added.

The air base at Tabouk is protected by U.S.-made Hawk ground-to-air missiles, a knowledgeable source said, but the Hawk system's radar does not "see" far enough to detect penetrations along the coast of the Gulf of Aqaba or, farther south, the Red Sea.

Israeli jets entering Saudi skies from the Gulf of Aqaba thus escape detection except by ground spotters as long as they remain at a safe distance from the air base. As far as is known, there has been no political decision by the royal family in any case to try to stop the overflights.

With the bulk of the Saudi Air Force and other military on the other side of the kingdom, particularly near Dahran, it is unclear why the Israeli government sends its jets into Saudi Arabian airspace. According to reliable foreign experts, there is little at Tabouk that requires repeated aerial photo reconnaissance.

"It might just be a question of space," one source said. "It's something they Israelis lack."

The rhythm of the Israeli violations is uncertain. Saudi and foreign sources here indicated that they are irregular but frequent enough that the Saudi military no longer considers them a major occurrence -- just a steady irritation.

"The Israelis do it all the time," said a European diplomat. "They are always down there."

Although occasional mention of the overflights turns up in the Arab world press, they are not regularly reported by the Saudi government and there have been no known public complaints. Similarly, the Saudi government remained silent late last month when an Israeli missile boat ran aground on a Saudi beach and the incident was settled through U.S. intermediaries.

Such discretion contrasts sharply with the attitude of Palestinian guerrillas in Lebanon, who regularly report Israeli reconnaissance flights and underline the violation of Lebanese airspace. It fits in with Saudi ways, however, and also spares the kingdom the embarrassment of admitting its vulnerability to an Arab world looking to it increasingly for leadership.

The Saudi rulers' neighbor and fellow monarch, King Hussein of Jordan, also refrains from public condemnation of Israeli penetrations into Jordanian airspace, which are reported by U.S. diplomats in Amman to be a frequent occurrence to the irritation of the outgunned Jordanian Air Force.