he Soviet Union is sending as many as 1,000 troops to help operate the new SA5 long-distance, surface-to-air missiles that Syria is receiving to improve its air defense system wiped out by Israel last June, according to diplomatic sources.

The troops' arrival will double the number of uniformed Soviet military personnel in Syria, which since October 1981 has been linked with Moscow by a treaty of friendship and cooperation.

Syrian motorists recently have reported seeing Soviet troops in uniform in camouflaged trucks traveling from the Mediterranean port of Tartus, where Soviet equipment arrives, to the city of Homs.

The SA5s--which the Soviets reportedly never have supplied previously to another country--provide the Syrians with the range needed to hit high-flying Israeli reconnaissance and airborne control aircraft that proved so useful in the June dogfights.

The state-controlled media recently have warned consistently that Israel may seek to destroy the antiaircraft missiles. If the Israelis attacked the missile sites, there would be a risk of Soviet casualties.

Even with the new missiles, replacement of some of the 90-plus aircraft lost last June and Soviet resupply of armor, the Syrians appear genuinely afraid of a fresh Israeli attack. Officials said that the 30,000-man Syrian force in Lebanon has been given orders to keep on maximum alert.

It is unclear whether the Syrians exaggerate their fears of an Israeli attack to justify obtaining the Soviet-made equipment. In some diplomatic quarters, the Syrians are believed to be convinced that the Israelis will strike in the spring when the fertile plains of the narrow Bekaa Valley in eastern Lebanon have dried and could support tank operations.

Syrian and Soviet media have played up suggestions that the Israelis recently have moved more tanks and 155-mm and 175-mm long-range artillery into the southern half of the Bekaa Valley where the bulk of Syria's 30,000-man force in Lebanon is stationed. Western intelligence sources were unable to determine whether the Israelis have brought in reinforcements or were simply rotating men and materiel.

According to another possible scenario, Israel might launch preemptive air strikes against the SA5 sites, at least two of which have been identified. One site is located near Homs and guards Syria's industrial heartland and the entire length of the Bekaa Valley. The other site is located northeast of Damascus, and preliminary intelligence reports suggest that a third is being constructed near the Jordanian border.

Diplomatic sources said that the Syrians do not appear to be looking for a fight with the Israelis but rather wish to increase their degree of preparedness.

Specialists said the SA5s--which have an effective range of 150 miles and can operate up to 95,000 feet--could help neutralize the Hawkeye and other electronics-packed, U.S.-supplied aircraft credited with wiping out Syrian air defenses last June 9 and 10. Israeli missiles homed in on the Syrian missiles' radar, and the Syrian Air Force lost about a third of its 270 Soviet-supplied fighters in the debacle.

Syrian officials have justified acquisition of the SA5s--and updated versions of the SA2s, SA3s and SA6s destroyed in the June fighting--by insisting that Syria is seeking to restore partially the strategic balance with Israel. The Soviets are far from having replaced all the materiel destroyed last year, when the Syrians lost hundreds of tank crews and about 60 pilots. A combat pilot requires five years of training.

The bitter recriminations that characterized Syrian-Soviet relations during the summer have subsided, and no longer do Syrians accuse the Soviets of having failed to provide materiel matching the U.S. equipment used by Israel. The Soviets have ceased complaining that the Syrians incorrectly utilized the more than adequate supplies of Soviet weaponry available to them. Even the Israelis have toned down some of their more boastful claims.

If hostilities do resume, specialists suggested that the Syrians would likely seek to avoid having their main battle force chewed up by the better equipped Israelis. Syria instead would seek to inflict the maximum number of casualties on the Israelis, who already are questioning the cost of their military involvement in Lebanon.

Disputes over the future of southern or eastern Lebanon could provide a trigger for confrontation. Israeli tanks and troops face the Syrians across a no-man's land in the Bekaa, and talks on a withdrawal of the two sides' forces have made little progress.

Syria has not formally stated its policy on the future of Israeli forces in southern Lebanon. From what diplomats have been able to piece together, however, they believe that Damascus is likely to tolerate Israeli demands for a 25-to-30 mile deep security zone on the condition that the Lebanese Army, rather than the Israelis or their Lebanese militia surrogates, are in charge of thwarting cross-border attacks on Israel.

The Syrians also are believed to be willing to allow international peace-keeping troops now stationed in Lebanon to patrol the security zone if they were there temporarily and under Lebanese government control. Israeli requests for early warning stations have drawn Syrian criticism and renewed statements about the "interdependence" of Syrian and Lebanese defenses.

Syrian officials, convinced that the United States is unwilling or unable to force Israel to withdraw from Lebanon, see no reason for hastening their own departure.

Relations between Damascus and Washington, frozen during Alexander M. Haig Jr.'s 18 months as secretary of state because of what Syria considered his unstinting hostility, have improved to the degree that contacts have resumed and become routine. But there has been no narrowing of the gap of policy differences despite indications during the summer that Syria might be seeking closer relations with the United States.

Initial Syrian interest in President Reagan's Sept. 1 peace initiative has given way to skepticism largely because of Washington's inability to bring about Israeli troop withdrawal from Lebanon.

In addition, Syria's President Hafez Assad looks askance at U.S. efforts to persuade longtime rival King Hussein of Jordan to enter peace talks on the future of the Israeli-occupied West Bank and Gaza Strip with the tacit blessing of the Palestine Liberation Organization's leadership. Syria traditionally has feared that the United States, the Palestinians and Hussein would strike a deal with Israel that would leave Syria isolated.