A solemn President Carter arrived here tonight for his first U.S.-Soviet summit meeting and pledged to "enlarge cooperation and understanding" between the superpowers and reduce the dangers of nuclear war.

After an eight-hour flight from Washington, Carter stepped out of Air Force One to tarmac reception by Austrian officials, who are scheduled to meet Soviet President Leonid Brezhnev at the same spot Friday morning.

Carter's brief remarks stressed the importance of the Strategic Arms Limitation Treaty (SALT II), which is to be signed here, as a step toward international security and peace.

He was careful, however, not to raise expectations for a new era of broad Soviet-American accord.

It was left to Austrian President Rudolf Kirchschlaeger, speaking "as one of the about four thousand million human beings in this world," to express fervent hope that the Carter-Brezhnev meetings will lead to greater trust between the world's most powerful nations and thus contribute to "detente" and "a reduction of armaments."

Speaking at planeside in Washington and on arrival here, Carter avoided the use of the word "detente" and spoke more of improved communications than of anticipated amity.

He approaches the long-awaited meeting with Brezhnev "with hope, but without false expectations," he said as the journey began.

Saying that there are "significant differences" between the two nations, Carter declared that "we will make clear to the Soviet Union our views and our purposes in the world so that there can be no dangerous misunderstandings as we pursue our separate courses."

The emphasis on differences and "separate courses" ahead was a far cry from the expression of soaring hopes and good feeling that accompanied the signing of the SALT I treaties by President Nixon and Brezhnev seven years ago.

Unlike Nixon, Carter faces an arduous struggle to obtain Senate ratification of his arms control agreement with the Soviets. He is under heavy attack by some anti-Communist and conservative elements and has been charged with "appeasement" by Sen. Henry M. Jackson (D-Wash.).

Every statement and gesture during Carter's summit is likely to be affected by the fight at home, leading to emphasis on hard-headed security benefits from SALT II and de-emphasis of political cooperation with the Soviets.

Soviet officials also appear to have modest expectations for accomplishment here. An official in Brezhnev's advance party said the probable achievement, in addition to the signing of the treaty, would be an ice-breaking of sorts with Carter and his senior aides.

"We hope they will see what kind of people we are," said the official.

Brezhnev's schedule for Friday, which includes several events not previously announced, has generated hope that the Vienna meetings Saturday, Sunday and Monday will find the Soviet leader well enough for the substantive talks as well as ceremonial observances.

Soviet officials said Brezhnev, 72, will place two wreaths and attend the opera on Friday as well as pay a joint call with Carter on Austrian President Kirchschlaeger.

Secretary of State Cyrus R. Vance told reporters Brezhnev "has his ups and downs" but that his condition is "improved." Vance said this "will help," apparently referring to the prospects for the summit.

The secretary of state also praised the U.S. negotiating team in Geneva, which completed the last details of the SALT II treaty in talks with the Soviet team there on Wednesday. Vance described as "the unsung heroes of SALT" the Geneva negotiators, who hammer out highly technical issues and put into treaty language the basic political agreements of their U.S. and Soviet superiors.

Vance said the final technical items resolved in the negotiations, which lasted nearly seven years, involved the definition of cruise missiles and a section dealing with the release of re-entry vehicles, or separately targetable nuclear warheads.

Informed sources said the final compromise on the latter issue would prohibit the Soviets from testing procedures for the launching of more than 10 live war heads from a single missle. They demonstrated such procedures in a missile test in December.

The compromise is said to require that the launch of any objects above 10 from a single missile be clearly distinguishable to U.S. rader as dummy warheads or chaff, so that there could be no cause for concern that the SALT rules are being violated.

Ralph Earle, the chief U.S. negotiator at Geneva, is flying here Friday with the completed SALT II treaty. The document is to be signed by Carter and Brezhnev at mid-day Monday.

Carter is expected to spend most of the day Friday resting and conferring with aides. He is to meet Brezhnev for the first time during the protocol call on the Austrian president. Later the U.S. and Soviet leaders will attend the Vienna State opera's performance of Mozart's "Abduction From the Seraglio."

Carter's substantive discussions with Brezhnev and the Soviet summit delegation are scheduled to take place during the late morning and late afternoon on Saturday and Sunday, and before the signing of the SALT treaty at 1 p.m. Monday. CAPTION: Map, U.S. Verification System, By Richard Furno - The Washington Post