New Canadian F18 Hornet fighter planes will fly missions from bases near the Arctic Circle for the first time next year as part of joint U.S.-Canadian actions to modernize and strengthen the obsolescent tripwire air defense system guarding North America from Soviet attack.

Enthusiastic young fighter pilots are in training here now for the new mission. But it makes other Canadians ask whether their country is becoming more tightly bound to U.S. military strategy and therefore more vulnerable to direct involvement in the nuclear arms race.

Military flight test officials here said F18 fighters had been "cold-soaked" at 40 degrees below zero in Yellowknife, one of the three bases in the farther north, last winter as part of the preparations for the exercises in the High Arctic region and that they were "pleasantly surprised" when the planes started without difficulty.

The military maneuvers are scheduled to coincide with negotiations for a five-year renewal of the 28-year-old North American Aerospace Defense (NORAD) pact between Canada and the United States.

The normally routine talks are likely to be complicated by concerns in Canada's influential peace movement that the air defense agreement unwittingly could commit Canada to involvement ultimately in President Reagan's Strategic Defense Initiative, popularly known as "Star Wars."

A 3,000-mile string of radar stations was built in Canada's far north in the 1950s to detect any oncoming Soviet bomber attack. But that early-warning line is generally conceded to be obsolescent and fragmented, in need of modernization.

Advocates of arms limitation say they suspect, however, that a renovation could result somehow in antiballistic missile weapons eventually being positioned on Canadian soil.

Secretary of State George P. Shultz sought to dispel those anxieties on a visit to the Canadian west last week, saying Star Wars and the ongoing upgrading of North American air defenses are separate matters.

But other Reagan administration officials acknowledge privately that they cannot give Canada any unequivocal assurances that the Star Wars plan, which is still largely undefined, will not encompass NORAD in the future.

NORAD has taken on renewed importance now however, because of the development of cruise missiles and low-flying Soviet bombers, similar to the U.S. B1. These planes are capable of eluding conventional radar and striking with missiles from as far away as 1,500 miles.

The United States has pressed Canada, which spends less per capita on defense than any other member of NATO except Luxembourg, to help shoulder the cost for renovating the air defense system. During former prime minister Pierre Trudeau's last days in office, Reagan administration officials privately rebuked his government for its "shameful" contribution.

U.S. officials, however, did prevail on Trudeau to modernize Canada's small Air Force of aging, outmoded fighters through purchase of 138 F18s to replace aging F101 and F104 fighters. They also persuaded him to buck public opposition and allow testing of cruise missiles in Canada as its equivalent gesture to the deployment of cruise and Pershing II missiles in Europe.

The current prime minister, Brian Mulroney, who has professed a desire to increase Canada's defense commitments, agreed last March during talks with Reagan in Quebec that Canada would pay 40 percent of the estimated $1.5 billion cost to overhaul the outmoded Distant Early Warning (DEW) radar line.

But Mulroney, faced with a big budget deficit and strong opposition to shifting funds from Canada's elaborate social and medical programs, is not now regarded as likely to expand defense spending substantially.

That DEW line project is part of an elaborate effort that is aimed at creating a "cocoon of microwave coverage around North America," according to Maj. Gen. Robert W. Norton, Canadian regional commander of NORAD at the base here.

The heart of the system in times of war and crisis will be U.S. F15 interceptors and E3 Airborne Warning and Control System (AWACS) aircraft. The decision by Canada to participate directly in the effort by basing its F18s in the far north is "a simple statement of nationhood," Norton said.

The reference was to Canadian unease over protecting the sovereignty of a vast and largely undefended country, the second largest nation in land area after the Soviet Union.

Those concerns boiled over last summer when the United States sent a vessel through the Northwest Passage, which Canada regards as its sovereign territory, without seeking formal permission from the Canadian government.

Apart from participation in NORAD, Canada's defense of its frozen, sparsely settled Arctic region has been largely symbolic, including weekly overflights of radar and communications stations and token patrols by a widely scattered 1,500-member militia of Canadian Rangers, a subunit of military reserves that is recruited from among residents who carry small arms.

Canada's all-volunteer armed forces have remained, under Mulroney, at about 83,000 personnel in a population of 25 million.

The "partial squadron" of Canadian fighter planes that is to be sent north next year, from this and other bases 1,500 miles away, will be stationed at airfields around Whitehorse, Yellowknife and Frobisher Bay, Norton said. Military officers said about four to six planes will be at each site.

Norton said support equipment would have to be brought in for the exercises and two or three more airstrips probably would have to be built eventually. He said Canada does not plan to station the fighters permanently at the northern bases. "What is envisaged is we would have selected airdromes in northern Canada from which we could very quickly set up an operation with the CF18s," he said.

Here in this pine-forested, rolling terrain of lakes, rivers and abundant wildlife, the United States tests cruise missiles and British, Canadian, U.S. and West German squadrons engage in computer-simulated air battles with mock attackers.

Long criticized by allies for making only a paltry military effort, Canadian officers told reporters on a tour here that Canada contributes significantly to NATO by allowing the open spaces to be used as an immense playing field for rehearsing aerial dogfights, strikes against "enemy" airfields and testing aircraft that fly as low as 50 feet.

The Canadians are clearly eager to do more of that. They are discussing with Spain and Australia the possibility of their making similar use of the terrain and are also making a concerted effort to persuade NATO to establish a similar base at the World War II-vintage Goose Bay, Labrador, air field on Canada's eastern coast. The base currently is underused and a financial burden for Canada to maintain.

Military planners envision a flying area for that hoped-for NATO facility of 32,000 square miles -- over sparsely populated settlements and the world's largest herd of caribou in Labrador and northern Quebec.

"It allows us to make a contribution to NATO, which is one of the best we could make," said Maj. James Ongman of Canada's directorate of air plans. "It means that airfield will remain in Canada for a long time," he said.

Canada is in competition with Turkey for NATO's first integrated tactical fighter training facility. Canadian officers said they did not expect NATO officials to decide where they will locate the base before next summer.