The impact of the U.S.-Soviet treaty eliminating intermediate-range nuclear forces (INF), to be signed here Tuesday by President Reagan and Soviet leader Mikhail Gorbachev, will probably be felt first at a military base in the city of Kapustin Yar, where the Soviet Union launched its first ballistic missile 40 years ago.

One of the INF treaty's most unorthodox provisions will unfold at the base, 660 miles southeast of Moscow, within a few days after the pact takes effect, when the Soviets begin launching unarmed SS12 and SS20 missiles eastward virtually around-the-clock just to get rid of them.

Similar launches of powerful, highly accurate Pershing II missiles, without warheads, from Cape Canaveral east over the Atlantic are contemplated by the United States if studies show this is the cheapest, safest way to destroy the $6 million rockets within the treaty's three-year deadline.

The 2,800 U.S. and Soviet missiles to be destroyed under the INF treaty are only a fraction of those in the countries' nuclear arsenals, and their destruction will have only modest impact on the overall nuclear balance. But no previous arms agreement has called for destruction of so many weapons in such a brief period. Because of this and because of the novelty of its on-site verification procedures, the INF treaty is a departure for the superpowers.

Every U.S. and Soviet land-based missile designed to fly between 300 and 3,600 miles will be eliminated under the treaty, including Soviet SS4, SS12, SS20 and SS23 missiles, and U.S. Pershing IA, Pershing II and ground-launched cruise missiles, most of which were produced and deployed within the last decade.

None of the Soviet arms are aimed at the United States. All are pointed at U.S. allies and China. But all of the U.S. arms governed by the pact are aimed at the Soviet Union.

To swiftly wipe out whole categories of medium-range and shorter-range missiles under the treaty's elaborate requirements, both sides will take novel steps and encounter odd headaches.

For example, they will face environmental challenges in burning tons of highly toxic rocket propellant. They will have to arrange for disassembly of perhaps 2,000 modern nuclear warheads and missile guidance mechanisms, and return the radioactive materials to weapons plants or nuclear reactors. And they will have to build special housing for military inspectors to be stationed outside highly sensitive military facilities in both countries for 13 years.

It took six years of halting, fitful superpower negotiations marked by angry denunciations and florid rhetoric in Moscow and Washington to produce the treaty, 31 pages of legalistic text.

Two "protocols" or supplements to the treaty, totaling another 39 pages, spell out in extraordinary detail how the INF weapons must be dismantled and how each side can ensure the other has met its obligations. These procedures call for the most intrusive inspections of any arms pact since the dawn of the nuclear age, costing both governments the equivalent of hundreds of thousands of dollars annually.

"The inspection protocol makes the tax code look simple," one official said.

For example, when the Soviets slice the back end off certain trucks to make them incapable of carrying SS20 missiles, U.S. inspectors will be watching to ensure that the pieces are 39 inches long, not 38. When Soviet SS12 and SS20 missiles are launched from Kapustin Yar, U.S. officials will be there to inspect them beforehand and observe them arcing high above the Kazakh plain.Revealing Locations

In an unprecedented, 100-page burst of candor, the two sides have also provided spare descriptions of every site where INF weapons are produced, assembled, maintained, stored, maneuvered for training and deployed for war. Included are sites in at least eight states (including Maryland), as well as eight European countries and at least four Soviet republics, newly subject to intrusive inspections.

Together with treaty provisions allowing dozens of inspections annually, the detailed disclosure of previously sensitive military data is widely considered one of the treaty's most significant accomplishments.

The disclosure runs counter to a decades-old U.S. policy of refusing to acknowledge or disclose the presence of nuclear weapons anywhere in the world. It also breached Soviet policies that one negotiator said called for anyone who supplied such data to be shot for "committing treason."

This legacy of military secrecy had to be overcome, both sides realized, in order to establish on-site inspection procedures that would eliminate the risk of militarily significant cheating. Concerns about cheating were particularly high because all but a few INF missiles can be trucked from one site to another to avoid detection.

Previous nuclear arms agreements have constrained large, intercontinental ballistic missiles deployed in concrete-reinforced silos, readily identifiable in reconnaissance satellite photos. But the INF weapons are so compact that -- until now -- neither side knew how many the other had or where they were deployed at any given moment.

Secretary of State George P. Shultz wrote Congress last May, for example, that the Soviet Union may have "as many as 200-400 (or more)" SS20 missiles in storage, besides those deployed at known missile bases. U.S. officials said newly furnished Soviet information lists far more than 400 stored SS20s.

The decision to eliminate all INF weapons, those warehoused as well as those deployed at readily observable missile bases, is a calculated risk. The provision blocks either side from hiding hundreds of missile-carrying trucks in warehouses and garages for sudden use in war.

But neither side can offer complete assurance that stored missiles have been eliminated, simply because hiding them is too easy and finding them too hard.

"Nothing is 100 percent perfect," Shultz said last month. "It's possible" for the Soviets to cheat under the treaty provisions, "but I think, if it occurs, it would be in very small proportion."

Chief U.S. arms negotiator Max M. Kampelman said Friday, "You have to ask yourself -- will they cheat? And our conclusion was {that whenever} they can cheat -- and you assume they will -- it won't hurt us. And so we say under those circumstances, we recommend this treaty."

These statements signal an important shift in the administration's position on verification that is symbolized by the INF treaty.

Instead of airtight procedures for verifying Soviet compliance, a goal conservatives have long espoused, the treaty's procedures make cheating difficult, but not impossible. Instead of ruling out any risk of the slightest Soviet violation, the treaty aims to block only militarily significant cheating.

"This treaty is ultimately based on the same verification standards applied by Reagan's predecessors, both Democrat and Republican," said James P. Rubin, assistant director of the private Arms Control Association here. "All the administration's talk about perfect verification turns out to have been empty rhetoric, and rightly so." Initial Inspections

One to three months after INF treaty ratification by the U.S. Senate and the Supreme Soviet, teams of inspectors will visit every site listed in the treaty's 100-page "memorandum of understanding" or appendix to verify its accuracy.

U.S. officials said both sides viewed these preliminary inspections apprehensively, causing each to delay the exchange of certain data to mutual benefit.

In recent weeks, for example, the Defense Department moved items covered by the treaty away from one highly sensitive facility, so that it would not be listed and inspected by the Soviets. U.S. intelligence officials said the Soviets dismantled some INF facilities and moved missiles to new locations to evade U.S. inspections.

"These actions were militarily sound and completely legal," even if they were not in keeping with the pact's new spirit of openness, one official said.

With initial inspections completed, each country will officially declare where its missiles are to be eliminated. The likely U.S. sites are Army bases in Pueblo, Colo., and Tooele, Utah, where the rockets would be burned in specially designed pits.

Alternatively, the two sides could strap the missiles down and simply ignite the propellant, providing a fiery symbol of arms under control. But specialists said these techniques may create too much air pollution, causing both sides to consider destroying the rockets by launching from existing test ranges.

The treaty allows up to 100 rockets to be destroyed this way, falling harmlessly back to Earth. But all launches must be completed within 60 days after the treaty takes effect, and no more than four missiles can be launched each day. The United States insisted on these constraints to block the Soviets from gaining any military advantage from the launches.

No other flight tests are allowed, a constraint that can be readily observed by U.S. satellites and ground radars on the perimeter of the Soviet Union. The aim of this provision, according to Shultz and other officials, is to limit the military value of secretly produced missiles.

"If there are no tests, before long the system becomes obsolete," Shultz said. Watchdogs at the Gate

Secret production of the Soviet SS20 missile at an existing factory in Votkinsk, near the Ural Mountains, is to be blocked by stationing U.S. inspectors outside the main rail entrance to the plant for 13 years. Somewhat like interstate highway inspectors, the Americans will weigh, measure and perhaps x-ray anything large enough to look like an SS20.

In so doing, they will pick up fresh information about another, larger Soviet missile produced at the plant, the SS25, which is technically not covered by the agreement. The Soviets allowed such inspections when the United States agreed to permit the Soviet Union to station a weighing and measuring team outside a former Pershing II production facility in Magna, Utah, which now produces MX and Trident II missiles.

U.S. negotiators persuaded the Soviets to eliminate all shorter-range missiles within 18 months, and eliminate all but 180 deployed medium-range missiles within 2 1/2 years. Otherwise, each side pursues its own schedule, waiting either until the last minute or racing to finish first.

Besides eliminating missiles, the two sides are also required to dismantle and destroy all related missile launching equipment within three years. Initially, the two sides planned to destroy hundreds of mobile missile-carrying trucks, but the Army decided it wanted to keep the tractors used to pull the Pershing IIs through German forests, and the Soviets decided they wanted to give their SS20 flatbeds to civilians.

The two sides then entered protracted negotiations over how much of the flatbeds must be lopped off to make them too short and too weak to carry an SS20, finally settling on one meter.

At the end of the three-year period for eliminating missiles, launchers and support facilities, each side has the right to conduct an additional inspection of the missile bases, factories, storage sites and training areas to ensure that the other has met its obligations.

In the meantime, each country is allowed to send inspection teams to 20 missile bases and related facilities annually to ensure that dismantling is proceeding according to promise. An argument is under way within the administration whether U.S. teams should be run by the military or the Arms Control and Disarmament Agency.

If the inspection is conducted on Soviet territory by the United States, for example, the team will first be sent to Moscow or Irkutsk with measuring devices, radiation detectors and special dual-lens cameras that instantly produce two identical prints -- one for the Soviets and one for the Americans. At least two of the 10 inspection team members must speak Russian.

The Soviets are permitted to inspect the equipment to ensure it has no hidden espionage capabilities. They must then arrange transportation within nine hours to any site the team identifies, weather permitting. Photos on Demand

Once at the site, the Soviets take the pictures, but must photograph whatever the U.S. side demands, an unusual arrangement that reflects lingering uneasiness on both sides about providing unrestricted access. Soviet inspections on U.S. territory are governed by identical rules.

The Soviets agreed to a U.S. demand for 15 such inspections annually for the first five years after all INF weapons are eliminated, and 10 inspections annually for the next five years. U.S. officials believe that any Soviet missiles remaining after 10 years will be obsolete.

Two brief, legally binding notices in the treaty documents will provide U.S. and Soviet assurance that military inspections on the territory of their allies will comply with appropriate national laws. No more than half of the inspections can be conducted in any single country each year.

Conservative critics of the treaty are expected to focus largely on the fact that it fails to permit U.S. inspections of suspicious Soviet activities at sites not listed in the 100-page appendix.

The administration initially advocated U.S. inspection rights anywhere in the Soviet Union, but as the Soviets moved closer to accepting the proposal the administration backed away in fear that the Soviets would abuse a similar right to inspect anywhere in the United States.

Some officials said these fears were overblown, because sensitive equipment could be shrouded from Soviet view. But the administration decided, in the words of senior State Department adviser Paul H. Nitze, it would be "detrimental to U.S. security interests to give Soviet inspectors such unlimited access."

If an apparent Soviet violation is detected at a site outside the treaty terms, the administration has little recourse but to call a meeting of the Special Verification Commission, established by the treaty to resolve U.S. and Soviet compliance concerns.

Pentagon officials resisted establishing the commission because of its similarity to another U.S.-Soviet group, known as the Standing Consultative Commission (SCC), established by the 1972 Antiballistic Missile Treaty.

The administration has resisted using confidential meetings of the SCC to resolve disputes, preferring instead to make public allegations of Soviet cheating. But the new verification panel appears on paper to be little different from the little-used SCC.