For more than a year, the cameras in America's spy satellites have been regularly trained on three empty launch silos at widely separated missile test ranges in the Soviet Union.

The holes have remained vacant since they were completed in early 1979, but one day soon, U.S. intelligence analysts believe, they will be filled and used to test the first examples of a new -- and fifth -- generation of Soviet rockets.

If present U.S. intelligence estimates hold true, the new tests could have an effect on North Atlantic Treaty Organization politics, the strategic arms limitation treaty (SALT II) and -- should the tests come before November -- the U.S. presidential election.

Key Pentagon and civilian intelligence analysts expect:

A new Soviet intermediate-range missile deployed for use against China and Western Europe.

A new, solid-fueled ICBM that could be the Soviet answer to the proposed U.S. MX missile.

More accurate Soviet warheads that could cover a wider range of U.S. targets.

The most controversy could develop over an expected Soviet testing of a new intermediate-range ballistic missile to be targeted against China and Western Europe. One civilian analyst said recently that too much has been made of the SS200, an IRBM that was used as the reason for NATO's decision last December to introduce new American long-range missiles into Europe.

"The SS20 does not have the accuracy, range or yield," the analyst said recently, "that has been attributed to it publicly . . . and its deployment has been slow, indicating the Soviets have something better on the way."

A Pentagon official described the SS20 in similar terms and added that he thought it was an attempt to put something quickly in position for possible use against the Chinese. The SS20, he noted, consists of two stages of a longer-range mobile SS16 ICBM that the Soviets agreed to discard under the SALT process.

When a "really new" Soviet ICBM appears, the Pentagon official added, "we will have some explaining to do to people who were convinced the SS20 was really new."

Each year, the U.S. government engages in a public debate on the growing number of Soviet missiles and their explosive power as compared with the U.S. force.

The vast differences between the ways the two superpowers develop, produce, test and maintain their ICBM systems are never part of the discussions. Nonetheless, many analysts say why the Soviets build new missiles is as important as the numbers in measuring the nuclear arms race.

Take the differences in ICBM building.

The United States develops one model at a time, putting each system up for bids from any rocket-building company. Since the early 1960s, three versions of the Minuteman missile have been built. Today a new ICBM, the MX, is under development.

The Soviet system is entirely different. And the system as much as anything else is responsible for the fact that the Soviets have produced more than a dozen different ICBMs during the time the United States designed only three.

At its heart are what the Soviets call the missile design bureaus -- organizations gathered together normally under the direction of one top scientist. The bureaus compete with each other, developing different types of missiles and trying to sell them to the Soviet military. Once a design is finished, they immediately move on to the next.

Each bureau develops a specialty and the rockets its designs and builds over time normally follow a pattern.

M. K. Yangel, until his death, fathered the big rockets. His SS7 was followed by the giant SS9, which now has given way to the equally large SS18.

V. N. Chelomei developed smaller but more accurate land-based missiles, first the SS11 and later the SS19. Chelomei's specialty, however, has remained naval missiles.

A relative newcomer, N. Adiradze, has focused on solid-fueled missiles since, unlike the United States, all the other Soviet missiles are liquid-fueled. His SS13 was produced in small numbers while the follow-on, mobile SS16 was tested but never deployed, and finally outlawed by SALT II.

Although both President Carter and Soviet President Leonid Brezhnev signed SALT II in June 1979, neither country has ratified the pact. Both nations, however, have to date abided by its provisions.

In the numbers of launchers, warheads and amount of deliverable power, the present 1,400-missile Soviet landbased strategic force already far outdistances the 1,054-missile U.S. ICBM force.

Why, then, are the Soviets planning to test even more?

The answer, U.S. analysts say, is partly practical necessity. Most of the current Soviet ICBMs, highly respected as they are, have weaknesses in guidance systems or nuclear material use for maintenance (since they are almost all liquid-fueled), that Moscow is anxious to cure.

Mainly, however each new generation of Soviet ICBMs has been turned out by the built-in momentum of the bureaucratic design system. Much as General Motors, Ford and Chrysler turn out new models once the old ones are finished, so the Soviet missile builders go on in much the same fashion.

Thus U.S. analysts have confidence that as the eight year since the last tests nears, the new generation is bound to appear.

What the analysts are far less certain of, however, is what the new missiles will be like.

One development generally being ruled out is a sharp expansion in the number of separate bombs -- called reentry vehicles -- carried by a single giant Soviet rocket.

But there is almost universal belief that the missiles will introduce some new Soviet guidance system -- one that will bring their accuracies closer to those now built into U.S. missiles.

If the Soviets adhere to the SALT II agreement, only one of the new models will be substantially different from the ICBMs now deployed. But some analysts believe a second design may be a somewhat smaller version of a current missile. The Soviets, they point out, requested during the SALT negotiations that they be permitted to reduce the size of an existing missile by 15 percent without having it called their one new missile.

In the end, the treaty allowed the size reduction only if it were accompanied by a drop of one reentry vehicle from the number the existing missile carries.

Eight years ago, the Soviets began testing the giant SS18 ICBM in the midst of the 1972 U.S. presidential campaign. The Nixon-McGovern race was unaffected by those tests even though most government experts were surprised by the size of the rockets, particularly since SALT I had just been signed and ratified.

In 1972, however, word of the Soviet tests hardly got beyond the U.S. intelligence community.

This year, however, if the Soviets hold to their expected testing pattern, the political implications are bound to be different.

The Reagan forces have links to the U.S. intelligence establishment and word of the first test will reach them within "hours," according to one Pentagon analyst. Any Soviet deviation from SALT II limitations will immediately be made an issue, one Republican congressional staff aide vowed.

The United States and the Soviet Union follow different courses in their testing of land-based missiles once they have been built and deployed to operational silos.

Each year, the U.S. Strategic Air Command selects 10 to 12 of its deployed 1,000 Minuteman missiles, takes them out of their silos and ships them and their crews to Vandenberg Air Force Base, California.

There the missile is placed in a special test silo. The nuclear warhead is removed and sent to a Department of Energy laboratory of reliability testing. A simulated warhead is put in the nose of the missile and it is fired out to the Kwajalein lagoon in the Marshall Islands, site of a sophisticated U.S. missile test center.

No U.S. land-based missile has ever been fired from operational silos -- the last attempt to do that being stopped by congressional members of the far-western states who didn't want the Minutemen fired over their constituents.

The Soviets, on the other hand, fire an average of 20 test shots a year with each of the many types of missiles they have currently deployed. Many are fired from their operational silos as part of their program to give crews experience in handling the missiles.

One reason for the extensive testing is that the Soviet missiles -- except for the SS13 are liquid-fueled. And though liquid propellants provide a steadier, more efficient fuel, they are also more volatile and need to be tested more often for reliability.

In a year's time, according to intelligence sources, the Soviets will test upwards of 100 operational model ICBMs while the United States will test a dozen or fewer. It is one of the reasons the Soviets maintain much larger reserve stocks of their missiles as compared to the United States.

Another key difference in the U.S. and Soviet ICBM systems is the number of missiles each side keeps on operational alert.

Though the exact U.S. figure is highly secret, 80 percent to 90 percent of the U.S. Minuteman force is supposed to be ready to go at any time. All the operational silos are manned 24 hours a day.

The Soviets operate at a far lower alert rate. One analysts put it around 25 percent of their force.

This low rate, one analyst said, began when the first Soviet ICBM, the SS6, was propelled by a liquid fuel that was unstable and had to be pumped into the missile just before firing.

Even though their current liquid fuel can be stored in the missile over long periods of time, the alert rate has not substantially increased.

"They don't seem to fear a bolt from the blue," was the way one analyst describe the situation.