While President Reagan's "Star Wars" vision of a future nuclear defense has turned most eyes toward space, the Army has been pressing ahead with a much more pedestrian ground-based system that officials say could be in place to protect missile fields from incoming Soviet warheads as early as the late 1990s.

The Army's program, 19 years old, has popped only occasionally into public view. The most recent glimpse was last June, when the service announced with some fanfare that it in effect had intercepted and destroyed one missile warhead with another in the atmosphere.

Sometime in the next 18 months there likely will be another glimpse, when the Army plans to send a new sensing device into space from Shemya Island off Alaska.

The device will be sent aloft when the Soviets fire a test missile of their own. The idea is to see how well our sensor can track their warheads.

The Army plan has now been incorporated into the president's Star Wars program, or Strategic Defense Initiative, and is in a way its basis. The Army ballistic missile defense office (BMDO) this fiscal year will take up almost half of the $1.4 billion that Congress approved for the SDI.

More important, the Army weapons are relatively close at hand, while it will take seven to 10 years to determine the feasibility of space-based weapons.

The United States may thus be closer to a working defensive system than generally has been realized -- and the Army weapons are likely to be an important part of the forthcoming defensive arms talks in Geneva.

Lt. Gen. James A. Abrahamson, director of the SDI program, told Congress last March that "I would expect that sometime prior to . . . 1990 . . . we could begin to see that some of these things may indeed be deployable."

And a spokesman said more recently that the Army hopes to have "a technology demonstration" of its weapons to "support decision-making on an overall ABM or anti-ballistic missile system" by 1990.

The spokesman said the Army is at work on these devices:

* The rocket-launched sensing device to be sent up from Shemya, which was called DOT (for distant optical tracker) when initially tested. It is designed to give early information on incoming warheads. It is fired to a spot just above Earth's atmosphere and its infrared telescope relays data to the ground.

* An airplane version of DOT, called the airborne optical adjunct, which sends back the same type of targeting data when the Soviet warheads come closer to Earth's surface.

* A mobile radar system capable of receiving information from DOT and DOT's airborne cousin before it can "see" incoming warheads with its own antenna. This system then sends orders to launch interceptors and helps guide them to their targets.

* New fast interceptor missiles, with non-nuclear kill mechanisms. These interceptors, once launched, have built-in homing devices to help them hit warheads at different heights in the atmosphere.

Also scheduled to be awarded this year, according to the Army spokesman, are contracts to spell out how all these elements might be woven into a single system and to do preliminary work on a new higher-altitude interceptor.

This interceptor is to be a much smaller version of the device that successfully intercepted a warhead last June. The Army wants a dramatic reduction in size, from 1,000 pounds to less than 10, to make it viable as a weapon.

The Army weapons are intended to kill incoming warheads in their so-called terminal phase, as they leave space and enter the atmosphere about 60 miles above Earth's surface.

Officials are pushing them as a way of offsetting the Soviets' current heavy numerical advantage in large land-based intercontinental ballistic missiles.

So far, at least, the administration argues that the Army programs are well within the limits set by the 1972 antiballistic missile treaty with the Soviets.

It says the Soviets, by contrast, are violating that treaty in various ways.

The Soviets contend the opposite.

Under the terms of the ABM pact, the Soviet Union and United States each are permitted only one ABM site with no more than 100 missile launchers.

The United States in the early 1970s had such a system, called Safeguard, consisting of nuclear-tipped missiles that were to be guided to their targets by a giant radar at Grand Forks, N.D. The system was deactivated in 1975 because it was deemed ineffective.

The BMDO weapons would in effect replace it.

The Soviets, for their part, set up a rudimentary ABM system, called Galosh, around Moscow over 10 years ago. They have continued to work at modernization, and since 1980 have been upgrading the Moscow system with new radars and missiles.

In addition to the limits on sites and launchers in the 1972 ABM treaty, the two nations agreed not to "develop, test or deploy" either ground-based systems that were mobile or any components based in sea, air, or space.

The agreement permitted modernization of then-existing ABM systems but called for consultation if either country develops ABM components "based on other physical principles."

Abrahamson told Congress last year that, "Under the ABM treaty, we could in fact deploy 100 fixed, ground-based interceptors as the Soviets have now done."

Abrahamson and Franklin Miller, head of the Defense Department's strategic forces policy office, said the BMDO program, as well as other SDI basic research, fall well within what is permitted by the treaty.

(When BMDO projects run close to the treaty limitations, they are sometimes called experiments rather than tests and demonstrations rather than moves toward development.)

Since 1972, both sides have lodged numerous allegations of ABM treaty violations with a commission in Geneva which was established to work out such complaints. Some have been worked out, others remain as open issues between the two parties.

The Soviets, according to sources, protested the use of Shemya for the planned tracking experiment after The New York Times last May disclosed plans to build the rocket launching facility for the device.

U.S. officials countered by saying DOT launchings from Shemya were permitted under terms of the treaty since they were only experiments for a new ABM system and were gathering data for verification of other arms agreements.

The DOT data, sources said, will differentiate real Soviet warheads from decoys and provide U.S. intelligence with numbers and characteristics of those warheads. That information helps show whether Moscow is complying with current offensive arms agreements.

DOT also will provide infrared pictures of these warheads, which could be put into a computer and used for surveillance, targeting and even kill systems in a possible future U.S. ABM system.

DOT's cousin, the airborne optical adjunct program, gives early tracking and targeting information. It features an advanced infrared system mounted in a modified Boeing 767 airplane and is scheduled to be demonstrated in 1987, according to congressional testimony.

Because the aircraft flies at an extremely high altitude, it can gather data in advance of ground-based radar and turn the data over to a ground-based battle management facility. A less-sophisticated version, mounted in a KC135 aircraft, is already flying missions from the Shemya Island air base.

Another basic element for a new ABM system is a terminal-imaging, phased-array radar that would deliver a clearer, quicker picture of attacking warheads. The BMDO will award a contract for concept development of the radar this year.

Two other missile elements for an ABM system are undergoing tests.

A fast-burning land-based missile, nicknamed SR-Hit (for small radar homing intercept technology), has had three flight tests from White Sands Missile Range in New Mexico and, according to an Army spokesman, could lead to "a non-nuclear kill weapon for use in the atmosphere," probably less than 10 miles above missile sites being defended.

An interceptor, nicknamed HEDS (for high endoatmospheric defense system), would hit warheads 60 miles above Earth's surface, just as they re-enter the atmosphere from space. The "concept definition" contract for this weapon will be awarded in April, with a demonstration scheduled within four years.

Information on all these weapons is available in public records.