U.S. early-warning coverage of Soviet long-range nuclear missiles could erode slightly if the first of a new generation of satellites remains grounded until the end of the year because of Titan missile explosions, according to congressional and administration sources.

A constellation of three Defense Support Program (DSP) satellites, in fixed orbit 20,000 miles above the Soviet Union, now provides the first warning of a Soviet missile launch. The satellites' infrared sensors detect heat from rocket firings on Earth and quickly relay that information to U.S. ground stations.

The current DSP satellites are running out of electricity and are less capable than successors being built. Failure to begin replacing them could mean the early-warning network is "less than ideal" but does not jeopardize the ability to detect a Soviet missile attack, one source said.

The first replacement was delivered earlier this year to the Air Force, which intended to launch it atop a Titan 34D rocket. The launch was delayed while the Air Force tried to determine the cause of a Titan 34D explosion at Vandenberg Air Force Base last August, and then delayed again after a second Titan blew up last month.

The new generation of DSP satellites is needed "to replace those that have failed, worn out or are unable to meet the growing threat," according to congressional testimony by the Air Force two years ago.

The new breed is intended to have longer operational life, greater resistance to nuclear radiation from Soviet antisatellite weapons and to be resistant to electronic jamming while sending data to other satellites and to ground stations.

Experts on the U.S. early-warning satellites said an "ideal" system means having three operating DSP satellites and one reserve in orbit. The three give not only redundant early-warning data but also work together to pinpoint the infrared plume of a missile launch.

In general, the recent epidemic of U.S. space failures -- including the destruction of two Titans, a Delta rocket and the shuttle Challenger and loss of its crew of seven -- has not yet put the Defense Department "in dire straits," one official said recently.

More than 40 U.S. military satellites in nine systems are in space, according to Pentagon sources.

With the exception of U.S. photo-intelligence capabilities, jolted by the loss of two satellites in the last eight months, "we are not unhealthy," one official said. That could change, however, if the Titans and space shuttles cannot be launched again next year, sources said.

Two major new space-based systems face delays. Comprised of "Navstar" and "Milstar" satellites, they are to provide navigation, targeting, nuclear-explosion location and communications services in the next decade.

The first deployments of the 30 satellites that eventually will make up both systems were scheduled to begin later this year on the shuttle. The delays probably will prevent the systems from being totally operational by the 1990s, according to congressional and administration sources.

New satellites for systems already operating, such as the Defense Satellite Communications System (DSCS), the Defense Meteorological Satellite Program (DMSP) and the Navy's Fleetsatcom communications system also will be delayed. Those systems, however, have spares that can be used in event of malfunction of operating satellites.

Electronic and photo-intelligence satellites, which comprise the National Reconnaissance Organization's "black" programs, also face launch delays. The Titan 34D has been the prime launch vehicle for both kinds of satellites, although larger, more capable intelligence satellites have been designed to be carried in the shuttle.

The DMSP is the Pentagon's "single most important source of weather data" for tactical and strategic forces, according to information given Congress last year.

Its two operational satellites, which fly a near-polar orbit about 450 miles above Earth, cover the globe 14 times a day and send current meteorological, space environmental and oceanographic data to Air Force and Navy forecasting centers. Additional capability is being added to allow transmissions to local Air Force, Navy and Marine Corps operating units equipped with special receiving units.

The DSCS provides secure voice and data communications for Defense Department forces worldwide during wartime. In peacetime, the system also carries embassy and intelligence messages from around the world in a link that cannot be cut by any host country.

The DSCS is switching to a new generation of satellites, with the newer versions hardened against nuclear radiation and protected against jamming.

Seven DSCS II satellites, four operational and three spares, are in orbit. The spares, according to congressional testimony, "may be the only way to acquire additional capacity under hostile nuclear war conditions where replenishment launches may not be possible."

Fleetsatcom is the Navy's prime military communications network. Made up of four satellites spread around the Equator, the system transmits coded messages to and from the fleet around the world.

A replacement Fleetsatcom satellite scheduled for launch from Cape Canaveral May 22 aboard an Atlas-Centaur booster, the only large military rocket that has not failed recently, has been delayed for as long as a month.