The U.S. Air Force's RC135 reconnaissance plane near the flight path of the Korean Air Lines passenger plane shot down by a Soviet fighter last week was on a round-trip intelligence mission to assess Soviet air defenses, military sources said yesterday.

Although the Air Force clamped a tight secrecy lid on the RC135's flight, sources said the plane flew out of Eielson Air Force Base, 26 miles southeast of Fairbanks, Alaska, where it is part of the 6th Wing of the Strategic Air Command.

Air Force technicians in the rear of the RC135 were listening to and recording Soviet voice and electronic communications as they lumbered along in the military version of the Boeing 707 airliner, the sources said.

Air Force officials described the RC135's intelligence-gathering as routine. The United States and the Soviet Union, they said, constantly track each other's "electronic order of battle," which includes how radar stations react to intruders, and forward air defenses, such as how many fighters have moved to what bases and when.

"The RC135 is an unarmed plane," an Air Force source said. "It is in constant communication with the ground.

"It has all kinds of navigational aids to keep it over international waters because we know from experience that if one strays in Soviet airspace, it will be shot down," the source said.

Sources said that if the United States needs a plane to fly at the very edge of Soviet airspace or within it to gather crucial information, it would use a plane such as the SR71 Blackbird, not a defenseless RC135.

SR71 pilots rely on its speed and ability to fly at extremely high altitude to keep it safe from Soviet fighters such as the Su15 and MiG23s that rose to intercept the Korean 747.

Air Force sources with firsthand knowledge of RC135 missions from Alaska along the Soviet Union's Kamchatka Peninsula said the planes must be dispatched frequently to learn of any change in the alert status of Soviet air squadrons or other unusual military activity there and on the Soviet-controlled Sakhalin Island to the southwest.

Because the Soviets track the flights, these sources said, last week's pass near the Korean airliner's flight path would come as no surprise.

It is standard practice, they said, for U.S. military planes to try to "tickle" Soviet radar into action.

They said this amounts to flying close enough to air defenses to cause the Soviets to activate search radar and perhaps fire-control radar and to talk about what they are seeing and doing in response to the unidentified aircraft overhead.

This yields one piece of a giant mosaic of Soviet capabilities and possible intentions that the United States and its allies spend billions putting together day after day, year after year, from outposts throughout the world, the sources said.

Much of the electronic input is processed by the National Security Agency, headquartered in Fort Meade, Md.

Many Soviet test missiles land in waters near the Kamchatka Peninsula, off the Soviet Union's southeastern coast between the Sea of Okhotsk and the Bering Sea.

This makes this area extremely interesting to western intelligence analysts, who try to determine such factors as the missiles' accuracy and throw-weight to estimate the state of Soviet technology.

The RC135 cited after the Korean plane was shot down, sources said, was most likely involved only tangentially in gathering missile data.

More specialized intelligence-gathering, including that by planes flying out of Shemya Air Force Base at the western tip of the Aleutian Islands, focuses on missile activity, they said.

Japanese electronic eavesdropping stations on Japan's northernmost island of Hokkaido, sources said, easily could have overheard radio conversations of Soviet pilots flying near the Korean airliner at about 30,000 feet because such communications can be heard at great distances.

Normal radio conversations from ground stations, they said, are blocked by the earth's curvature and are beyond range of distant radio stations such as those in Japan.

Retired Gen. Russell E. Dougherty, a former commander of the Strategic Air Command and currently executive director of the Air Force Association, said RC135 intelligence-gathering provides "part of the puzzle" and is vital to U.S. security.

"You don't want to shadow box," he said. "You want to know as much as you can legitimately find out."