MANAGUA, NICARAGUA, JAN. 25 -- The downing of a cargo plane flying supplies to Nicaraguan rebels Saturday night suggests that the Sandinista armed forces have improved their ability to detect overflights and underscores a continuing Honduran role in the rebels' CIA-run resupply operation.

The incident also has raised a mystery over the identities of the pilot and copilot of the old DC6 four-engine plane, as well as questions whether recent supply flights violated a week-long suspension of air drops of U.S. lethal aid to the rebels, known as contras.

According to U.S. military observers in Central America, the Sandinistas have dramatically improved their radar capabilities in the last two years with the delivery of new Soviet equipment.

"The biggest development in the last two years has been in radar coverage," said a source who monitors the Sandinista military from another country in the region.

He said that in addition to their old Soviet early-warning radar, the Sandinistas now have "acquisition radar" -- used to lock onto a target -- fire-control radar and coastal radar.

Another military observer said in a recent interview that the Sandinistas had installed several radar sites in northern Nicaragua to detect aircraft flying over the country from the north.

The Sandinista commander of the 5th military region, Lt. Col. Roberto Calderon, indicated in a briefing in the southeastern town of San Carlos yesterday that the supply plane had been tracked along its flight path off Nicaragua's Atlantic Coast and into its drop zone.

"We detected the entry of the DC6" into Nicaraguan airspace, Calderon said. He said the plane had dropped 11 packages of about 800 pounds each to contra forces 27 miles northeast of San Carlos before it was hit twice by shoulder-fired C2M missiles. A western military observer identified the C2M as a portable, Soviet-made, infrared-homing, antiaircraft weapon known in U.S. military parlance as the SA7.

The Sandinistas also possess the more advanced SA14, which the Soviets began supplying to Nicaragua more than a year ago, the military observer said.

While the Sandinistas' ability to detect the overflights may have improved, the troops' performance in bringing down the planes generally has been a disappointment to military leaders here, the observers said. Before the downing of the contra DC6 shortly after 8 p.m. Saturday, the Sandinistas had shot down only one rebel supply flight: a C123 hit in daylight in October 1986, resulting in the capture of American airman Eugene Hasenfus.

Since then, more than 400 contra resupply flights have been flown over Nicaragua under U.S. aid programs for the rebels, according to U.S. officials.

According to Calderon, a captured survivor from Saturday's downed flight, Alejandro Sanchez Herrera, told interrogators that the pilot of the DC6 had been a Colombian whom he knew only as "Richard."

Sanchez identified the copilot as a man named Frixione and said the others on board were six cargo handlers and two parachutists who jumped with the air drop. He said that on seven contra supply flights he had made previously from the Honduran territory of Swan Island off the Caribbean coast, the pilots had been Nicaraguans. He said that Americans never flew on the supply flights he was on.

Contra spokesmen insist that all those on Saturday's flight were Nicaraguans and that the pilot was a former National Guard flyer named Donald Frixione. However, a woman who said she was a relative of Donald Frixione said today he had been killed in 1979 in combat.

According to Calderon, Sanchez told interrogators that he had been aboard contra supply flights on Dec. 18, 19, 20, 26, 28 and 30, and on Jan. 17 and 23. Neither Calderon nor Sanchez, during a brief appearance before reporters, specified what kind of supplies allegedly had been dropped on Jan. 17, when a week-long suspension of deliveries of U.S. lethal aid to the contras was in effect because of a Jan. 15-16 Central American peace summit in Costa Rica. U.S. officials have acknowledged drops of nonlethal aid and have pointed out that they were authorized.