MOJAVE, California (AP) -- Pilots spent four days last week flying a trio of airplanes at each other over the Mojave Desert, missing on each pass by as little as 300 meters (1,000 feet.)

Remarkable was where one of the pilots was not. Some 3,450 meters (11,500 feet) below the cockpit, one pilot sat safely on the ground as he coolly scrambled to avoid hitting his colleagues with the skeletal experimental aircraft he flew by remote control.

The flights are part of a NASA project to develop a collision-avoidance system that would allow fully autonomous, and not just remotely piloted, aircraft to operate in civil airspace.

The Federal Aviation Administration remains concerned the drones could pose a collision hazard to other, piloted aircraft. For now, the FAA must certify each drone flight, or series of flights, individually.

The FAA, Pentagon and NASA are studying the safety and reliability standards that drones must meet, FAA spokesman William Shumann said.

The FAA wants to make sure that drones can respond as quickly as piloted aircraft to instructions from air traffic controllers, Shumann said. It's also uncertain whether remote operators of drones would need to be licensed pilots, he added.

During 20 collision scenarios flown in restricted airspace over four days and completed last Friday, the ground-based pilots of the experimental Proteus saw nothing of the other two planes, save the stream of radar and other data that alerted them to their presence.

Each time the Proteus maneuvered in time to avoid colliding with the other aircraft -- an F/A-18 jet and a propeller-driven Beechcraft.

"We were seeing the targets earlier than the pilots in some cases," said Peter Siebold, a test pilot for Proteus builder Scaled Composites LLC. (A pilot and co-pilot were aboard the Proteus, but only as backups.)

Engineers equipped the Proteus with a radar system originally developed to help low-flying helicopter pilots avoid power lines. The plane also carried an instrument able to detect the transponders found in larger aircraft, though not gliders, hot air balloons and antique planes.

Eventually, engineers envision a system that combines radar, transponders, cameras and other instruments so drones can operate as safely as any other plane. Such a system is anywhere from 10 years to 15 years away.

"The demonstrations we're doing are not the definitive answer. They're a step on the way," said Glenn Hamilton, of the National Aeronautics and Space Administration's Dryden Flight Research Center.

Teal Group Corp. aviation analyst Richard Aboulafia said the FAA will come to accept drones with time. Continued military use helps, despite the many crashes that have brought down drones, including Predators and Global Hawks.

"It's a question of flight time," Aboulafia said. "What you will see is the FAA get a warm and fuzzy from the number of flight hours that have been trouble free."

NASA is fostering the development of drones for mapping, communications and reconnaissance of fires and other natural disasters.