Pilots spent four days last week flying a trio of airplanes at each other over the Mojave Desert in the United States, missing on each pass by as little as 300 meters.

　　Remarkable wasswheresone of the pilots was not. Some 3,450 meters 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 robotic drones, commonly called unmanned aerial vehicles, have garnered lots of publicity thanks to their high-profile, military roles in Kosovo, Afghanistan, Yemen and, now, Iraq. Yet beyond the hype, the planes still aren't approved for routine use over the United States.

　　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, saving 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.