Airline pilots and passengers may soon share the skies with robotic planes that constantly watch the Earth.

Yet these airborne automatons will keep a safe distance from their peopled counterparts thanks to a radar system designed to help prevent collisions. Provided the test next week of the new system runs smoothly, it will open up the friendly skies of national civil airspace to robotic vehicles, previously confined to restricted zones.

NASA researchers at the Dryden Flight Research Center in Edwards, California are poised to start testing on the collision-avoidance system April 3 using a vehicle called Proteus, a high-altitude plan that can fly long distances with or without an onboard pilot.

Without those capabilities, Hamilton told SPACE.com, the scientific and commercial uses of unmanned aerial vehicles will be constrained by severe Federal Aviation Administration (FAA) restrictions on their flight capabilities.

On its own, an unmanned aircraft isn't too smart, requiring a live person to control it remotely through a computer cockpit on terra firma. While the method works fine in keeping the vehicle flying, most unmanned planes lack the equipment necessary to scout out their surrounding airspace and watch for incoming obstacles.

"Some of them have forward-looking cameras able to give controllers a visual picture, and maybe even a video capability to slew the view back and forth," Hamilton said. "But in general the capability isn't there."

Proteus is outfitted with a 35-gigahertz radar system originally built by Canada's Amphitech International to ward off helicopters from nearby power lines. The radar's lightweight body and low power requirements make the instrument a prime candidate for use aboard robotic planes. Aboard Proteus, the radar is expected to monitor a radius of at least 6 nautical miles (11 kilometers). If any incoming obstacles detected, the readings will be relayed to the ground where a controller will decide whether a course correction is necessary.

"We're sort of taking the existing radar technology and tweaking it to detect other aircraft instead," said Michael Alsbury, a pilot and project engineer with Scaled Composites, LLC.

The data will be broadcast to ground control via direct line-of-sight connection and a satellite link that will allow researchers to track the craft over the Earth's horizon. The latter, Hamilton said, should give researchers a clue into the lag time between an object's detection and ground pilot reaction while an unmanned aerial vehicle is out of direct contact.

Although Proteus will be flown from the ground, Alsbury and another pilot will ride in the plane as it encounters near-collisions with a variety of aircraft, ranging from a slow-pokey hot air balloon to the F/A 18 jet approaching at nearly 575 miles per hour (926 kilometers per hour). The pair will ride along on 22 flights over the three-day test to monitor radar results and insure the aircraft's safety.

The driving force behind the flight tests next week is safety. Because of their inability to check for potential collisions, today's unmanned aerial vehicles must take off in restricted airspace -- Proteus uses space associated with Edwards Air Force Base in California -- climb to cruising altitude and then follow a strict flight plan approved by the FAA.

Proteus, for example, must climb to an altitude of about 40,000 feet (12,292 meters) before it can leave restricted airspace and follow it's meticulously prepared flight plan. The altitude puts it above those traveled by passenger jetliners, toping out at about 35,000 feet (10,668 meters).

Getting approval for one flight alone takes at least 60 days - the amount of time a plan must be submitted before a mission - and other restrictions, such as avoiding flights over cities, populated areas and farms, also affect robotic aircraft uses. Such restrictions are troublesome for NASA researchers hoping to use Proteus for Earth environment and atmospheric research, and make commercial enterprises to use unmanned vehicles as surrogate communications relays for major cities downright impossible.

"What we have to do is demonstrate that unmanned aircraft can operate with a comparable level of safety as manned craft," said Russell Wolfe, a project engineer with the Alexandria, Virginia-based Modern Technology Solutions, Inc. Wolfe choreographed each of the simulated near-collisions for the Proteus tests.

Wolfe told SPACE.com that robotic craft have to be able to at least measure up to the visual capabilities of human pilots aboard a plane, which the FAA requires to be a distance of about 3 miles (5 kilometers).

Last year, ERAST researchers were able to use the Amphitech radar to detect "cooperative" aircraft, vehicles carrying transponders that announce their presence in the vicinity and altitude, and can be used to find their location, Hamilton said. Next week, however, Proteus is expected to detect aircraft without using transponders and broadcast its data to ground controllers in real time, something not done in the earlier test.

"It's important because most vehicles flying below 10,000 feet (3,000 meters), experimental planes, gliders and balloons, don't have transponders," Alsbury said. "They're out there, and we need to be able to avoid them."

Wolfe said researchers hope to be able to see other aircraft well enough to keep a 500-foot safety bubble of empty air around Proteus. Only after unmanned aerial vehicles can be proven as safe as manned vehicles in the air, can they be used in civilian airspace across the country as forest fire lookouts, mobile phone relays, courier planes for mail and a host of other uses suggested for them, he added.

Being able to detect other aircraft and ward off potential collisions is only one of the hurdles to overcome before unmanned aircraft can make their way into civilian airspace.

NASA researchers are working alongside U.S. Air Force scientists to develop autonomous systems that could allow robotic aircraft to make their own landings and course corrections instead of relying on a human controller on the ground.

"But I think there are always going to be humans in the loop somewhere, the FAA would want it," Wolfe said.

NASA and other aerospace researchers are hope to develop unmanned aircraft safe enough to fly in civilian airspace by 2008, he added.