NASA’s ability to track spacecraft is crucial to the exploration of the stars. But the space agency is also looking earthward, hoping to build a radar system that could peer through trees, clouds and other obstructions to find lost planes and hopefully save lives.

The radar, called synthetic aperture radar or SAR, is meant to complement a global satellite system that listens for electronic emergency beacons sent by wrecked planes or people in distress. In remote areas, the beacons guide search and rescue personnel directly to the crash site. But the system, called Cospas-Sarsat, is all but useless if the emergency beacon is destroyed on impact or fails to activate.

Last April, researchers conducted their latest search and rescue SAR test in the mountains of California’s San Bernardino National Forest. With the help of the San Bernardino County Sheriff’s Department, plane and radar crews flew over a mock plane crash hoping to detect aluminum targets measuring six to eight-feet (1.8 to 2.4 meters) long. The area covered in the search was 360 square miles (579 square km). The test also hopes to find the wreckage of a real aircraft lost in the area last year.

Unlike conventional radar, which may require huge antennae to get the amount of detail search and rescue operations need, SAR is able to use a shorter antenna combined with computer processing to achieve the necessary detail.

The latest test used a radar with an antenna about two meters long attached to the belly of a NASA DC-8 plane that points off to the side, instead of directly below the aircraft. Because the SAR collects data while the plane is in motion, it creates what researchers call a "virtual antenna" a few kilometers long, allowing observers to see objects in much more detail than if the radar was standing still.

The SAR method is about 40 years old, initially used by the military to take pictures of the ground, then later as a remote-sensing tool in hydrology studies. Since then, scientists have worked to increase the radar’s resolution and speed up the time it takes to process the data.

"Now we can look at separate targets that are about three meters apart," said David Imel, project manager of Airsar, short for Airborne Synthetic Aperture Radar. Airsar is NASA’s testing ground for new radar technologies.

A computer analyzes the radar data and converts into long image strips that can then be pieced together into a coherent picture of a 100-square-mile area or more and offer up potential places to look for missing planes. The goal now is to identify the most effective methods and procedures to find aircraft remains.

Imel told SPACE.com that a team of scientists from the Jet Propulsion Laboratory in Pasadena, where his project is based, and the Dryden Flight Research Center in Edwards, where the DC-8 jet that carries Airsar is parked, are working with the Goddard team to develop the radar.

The people who make search and rescue operations their business hope the SAR technology will give them a way to close cases where crashed planes, and their pilots and passengers, have simply disappeared. According to records collected by the National Transportation Safety Board, at least 15 airplanes have gone down in the United States over the last six years and remain undiscovered. Most of them, board officials said, were small, personal planes.

"For me, personally, I want closure," said Lt. Mike Tuttle, manager of the San Bernardino Sheriff’s Department’s search and rescue operations, in a telephone interview. "But I also want to validate this technology."

Tuttle, who also heads the National Association of Search and Rescue, said the radar could put to rest a year-long search for Daniel Katz. Katz was a 24-year-old pilot whose small Piper II plane crashed into the San Bernardino National Forest in May 2001. Thick brush has hidden the wreckage from visual searches by air and the terrain is rough going for ground crews.

"Search crews are limited, particularly when they get fatigued," said Goddard consultant Roy Dreibelbis, a former Air Force helicopter rescue pilot and once-director of inland search operations for the Air Force Rescue Coordination Center. "The eyes can miss a lot."

But because the synthetic aperture radar can penetrate trees, brush and weather, it could find a plane wreck were human eyes can’t, he added.

A past test of the system, which has been under development since the early 1990s, discovered the remains of a small airplane in a mountainous region of Montana after conventional searches turned up empty. The SAR method identified 14 potential crash sites for search crews to check for the plane, which was subsequently found.

The SAR system is still years away from being an operational tool during search and rescue missions. Now the system is used in research to determine the most effective frequencies when searching for aircraft wreckage. It’s bulky, weighing on the order of two tons, and currently relies on the availability of the Dryden DC-8 research plane, and the condition of its radar instrument.

"Some of the biggest delay is just in getting the data here," Affens said. "It typically has taken months getting the information to Goddard for analysis."

Tuttle said the time element is crucial for search missions, and a real-time version of the SAR instrument devoted specifically to emergency search and rescue services is needed. That system, Goddard officials say, is still four or five years away since it currently takes weeks to analyze radar measurements. Eventually, a workable system could even be installed on an unmanned aircraft to keep search plane crews out of danger during severe weather conditions, they added.

In the meantime, researchers hope to test the current SAR instrument on a variety of landscapes, such as desert floors, rain forests and snow to better gauge its effectiveness.