SAR as a Saving Grace---Someday
The home team's newsletter somehow found its way to the bottom of the stack; when I finally got around to reading the last issue of the Geophysical Institute Quarterly, I found an interesting article on a new use for synthetic aperture radar.
Synthetic aperture radar, better known as SAR, is a research tool whose time has come. It's certainly come to Alaska, which has the only U.S. station for receiving SAR information from space. The huge dish-shaped antenna atop the institute's Elvey Building picks up SAR data sent by earth-orbiting satellites, which report on many aspects of ground and sea over which they pass even at night or during cloudy weather. SAR images are recorded by radio waves, not light waves as would be needed for ordinary cameras, and thus are especially useful for capturing information formerly hidden from remote-sensing instruments by the arctic winter.
It was another possible use for SAR that a team from the National Aeronautics and Space Administration's Goddard Space Flight Center investigated in interior Alaska during this past summer. The NASA group loaded a synthetic aperture radar aboard a DC-8 aircraft and set out to find downed and wrecked airplanes.
Because it was a test, they hunted only for carefully staged wrecks---debris gathered from genuine Alaska crashes and set out in known, marked locations in the Interior forests. (Jason Williams, a calibration engineer at the Geophysical Institute's Alaska SAR Facility, helped mark the locations by setting up radar reflectors at each corner of the test sites. "The reflectors return a predictable signal," the newsletter quotes Williams as saying. "They can be used to validate and correct SAR data"---that is, the reflectors made dandy calibration and position checkpoints.)
The technology certainly seemed worth testing for the purpose. A successful search for a downed aircraft depends on both good light and good weather, and the north rarely offers enough of either. Even when a small plane goes down during a dry spell in June, it may be hidden by trees.
Such difficulties can leave downed planes lost for years, but they need not be difficulties for synthetic aperture radar. SAR can gather images not only through moonless nights in December or cloud-choked mornings in August, but also through dense foliage in June. The particular kind of radar mounted on the Goddard DC-8, known as multipolarization SAR, has proven especially useful in distinguishing between natural features and human-made objects.
As tested, airborne SAR wouldn't do for search and rescue operations. For the summer's trials, the data gathered in Alaska were sent to the Jet Propulsion Laboratory in Pasadena, California, where the images were produced; then the images were sent to the Goddard Space Flight Center in Maryland, where they underwent enhancement and analysis. Presumably, if the technology is ever turned toward finding a real downed aircraft, that multistep process would be shortened.
The enhanced images look like black and white aerial photographs, except that the aircraft fragments look like flecks of brightness against the textured darker grays of the rolling terrain. The hills and valleys do show up very well, so well that the images of wreck locations could be used as guides for ground crews trying to reach the site.
Images of built-up areas show numerous bright flecks, because anything from junkyards to railroad tracks can generate the same kind of radar return as can a downed plane. But the location of most built objects is on the map, and these "known targets," as radar operators put it, can be distinguished from "unknown targets," just as an astronomer can pick out an asteroid appearing in a photograph of a known pattern of stars.
Nevertheless, airborne SAR works best in undeveloped places. For that reason and the number of small planes here, Alaska should be at the head of the line if and when a search-and-rescue SAR becomes available.
