Multimillionaire adventurer Steve Fossett went missing in a single-engine aircraft on Monday after he took off for what was supposed to be a two-hour trip. Rescuers are combing a 600-square-mile area in Nevada to try to find Fossett, but they are guessing at his location. Was his plane being tracked by radar before it vanished?
Not necessarily. The FAA's radar-based air traffic control system covers many parts of the United States but not all. Ground radar systems send out two types of radio signals to gather information: Primary radar bounces back from targets and tells controllers that something—a flock of birds, a helicopter, a storm cloud—is in a certain spot; secondary radar "interrogates" the plane's transponder radio to gather more specific details about the flight, like its current altitude. But mountainous terrain—like that found in western Nevada, where Fossett was probably flying—can interfere with both kinds of signals. And if those signals don't bounce back to the antennas on the ground, then there's no trace of the aircraft.
There's also no radar coverage for planes flying over oceans and other large bodies of water, like the Great Lakes. In addition, some weather conditions, like extreme heat or thunderstorms, can mask a plane from detection by primary radar. (Stealth planes are designed in strange shapes to avoid discovery; they supposedly reflect radar in all directions except back to the source.)
Even if the signals weren't blocked by Nevada's mountains, Fossett may not have been on anyone's radar screens. That's because the adventurer took off into Class G, or uncontrolled, airspace. In these sparsely populated areas, the FAA has no responsibility to manage air traffic and doesn't guarantee radar coverage. Still, the large military presence in Nevada means that Air Force or Naval radar might have picked up on Fossett's plane. According to some news accounts, rescue workers are analyzing radar information.
Aerobatic planes like the one Fossett was flying don't always have transponders. And even if the plane did have one, Fossett might have turned it off. A recreational pilot can fly with the transponder off in certain situations, like if he's far from a major airport and doesn't need air traffic control to point out where other planes might be. With the transponder off, Fossett's plane would have shown up under primary radar as an intermittent blip that appears every five to 12 seconds. A radar system might detect hundreds of targets at any one time, though, so to figure out where Fossett went, authorities would have to look at his takeoff time, figure out which of the blips was his plane, and then reconstruct his path.
Bonus Explainer: Can Slate readers help find Fossett using Google Earth? Probably not. Google Earth's images are anywhere from six months to three years old, so you'd have to wait until next year to spot the tycoon's plane. But Google's imagery providers have shared new data during some humanitarian crises—Hurricane Katrina, the Indian Ocean tsunami in 2004, and the 2005 earthquake in Pakistan.
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Explainer thanks Chris Dancy of Aircraft Owners and Pilots Association, Peter Dumont of Air Traffic Control Association, Steve Jacobs of National Air Traffic Controllers Association, and Tammy Jones of Federal Aviation Administration.