PHOTO: ANDY CAULFIELD/GETTY IMAGES

Delays ahead: Controlling air traffic on the ground could be made easier by sensors that determine where aircraft are by the disturbance they make in the Earth’s magnetic field.

Air-traffic control is a ­complex and high-stress business. Mistakes are not allowed. And it’s not ­limited to directing planes for takeoff and landing—a job that’s been ­memorably described as three-dimensional chess. It also includes keeping track of where planes are on the ground. According to safety watchdog Eurocontrol, in 2005 alone there were 600 occasions when people, cars, or planes crossed Europe’s airport taxiways when they shouldn’t have. In fact, the most deadly airline ­accident ever happened on the ground: in 1977, two 747 jumbo jets collided in the Canary Islands, killing 583 people.

Most big airports have expensive ground-radar systems to keep track of where the hundreds of moving planes are on the sprawling tarmacs. But ground radar sometimes reflects off buildings and terminals, leaving small gaps in coverage. Smaller airports like the one in Thessaloníki, Greece, can’t afford ground radar. Such blind spots on the ground at airports large and small have prompted European researchers to explore the use of fluctuations in the Earth’s magnetic field to better pinpoint where planes are on busy taxiways. The results, researchers say, show that using US $150 sensors can fill in the blind spots.

Aerospace engineers and ­physicists in Greece, Germany, the UK, and Austria came together to build and test the cigar box–size magnetic sensors. The basic components of each sensor are a small memory chip, a magneto­resistive sensor, and a signal processor. The ­sensing element consists of a thin nickel-cobalt film over a silicon wafer. In this case, the wafer is set up like a resistor. An electric current is passed through the wafer so that when an external ­magnetic field is applied to the sensor or ripples in a field wash over it, the value of that resistance changes ever so slightly.

The quantity of ferromagnetic metal in an aircraft ­introduces ­disturbances in the Earth’s magnetic field in the nano­tesla range. The ­sensor can ­distinguish the disturbance left by a moving aircraft on the ground from background noise and other objects well enough to ­pinpoint not only the location of the ­disturbance but also the cause.

Engineers at Saarland University in Saarbrücken, Germany, working with local electronics firms, made a ­version of the sensor that can be mounted in ­existing taxiway light housings and ­runways. The Earth’s magnetic field is not affected by buildings, fog, or rain, so the sensors just have to look for the right fluctuations, says Saarland ­experimental physicist Haibin Gao. That means the sensors work in the crevices where radar might not and see through weather that would blind a camera-based system. Each sensor covers about a 50‑meter range. Gao says it would be too much trouble to cover a big airport like Frankfurt with hundreds of these ­sensors; still, they would be needed only at key points and to fill radar gaps in order to be ­effective. Smaller airports with one or two runways, however, could ­distribute enough for complete coverage.

Test results from Frankfurt and Thessaloníki are encouraging, says Nikolaos Grammalidis, a researcher at the Informatics and Telematics Institute, whose team, based in Thessaloníki, developed the sensor’s filtering ­software. Grammalidis says that more filtering research needs to be done to account for, say, a large car passing at a ­considerable distance from the ­sensor, which might produce a signal similar to that of an airplane. But Saarland’s Gao says the magnetic-field sensor is poised to be a relatively inexpensive step in improving the picture of air ­traffic on the ground.