Air-traffic control is a Âcomplex and high-stress business. Mistakes are not allowed. And it’s not Âlimited to directing planes for takeoff and landing—a job that’s been Âmemorably 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 Âaccident 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 ThessalonÃki, 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 Âphysicists 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 magnetoÂresistive sensor, and a signal processor. The Âsensing 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 Âmagnetic 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 Âintroduces Âdisturbances in the Earth’s magnetic field in the nanoÂtesla range. The Âsensor can Âdistinguish the disturbance left by a moving aircraft on the ground from background noise and other objects well enough to Âpinpoint not only the location of the Âdisturbance but also the cause.
Engineers at Saarland University in Saarbrücken, Germany, working with local electronics firms, made a Âversion of the sensor that can be mounted in Âexisting taxiway light housings and Ârunways. 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 Âexperimental 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 50â''meter range. Gao says it would be too much trouble to cover a big airport like Frankfurt with hundreds of these Âsensors; still, they would be needed only at key points and to fill radar gaps in order to be Âeffective. Smaller airports with one or two runways, however, could Âdistribute enough for complete coverage.
Test results from Frankfurt and ThessalonÃki are encouraging, says Nikolaos Grammalidis, a researcher at the Informatics and Telematics Institute, whose team, based in ThessalonÃki, developed the sensor’s filtering Âsoftware. Grammalidis says that more filtering research needs to be done to account for, say, a large car passing at a Âconsiderable distance from the Âsensor, 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 Âtraffic on the ground.