Sensors that detect fluctuations in magnetic fields could prove a valuable supplement to air traffic control. While images of planes stacked above airports are more common, preventing accidents on the ground is an unexpectedly significant issue. Safety watchdog Eurocontrol reported over 600 risky crossing of European taxiways in 2005, and the deadliest air accident ever that occurred on the ground. The basic problem is that ground-radar systems are susceptible to having signals bounce off buildings, which results in gaps in coverage. Research findings suggest that sensors that detect magnetic field fluctuations can fill in those blind spots. The size of a cigar box and costing about $150, each sensor consists of a memory chip, a signal processor, and a magnetoresistive sensing element. A thin nickel-cobalt film is placed over an electrified silicon wafer that acts as a resistor. Ripples in a magnetic field cause the resistance to change. Although the metallic content of aircraft produces magnetic field disturbances in the nanotesla range, these sensors can distinguish moving aircraft on the ground from other objects. Covering a 50-meter range, they can fill in the gaps where radar does not work and see through weather that would compromise a camera-based system. Larger airports could install magnetic sensors at key points, while smaller airports could use enough for complete coverage. Although more filtering research is needed, preliminary results are encouraging.