Mode-S surveillance is arriving in European airspace just in time, according to Eu-rocontrol, and is already becoming essential for handling air traffic safely and efficiently in the most congested areas. In this new Mode-S (the "S" stands for "selective") surveillance scheme, the transponders do not reply to every interrogation from a ground-based secondary surveillance radar—they do so when their "names" are called. This technique reduces the clutter in the radio-frequency spectrum that's proving to be a problem with Mode-A/C (identification/altitude-readout) transponders: They all respond in unison anytime a radar covering their airspace sends out an interrogation.
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