Eddytor takes us to eWeek for a look at the FAA's air-traffic control system, which, after 20 years of continuous operation, is in desperate need of an overhaul. Recent crashes have caused major delays, but the system's scope and importance make it difficult to test upgrades and improvements. "Many technologies are used in air traffic control systems. Primary and secondary radar are used to enhance a controller's 'situational awareness' within his assigned airspace; all types of aircraft send back primary echoes of varying sizes to controllers' screens as radar energy is bounced off their skins. Transponder-equipped aircraft reply to secondary radar interrogations by giving an ID (Mode A), an altitude (Mode C) and/or a unique callsign (Mode S). Certain types of weather also may register on a radar screen."