Altitude reporting equipment and transponder systems are significant elements for the safe operation of aircraft in the National Airspace System (NAS). ATC monitors aircraft movements and ensures positive separation with identity and three-dimensional positional information obtained from individual aircraft altitude reporting equipment and transponder systems. Traffic Alert and Collision Avoidance Systems (TCAS), installed in nearly all commercial passenger air carrier aircraft and many other aircraft, depend on reliable and accurate altitude reporting equipment and transponder systems in surrounding aircraft. ADS-B systems are dependent on aircraft equipped with reliable and accurate position-reporting equipment and transponder systems.

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