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During space flight, the communications and tracking system of the Space Shuttle Orbiter uses -band and -band links to provide, in addition to tracking, reception of digitized voice, commands, and printed or diagrammatic data at a maximum rate of 216 kbits/s. The system provides a transmission capability for digitized voice, telemetry, television, and data at a maximum rate of 50 Mbits/s. -band links may be established directly with a ground station and both -band and -band links may be routed through NASA's Tracking and Data Relay Satellite System. A simultaneous capability to communicate with other satellites or spacecraft, using a variety of formats and modulation techniques on more than 850 -band channels, is provided. UHF is used for communication with extravehicular astronauts. Audio and television subsystems serve on-board needs as well as interfacing with the RF equipment. During aerodynamic flight following entry, the -band link can be supplemented or replaced by a UHF link that provides two-way simplex voice communication with Air Traffic Control facilities. TACAN is used as a navigation aid from shortly after blackout exit at about 150 000 ft (45.7 km) to about a 10 000-ft (3-km) altitude where a microwave scan-beam landing system assumes navigation aid functions. At about 2500 ft (760 m) to touchdown, radar altimeters provide accurate altitude determination.