permit changes to separation standards, thereby playing a significant role with respect to safety and efficiency.
For en route, TRACON, and tower operations, radar is the primary source of surveillance data used to calculate and predict the position, speed, and course of aircraft. Radar surveillance is not provided for oceanic air traffic control. Within the national airspace system, radar is considered a service delivered to controllers by a radar system consisting of the radar equipment itself, radar processing hardware and software, the display devices used to present the resulting data, and the interfaces among the system elements. The key elements of the radar systems for en route, TRACON, and tower air traffic control are summarized below.
There are two fundamental types of radar supporting air traffic control. Primary radar relies on reflection technology that provides data sufficient to calculate the range and bearing, but not the altitude, of a detected object. All en route centers and TRACONs are fed by primary radar. En route centers use the long-range air route surveillance radar (ARSR), which scans a wide area (generally a 250-mile radius). TRACONs use a shorter-range airport surveillance radar (ASR), which scans a narrower area (generally a 60-mile radius). In addition, busy towers are supported by a specialized primary radar, called airport surface detection equipment (ASDE), that detects ground objects. Radar is also a key sensor for detection of weather features.
Secondary radar, or beacon radar, usually collocated with primary radar, transmits an interrogation pulse; when the interrogation is received by an aircraft equipped with the appropriate transponder, the transponder replies with codes that indicate the aircraft's altitude and identification; the replies are received by the secondary radar. Secondary radars support both en route centers and TRACONs. Each en route facility is serviced by multiple radar sites, whereas most TRACONs are serviced by a single radar sensor.
At the en route center, primary and secondary radar data are processed by the HOST computer's radar data processor. The radar data processor software assesses the quality of radar data and blends radar inputs from multiple sites to provide controllers with the best available targets. The HOST radar data processor, together with computer display channel or display channel complex computers—depending on the facility—process the radar data to identify targets, calculate their positions, track their movements, correlate altitude and identification data with targets, transform the data to display coordinates, and display the resulting information (including target pixels, data blocks, and warnings of conflicts and unsafe altitudes) along with maps and other data seen on the controllers' plan view displays. The enhanced direct access radar channel is a backup processing