of azimuth and present target information on a radar display  located in a tower or center. This information is used  independently or in conjunction with other navigational aids  in the control of air traffic. ARSR is a long-range radar system designed primarily to  cover large areas and provide a display of aircraft while en  route between terminal areas. The ARSR enables air route  traffic control center (ARTCC) controllers to provide radar  service when the aircraft are within the ARSR coverage.  In some instances, ARSR may enable ARTCC to provide  terminal radar services similar to but usually more limited  than those provided by a radar approach control. ASR is designed to provide relatively short-range coverage  in the general vicinity of an airport and to serve as an  expeditious means of handling terminal area traffic through  observation of precise aircraft locations on a radarscope.  Nonprecision instrument approaches are available at airports  that have an approved surveillance radar approach procedure.  ASR provides radar vectors to the final approach course and  then azimuth information to the pilot during the approach.  In addition to range (distance) from the runway, the pilot  is advised of MDA, when to begin descent, and when the  aircraft is at the MDA. If requested, recommended altitudes  are furnished each mile while on final. PAR is designed to be used as a landing aid displaying range,  azimuth, and elevation information rather than as an aid for  sequencing and spacing aircraft. PAR equipment may be  used as a primary landing aid, or it may be used to monitor  other types of approaches. Two antennas are used in the PAR  array: one scanning a vertical plane and the other scanning  horizontally. Since the range is limited to 10 miles, azimuth  to 20°, and elevation to 7°, only the final approach area is  covered. The controller’s scope is divided into two parts. The  upper half presents altitude and distance information, and the  lower half presents azimuth and distance. PAR is a system in which a controller provides highly  accurate navigational guidance in azimuth and elevation  to a pilot. Pilots are given headings to fly to direct them to  and keep their aircraft aligned with the extended centerline  of the landing runway. They are told to anticipate glidepath  interception approximately 10–30 seconds before it occurs  and when to start descent. The published decision height (DH)  is given only if the pilot requests it. If the aircraft is observed  to deviate above or below the glidepath, the pilot is given  the relative amount of deviation by use of terms “slightly” or  “well” and is expected to adjust the aircraft’s rate of descent/ ascent to return to the glidepath. Trend information is also  issued with respect to the elevation of the aircraft and may  unit transmits a coded pulse sequence that actuates the aircraft  transponder. The transponder answers the coded sequence  by transmitting a preselected coded sequence back to the  ground equipment, providing a strong return signal and  positive aircraft identification, as well as other special data  such as aircraft altitude. The radar systems used by ATC are air route surveillance  radar (ARSR), airport surveillance radar (ASR), and  precision approach radar (PAR) and airport surface detection  equipment (ASDE). Surveillance radars scan through 360°  9-47 Functions of Radar Navigation Figure 9-43. Example of a head-up display (top) and a head-down  display (bottom). The head-up display presents information in front of  the pilot along his/her normal field of view while a head-down display  may present information beyond the normal head-up field of view.