control it manually for searching in a specific target sector. As we mentioned in chapter 1, the air search 3D radars determine altitude by scanning the vertical plane in discrete increments (steps). Although this may be done   mechanically,   most   frequently,   it   is   done electronically.   Figure   2-8   shows   the   radar   beam radiated  at  different  elevation  angles  as  electronic scanning changes the radiated frequency in discrete steps. Each elevation angle or step has its own particular scan frequency. A   computer   electronically   synchronizes   each radiated frequency with its associated scan angle to produce the vertical height of a given target. The 3D radars also use a range-height indicator (RHI) in addition to the PPI used with 2D radars. We will  discuss  both  indicators  in  further  detail  in  the section  on  radar  indicators. CARRIER-CONTROLLED   APPROACH (CCA) AND GROUND-CONTROLLED APPROACH (GCA) RADARS Carrier-controlled   approach   (CCA)   and ground-controlled   approach   (GCA)   systems   guide aircraft   to   safe   landings,   even   under   conditions approaching zero visibility. Radar is used to detect aircraft and to observe them during their final approach and  landing.  Guidance  information  is  supplied  to  the pilot in the form of verbal radio instructions, or to the automatic  pilot  (autopilot)  in  the  form  of  pulsed  control signals. The primary approach systems in the Navy are the AN/SPS-46(V)  Precision  Approach  Landing  System (PALS)  for  CCA  and  the  AN/FPN-63  Precision Approach  Radar  (PAR)  for  CGA. AN/SPN-46(V) PALS The  AN/SPN-46(V)1  system  provides  safe  and reliable final approach and landing for PALS-equipped Figure  2-8.—Electronic  elevation  scan. carrier-based  aircraft,  during  daylight  or  darkness.  It  is rarely  affected  by  severe  weather  and  sea  state conditions,  and  is  not  affected  by  low  ceiling  and visibility  problems. The AN/SPN-46(V)2 system is installed at selected naval air stations (NAS). It is used for the PALS training of flight crews, operator and maintenance personnel, and  the  PALS  certification  of  aircraft. The  AN/SPN-46(V)1  system  replaces  the AN/SPN-42A   Automatic   Carrier   Landing   System (ACLS) on CV/CVN class ships. The AN/SPN-46(V)2 system replaces the AN/SPN-42T1/3/4 at various naval air  stations. General  Theory  of  Operation The AN/SPN-46(V) PALS allows simultaneous and automatic  control  of  two  aircraft  during  the  final approach   and   landing   phase   of   carrier   recovery operations.  Designed  primarily  as  an  “automatic” landing system, it also has manual control capabilities. The  AN/SPN-46(V)  has  three  modes  of  operation  that are identified, based on the type of control (automatic or manual)  and  the  source  of  information  (display  or voice). Mode  I  (automatic   control).—The   Central Computer   Subsystem   (CCS)   processes   flight information from the radar/ship motion sensor (SMS), wind speed and direction equipment, and other ancillary equipment. It then transmits command and error signals to each aircraft via the Link 4A. The aircraft receives these command and error signals and translates them into control actions that maintain the aircraft within a narrowly  prescribed  flight  envelope. Mode  II  (manual  control  with  display).—The aircraft   cockpit   display   receives   command   and error  signals  that  direct  the  pilot  to  take  proper actions. Mode III (manual control with voice).—The air traffic  controller,  using  the  processed  flight  data transmitted  to  the  operator  control  console  (OCC), provides  the  pilot  with  voice  communications  for  a manual approach. Configuration The AN/SPN-46(V)1 system consists of 26 units categorized   into   four   major   subsystems:   display (units  1  and  2),  ancillary  equipment  (units  3-11), central computer (units 12- 16), and radar/SMS (units 17-26).  A  pictorial  flow  diagram  of  the  system  is 2-15