stabilized deck-coordinated system referenced to the touchdown  point  on  the  flight  deck. 3.  Digital  computer.    This is a general-purpose computer  used  to  provide  functions  for  radar  data stabilization,   data   filtering,   and   computations required  for  control  of  the  aircraft. 4.   Data   link   monitor.   This   subsystem   con- tinuously  checks  data  link  transmissions  for  errors.  If the messages do no check properly, the monitor will switch the system to either Mode II or Mode III, or will  generate  a  waveoff  signal. 5.  Control  console.  This  console  monitors  and controls  the  various  functions  of  the  landing  system. RADAR BEACON AN/APN-154B The  radar  beacon  is  located  on  the  aircraft  and  is used  to  receive  the  Ka-band  signal  interrogations from the LCC radar. The radar beacon then transmits X-band   replies   to   the   carrier   to   provide aircraft position data. APPROACH  POWER  COMPENSATOR AN/ASN-54 The  approach  power  compensator precise (APC) automatically  adjusts  throttles  to  maintain  the angle-of-attack,  and  thus,  the  airspeed  during  aircraft landing  approach. It  can  be  used  for  all  carrier landings and is required for Mode I approaches. For Mode  II  and  Mode  III  approaches,  the  APC  is optional. Data from the angle-of-attack transducer, normal  accelerometer,  and  the  stick/stabilizer  position are  used  to  control  an  electromechanical  servo actuator. This servo actuator is coupled to the throttle linkage  on  the  engine  fuel  control. ATTITUDE REFERENCE INDICATOR ID-1791/A The ID- 1791/A (VGI) is used to display the glide path errors from either the data link or monitor link on cross pointer needles. This indicator also determines and displays backup pitch and roll attitude, as well as displaying side-slip and turn rate. DISCRETE MESSAGE INDICATOR 128AV66836 This  indicator  is  located  on  the  aircraft.  It displays nine status indications from the one-way link system, autopilot, and the beacon radar. WARNING INDEXER PANEL Three  warning  lights  on  the  warning  indexer panel informs the pilot of the status of the approach power  compensator,  the  status  of  the  AFCS,  and whether  a  waveoff  has  been  initiated.  The  three indicators are the APC STBY, AFCS OUT, and the WAVEOFF   indicators.   The   APC   STBY   will illuminate when the APC is in standby. The AFCS OUT  will  illuminate  when  the  autopilot  system  is  not operating   properly. The  WAVEOFF  indicator  will illuminate when a wave-off has been initiated. When either the AFCS OUT or the WAVEOFF indicators illuminate  while  in  Mode  I  approach,  the  pilot  is required  to  immediately  take  over  control  of  the aircraft. APPROACH INDEXER 1284V653-1 This unit is located on the aircraft. It provides an additional angle-of-attack in the pilot’s field of view. This  unit  is  used  when  flying  cross  pointers  on  a Mode II approach or when monitoring display on a Mode  I  approach. PRINCIPLES  OF  OPERATION Learning Objective:  Recognize  the  operating principles  of  the  automatic  carrier  landing system. The   all-weather   combination   AFCS/ACLS provides   automatic,   semiautomatic,   or   manual operation  for  aircraft  carrier  operations  with minimum use of airborne electronic subsystems. The aircraft control commands are generated by shipboard computers  so  that  the  necessary  pitch  and  bank signals can be received by the AFCS via the one-way data link system. This closed-loop operation between aircraft  and  carrier  provides  automatic  control  to touchdown.  This  system  provides  a  final  approach and  landing  for  carrier-based  aircraft  during  daylight or   darkness,   with   minimum   interference   for conditions of severe weather and sea state, and no limitation  due  to  low  ceiling  and  visibility. There are three modes of operation of the ACLS that can be selected by the pilot—Mode I, Mode II, and Mode III. Mode I is a fully automatic approach from  entry  point  to  touchdown  on  the  flight  deck. Mode II requires manual control of the aircraft. In this  mode,  the  pilot  controls  the  aircraft  by  observing 9-3