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