The barometric switch in the RAWS auto- matically resets its altitude reference pressure to ground level before flight. When 700 feet above takeoff altitude has been reached, the switch actuates and changes the RAWS reliability circuit. At this point, the radar altimeter reliability signal is inhibited. This prevents nuisance warnings when the aircraft is flying above the operating range of the switch. AUTOMATIC DIRECTION FINDER (ADF) The Direction Finder Set, AN/ARN-83, is known as a low frequency automatic direction finder (LF/ADF). This system is a radio navigational aid that operates in the low-to-medium-frequency range. It continuously indicates the bearing to a selected radio station, acts as a manual direction finder, and as a conventional low-frequency radio receiver for voice and unmodulated transmissions. The LF/ADF is sometimes referred to as a radio compass. Major Components The AN/ARN-83 direction finder set consists of four major components. The y are the R-1391/ ARN-83 receiver, the C-6899/ARN-83 control panel, the AS-1863/ARN-83 loop antenna, and the ADF sense antenna. RADIO RECEIVER R-1391/ARN-83.— The radio receiver is remotely controlled from the ADF control. It does all processing of signals received by the loop and sense antennae and sends bearing information to the navigation indicator group. DIRECTION FINDER CONTROL PANEL C-6899/ARN-83.— The control panel controls all functions of the system (fig. 2-11). Selection of the three modes of operation is accomplished here. Manual control of the loop antenna is done using the LOOP knob. The tune knob tunes the receiver for the Figure 2-11-Direction finder control panel C-6889/ARN-83. station, and it is visually indicated through the frequency dial. The tuning meter indicates the strength of the receiver signal. The BFO switch causes a tone to be produced for tuning purposes. LOOP ANTENNA AS-1863/ARN-83.— This antenna is a flat, one-piece, sealed unit. It consists of four ferrite-cored coils arranged in a rectangle. Two coils line up parallel to the fore-aft axis of the aircraft, and the other two are perpendicular to the axis. Each pair of coils provides a signal that is sent to the receiver for processing. ADF SENSE ANTENNA.— The ADF sense antenna element is basically an aluminum panel connected to the input of the lightning arrester. This panel is encased in fiber glass and mounted flush with the fuselage. The loop antenna is physically mounted on the sense antenna. Functional Description There are three functional modes with the LF/ADF. These modes are (1) ADF mode, (2) loop mode, and (3) antenna mode. The direction finder set provides audio to the intercommunication system in all three modes of operation. ADF MODE.— In the ADF mode, the loop antenna signals are mixed with the nondirectional sense antenna signal in the receiver. This signal is then detected for audio to be sent to the headsets. The bearing coordinate data is produced by the receiver error correction servomotor network. This is then sent to the navigation indicator for display. LOOP MODE.— In the loop mode, the fixed loop antenna RF signal is modulated and detected in the receiver. The resulting audio error signal is used to produce an audio tone signal that is fed to the intercommunication system. The servomotor net work is then manually controlled to null the audio tone. At the null, the output of the receiver will represent relative bearing to the radio station. ANTENNA MODE.— This mode causes the system to act just like a normal radio receiver. It receives a voice or continuous wave (CW) signal at the sense antenna, which is detected and amplified by the receiver. The receiver then sends the audio to the ICS. In this mode of operation, there is no bearing data signal being developed. 2-12