is assured. The TACAN set receives coded AM RF pulse pairs from the ground station. It then detects, decodes, and demodulates the signal to extract the bearing and identity signals. The bearing signal is then sent to the indicator group, and the identity signal is sent to the ICS group. TRANSMIT RECEIVE MODE.— In the T/R mode, the system provides range and bearing to the ground station and station identity information. To measure the slant-range to the ground station, the TACAN transmits an interrogation signal to the station. The station then sends a reply signal, which is detected and decoded by the TACAN set. The range is computed by measuring the elapsed time between transmission of the interrogation pulse and the reception of the reply. The range and bearing information is then sent to the indicator group, and the station identification signal is sent to the ICS group. AIR-TO-AIR MODE.— The TACAN set is used to measure range between two or more similarly equipped aircraft in the air-to-air mode. To accomplish this, each TACAN set transmits interrogation pulses, receives interrogation pulses from other TACAN sets, and transmits a reply pulse when interrogated. Interrogation signals consist of a coded pulse pair, and the reply is a single pulse. The system will measure the elapsed time between the transmitted interrogation and the reply received to compute range. When using this mode, one aircraft must use a channel that is either 63 channels higher or lower than the other aircraft. Self-Test There are two types of self-test used by the TACAN system. They are the readiness monitoring and the interruptive self-test. READINESS MONITORING.— Readiness monitoring is continuously ongoing self-test. The self-test module is monitoring critical system performance parameters without interrupting system operation. If any one of the continuously monitored parameters degrades beyond limits, a NO-GO indicator will light on the control panel, as well as on the RT faceplate. When the power supply reaches + 125°C (±5°C), the self-test module will light the TEMP indicator on the RT faceplate. INTERRUPTIVE SELF-TEST.— This self-test is initiated by the operator depressing either of the BIT switches on the control panel or the RT. The test can be performed on any of the channels or in any of the modes of operation. If there are no faults found, range will display 1.8 nautical miles, bearing will display 4 degrees, and the GO indication will be displayed for 9 seconds on the TACAN control panel. If there is a fault, the NO-GO indicators will light in the same manner as in readiness monitoring. LONG RANGE NAVIGATION (LORAN) The name loran is an appropriate description of the hyperbolic system of electronic navigation. It provides lines of position over the surface of the earth. Over water, usable loran signals can be received at ranges up to 2,800 miles. This is done with low-frequency radio waves. At these operating frequencies, radio waves are capable of following the curvature of the earth. Loran lines of position can be crossed with each other, or with lines of positions determined by other means, to provide fixes.    Loran lines are stationary with respect to the earth’s surface. Their determ- ination is not dependent upon compass or chronometer, and it is not necessary to break radio silence to obtain them. Loran signals are available for reception in all types of weather, except during very severe electrical storms. For more information on the theory of operation of loran, refer to Aviation Electronics Technician 2 (Organizational), NAVEDTRA 12330, chapter 3. Major Components The AN/ARN-81 loran receiving set consists of three major components. They are the R-1336/ ARN-81 receiver, the IP-796/ARN-81 display indicator, and the C-6604/ARN-81 control indicator. The receiver processes the input signals and routes them to the display. The display provides a means to align the pulses to determine the aircraft position. The control indicator provides selection of power, operating modes and channels, and delay times. System Functions The loran receives signals through the ADF sense antenna. The signals are routed from this antenna to the R-1336/ARN-81 receiver. At the receiver, the signals are amplified, heterodyned, and detected to provide video output signals, which are applied to the video amplifier. This signal is then applied to a high-gain amplifier that supplies two outputs. One of 2-15