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