TM 55-1510-219-10 7.    Double needle on RMI Read course to station. (2) To operate set for sense antenna direction finding: 1.  Mode selector ANT. 2.    Range switch Select. 3.    TUNE  control  rotate  for  maximum reading on tuning meter. 4.    GAIN control As required. (3) To   operate   set   for   audio-null   direction finding: 1.    Mode selector ANT. 2.     BFO-OFF switch BFO. 3.    Range switch select. 4.    TUNE control Tune desired station. 5.    GAIN   control   Adjust   for   minimum audio output. 6.    Double needle switches (RMI, fig. 3- 9) As required. 7.    BFO-OFF switch OFF. 8.    Mode selector LOOP. 9.    LOOP  switch  L  or  R.    Turn  left  or right until a null is reached (minimum sound in headsets). 10.    Double   needle   on   RMI   (fig.      3-9) Read course to station. NOTE The  true  null  and  direction  to  the radio station may be indicated by either   end   of   the   single   needle. This ambiguity must be solved to determine  proper  direction  to  the station. (4) Shutdown.  Mode selector OFF. 3-26.  TACAN SYSTEMS. a. Description.          Two     Tactical     Air     Navigation (TACAN) systems are provided.  One is dedicated to the INS and is used only for position updating; the other is used    in    conjunction    with    other    avionics    systems, including  the  flight  director  system  and  the  autopilot. TACAN   is   a   radio   navigation   system   which   provides aircraft  distance  and  bearing  information  relative  to  a TACAN ground station.  Both systems operate in the L- band frequency range of 962 to 1213 MHz. Their  range,  though  limited  to  line-of-sight,  is  designed to provide reliable reception of a TACAN ground station at a distance of 170 nautical miles at an aircraft altitude of   20,000   feet.      The   normal   time   required   for   the systems to lock on to a selected ground station signal is three    seconds.        The    avionics    TACAN    system    is protected    by    the    2ampere    TACAN    circuit    breaker located  on  the  overhead  circuit  breaker  panel  (fig.    2- 26). b. Avionics  TACAN  System.    The  AN/ARN136(V) avionics    TACAN    system    consists    of    the    RT1321 receiver-transmitter and the CP-1398 azimuth computer, both   of   which   are   located   in   the   right   nose   avionics compartment;  the  ID-2218  range  indicator,  located  on the instrument panel, the C-11265 control panel, located in  the  pedestal  extension;  and  an  antenna  located  on the  top  surface  of  the  aircraft  fuselage.    The  avionics TACAN system operates in conjunction with TACAN and VORTAC  ground  stations  to  provide  distance,  ground speed,  time-to-station,  and  bearing-to-station  data.    It operates  in  the  L  band  frequency  range  on  one  of  252 pre-selected frequencies, 126 X mode and 126 Y mode channels.      Course   deviation   from   TACAN   stations   is displayed   on   the   HSI.      Distance,   time-to-station,   and ground   speed   are   displayed   on   the   TACAN   digital display  (fig.    3-17).    The  ground  speed  and  time-to- station  are  accurate  only  if  the  aircraft  is  flying  directly toward  the  ground  station  at  a  sufficient  distance  that the slant range and ground range are nearly equal. The  avionics  TACAN  system  may  be  operated  by the flight director system or connected to and used with the  autopilot  system.    When  employed  as  the  primary means  of  navigation,  aircraft  flight  may  be  controlled manually   or   by   the   autopilot.      Indications   of   aircraft heading and bearing to ground stations are displayed on the course deviation indicators (HSIs).  Relative bearing to  a  station  is  displayed  by  the  RMI  bearing  pointer. TACAN distance, ground speed, and time-to-station are all  displayed  on  the  TACAN  indicator  located  on  the copilot's instrument panel (fig.  2-28). The   TACAN   control   panel   (fig.      3-17)   enables selection of the TACAN frequency (channel) to be used, and  provides  for  self-test  of  TACAN  circuits.    X  or  Y channel is selected by the X/Y switch.  Most TACAN and VORTAC stations are operated on the X mode.  When Y mode stations are operational, air navigation charts will designate the Y mode stations.  A toggle switch provides system    power    ON/OFF    control.        Audio    control    is provided by a rotary control placarded VOL. c. INS  TACAN  System.    The  INS  TACAN  system is  of  the  same  type  as  the  avionics  TACAN  system. This set, inaccessible to pilot control, is coupled directly 3-31