7. 8. 9. 10. BFO-OFF switch - OFF. Mode selector switch - LOOP. Loop switch - L or R. Turn left or right until a null is reached (mini- mum sound in headsets). 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   procedure:   Mode   selector switch (ADF control panel, fig. 3-18) - OFF. 3-28.  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, and provides   only   DME   information   to   the   INS.The other is used in conjunction with other avionics sys- tems,  including  the  flight  director  system  and  the autopilot. For normal navigation 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  pro- vide reliable reception of a TACAN ground station at a distance of 170 nautical miles at an aircraft alti- tude  of  20,000  feet.  The  normal  time  required  for the systems to lock on to a selected ground station signal is three seconds. Both systems are protected by  a  2-ampere  circuit  breaker,  placarded  TACAN, located  on  the  overhead  circuit  breaker  panel  (fig. 2-26). b.  TACAN  System  (Non-INS  dedicated).  The TACAN  system  (Non-INS  dedicated)  consists  of  a range unit (which includes the system transmitter) and  a  bearing  unit,  both  located  in  the  right  nose avionics   compartment;   a   distance   indicator   (fig. 3-19)  located  on  the  instrument  panel;  a  control unit  (fig.  3-20)  located  in  the  pedestal  extension; and an antenna, located on the top of the fuselage. The  TACAN  system  (non  INS  dedicated)  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 preselec- ted frequencies, 126 X mode and 126 Y mode chan- nels.  Course  deviation  to  TACAN  stations  are  dis- TM  55-1510-221-10 played on the HSI’s. Distance, time-to-station, and ground  speed  are  displayed  on  the  TACAN  digital display  (fig.  3-19).  The  ground  speed  and  time-to- station  are  accurate  only  if  the  aircraft  is  flying directly toward the ground station at a sufficient dis- tance  that  the  slant  range  and  ground  range  are nearly equal. The (Non-INS dedicated) TACAN sys- tem may be connected to and used with the autopi- lot system. When employed as the primary means of navigation, aircraft flight may be controlled manu- ally or by the autopilot. Indications of aircraft head- ing and bearing to ground stations are displayed on the  horizontal  situation  indicators.  Magnetic  bearing to a station is displayed by the RMI and pilot’s HSI 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.  2-29).  TACAN  distance  is  displayed  on  the HSI’s. The TACAN control panel (fig. 3-20) enables selection  of  the  TACAN  frequency  (channel)  to  be used, and provides for self-test of TACAN circuits. At  the  present  time,  most  TACAN  and  VORTAC stations are operated in the X mode. When Y mode stations are operational, air navigation charts will designate  the  Y  mode  stations.  The  small  (outer) control provides system power ON-OFF and station identifier tone, volume and control. c. INS TACAN System. The INS TACAN sys- tem  is  coupled  directly  to  INS  circuits.  It  is  dedi- cated  only  to  updating  the  INS,  is  activated  when the INS is operational, and is controlled only by the INS. The INS TACAN consists of a range unit and a distance indicator, both located on the INS equip- ment rack and both identical to counterparts in the (non-INS  dedicated)  TACAN  and  antenna,  located on the underside of the fuselage (fig. 2-1). No con- trols are required or provided for the INS TACAN system. d. Controls/Indicators and Functions. (1.)  TACAN  control  panel. 1. 2. 3. 4. 5. 6. TEST SWITCH. Activates system self-test. CHANNEL  INDICATOR.  Dis- plays  selected  TACAN  channel. X-Y  SWITCH.  Selects  X  or  Y mode for TACAN channels. VOL CONTROL. TACAN  volume. Adjusts OUTER  CHANNEL  SELECTOR KNOB.  For  manual  selection  of tens and hundreds part of channel number. INNER  CHANNEL  SELECTOR KNOB.  For  manual  selection  of units part of channel number. 3-35