TM 1-1510-225-10 3A-43 display.  In this case the receiver will be tuned to the last  valid  frequency  displayed  in  the  active  frequency display.    Taking  the  receiver  control  unit  out  of  the program  mode  with  dashes  in  the  standby  frequency display   will   unprogram   that   channel.      Pressing   the frequency    transfer    switch    will    cause    the    channel number   to   stop   flashing   and   the   frequency   to   start flashing.      The   frequency   can   then   be   changed   by rotating   the   tuning   knobs.      Pressing   the   frequency transfer switch again will cause the frequency to stop flashing and the channel to start flashing. The  receiver  control  unit  will  be  returned  to  the standby frequency entry mode by pressing the  CHAN switch  or  by  a  period  of  no  activity  for  20  seconds.   The frequency mode prior to channel or program mode will    be    resumed,   with   the   receiver   tuned   to   the frequency in the active frequency display. 3A-25.  TACAN SYSTEM (KTU 709). a.   Introduction.  The TACAN system is a polar coordinate    UHF    navigation    system    that    provides relative  bearing  and  slant -range  distance  information with respect to a selected TACAN or VORTAC ground station.  The effective range of the TACAN is limited to line  of  sight.    Actual  operating  range  depends  on  the altitude of the aircraft, weather, type of terrain, location and  altitude  of  the  ground  transmitter,  and  transmitter power.        The    TACAN    system    is    protected    by    a 3-ampere  circuit  breaker,  placarded   TACAN, located on the right sidewall circuit breaker panel, Figure 2-6. b.  VHF  Navigation  Receiver/TACAN  System Control  Unit  (KFS  579A)  Controls  and  Functions.   Refer to Figure 3A-20. (1)   Active  Frequency  Display.    Displays the active  frequency  or  channel  (frequency  or  channel  to which the receiver is tuned). (2)   Photocell. The built-in photocell automatically controls display brightness. (3)   Frequency Transfer Switch. The frequency transfer switch is a push-button switch that transfers   the   frequency   or   channel   in   the   standby display  to  the  active  display  and  the  frequency  in  the active  display  to  the  standby  display  each  time  it  is pressed. Pressing  the  frequency  transfer  switch  for  more than  2  seconds  while  in  the  standby  entry  mode  will switch   the   receiver   control   unit   to   the   active   entry mode. Momentarily    pressing    the    frequency    transfer switch  while  in  the  active  entry  mode  will  return  the receiver control unit to the standby entry mode. (4)   TACAN  Tens  and  Hundreds  Digits  and NAV Megahertz Digits Tuning Knob.  The TACAN tens and hundreds digits and NAV megahertz digits knob is the  smaller  of  two  concentric  knobs  that  are  used  to set the frequency or channel in the display.  Rotation of  the  smaller  tuning  knob  sets  the  two  digits  to  the right  of  the  decimal  point  in  the  standby  frequency display   in   50   kilohertz   increments   or   the   X   and   Y TACAN  channels.    X  channels  are  selected  with  the knob in and Y channels are selected with the knob out.   The   numbers   will   roll   over   at   the   upper   and   lower frequency limits. 1.     Active Frequency Display 2.     Photocell 3.     Frequency Transfer Switch 4.     Megahertz Tuning Knob 5.     Kilohertz Tuning Knob 6.     Channel Switch 7.     Power and Volume Control 8.     Standby Frequency Display Figure 3A-20.  VHF Navigation Receiver/TACAN System Control Unit (KFS 579A)