TM 1-1510-218-10 3B-25 display  brightness  to  compensate  for  changes  in  the ambient light level. Pilot selection of #2 NAV to tune the TACAN will remove  the  TACAN  derived  steering  information  from the   #1   HSI   display.      This   action   allows   only   the steering  information  obtained  from  the  #1  NAV/TAC control  to  be  displayed  on  the  #1  HSI  display.    Pilot selection of #1 NAV/TAC  DME HOLD will also remove the steering information from the #1 HSI display. A  DME  HOLD push switch located on the pilot's instrument panel is utilized to select DME-HOLD mode of   operation.      A   DME/TAC  source  selector  switch, located   on   the   pilot's   audio   control   panel,   permits selection of DME or TACAN mode of operation.  With the switch in the TACAN mode, bearing information is derived from the TACAN signal of the TACAN  station.   The  KTU -709  system  is  now  serving  as  a  navigation receiver  as  well  as  a  DME.    Distance  and  bearing information  will  be  displayed  on  the  pilot's  HSI  along with  a  display  of  range/  groundspeed/time-to-station data on the DME slave indicator. CAUTION Since  the  TACAN  ground  system  does  not provide guidance during Instrument Landing System (ILS) approaches, any ILS approach  must  be  flown  in  the  VOR/ILS mode, using the TACAN for DME only. b.    DME   Slave   Indicator.      Allows   display   of nautical     miles,     groundspeed,     and     time-to-station.   Refer to Figure 3B-15. 1.  NM (Nautical Miles) 2.  KT (Knot) 3.  MIN (Minutes) 4.  Photocell Figure 3B-15.  DME Slave Indicator (1)   NM .        Displays    nautical    miles    to    the station. (2)   KT.  Displays aircraft groundspeed. (3)   MIN.  Displays time-to-station. (4)   Photocell. Allows for automatic brightness control of the display. c.     Operation. CAUTION Power    to    the    KTU-709    TACAN    system should be turned on only after engine start- up.        This    procedure    will    increase    the reliability of the solid state circuitry. (1)   Operating Procedure. 1.   OFF/VOL control – Turn clockwise. 2.   DME/TAC source selector switch – As required. 3.   Frequency   selector   –  Select  desired frequency, and press the mode button as required. NOTE Prior to station lock-on, dashes will appear in  the  window  of  the  KDI -773  DME  slave control  indicator,  as  the  system  searches for   the   station.      Search   time   is   usually about  one  second.    Once  the  system  has locked     on,     the     distance     read-out     will appear,  followed  quickly  by  groundspeed and time-to-station computations.  4.   DME/TACAN    audio    switch    –    On.    Verify station. 5.   VOL control – As required. 6.   DME HOLD switch – As required. (2)   Shutdown Procedure.  OFF/VOL switch – OFF. 3B-24.  GLOBAL POSITIONING S YSTEM (KLN 90B). For     information     on     the     Global     Positioning System, refer to Chapter 3, Paragraph 3-7.