TM  1-1520-240-10            3-3-1 SECTION  III.  NAVIGATION EQUIPMENT 3-3-1.  VHF NAVIGATION AND INSTRUMENT LAND- ING SYSTEM (AN/ARN-123). The VHF navigation and instrument landing system (AN/ ARN-123) is an integrated navigation receiver consisting of a 160 channel VOR receiver, a 40-channel localizer receiv- er, a 40 channel glideslope receiver, and a market beacon receiver. Each receiver operates independently. The sys- tem consists of a control panel on the console, a receiver on the avionics shelf, and three antennas on the fuselage. The set includes VOR and marker beacon self-test capabil- ity. a.  VOR/Localizer   Receiver.  The   VOR/localizer   re- ceiver  processes  signals  in  the  frequency  range  108.00 through 117.95 MHz. The VOR/localizer information is pre- sented on the course deviation bar of the HSI on the pilot’s and copilot’s instrument panels. VOR bearing information is displayed on the No. 2 pointer of the indicator. If the received signal is weak or the set malfunctions, a red NAV flag will appear on the indicator. The VOR/localizer antenna is on the bottom of the helicopter at sta. 79. b.  Glideslope Receiver.  The glideslope receiver sec- tion of the AN/ARN-123 processes signals in the frequency range of 329.15 through 335.00 MHz.  Glideslope informa- tion is displayed on the HSI. The set will also cause a red GS warning flag to appear on the indicator when the re- ceived signal is unreliable or the glideslope receiver mal- functions. The glideslope antenna is on the forward pylon above the windshield. c.  Marker Beacon Receiver.  The marker beacon re- ceiver is a fixed -frequency receiver (75 MHz) which re- ceives signals from a ground marker beacon transmitter to determine helicopter position. The audio output of the marker  beacon  receiver  is  applied  through  the  inter- phone system to the headsets as a direct input (no inter- phone  control  box  switch  setting  necessary).  Three marker beacon lights are on each HSI MODE SELECT panel.  The  lights  are  labeled  MKR  BCN  and  indicate marker  beacon  passage.  Also  specific  tones  will  be heard over the interphone while flying over each beacon. Table 3-3-1 presents the indication as each beacon is passed. The lights are tested by momentarily pressing any one of three lights. The marker beacon antenna is under the fuselage is under the fuselage forward of the forward cargo hook. Power to operate the set is from the DC essential bus through the NAV VOR circuit breaker on the No.1 PDP. Power is supplied by the 26-volt AC instrument bus through the NAV VOR circuit breaker on the No. 1 PDP. Table  3-3-1.   Marker Beacon Indicator BEACON LIGHT COLOR TONE (HERTZ) Outer (O) Blue/Green 400 Middle (M) Blue/Green 1300 Inner (I) Blue/Green 3000 3-3-2.  OPERATION CHECK/SELF - TEST. NOTE This test requires an external VOR RF signal generated by a ground station, or ramp test set. a. Set the course indicator OBS control for a 315 degrees indication under the course index. b. Move the control unit VOR/MB switch to the test position (down). c. The CDI deviation needle should indicate center "2 dots. d. The VOR/LOC flag should be buried. e. The  RMI  should  point  to  the  315  degrees  ra- dial("5 degrees). f. The marker beacon lamp(s) should illuminate. 3-3-3.  Controls and Function, VHF Navigation and Instrument Landing System Set.  (fig. 3-3-1) CONTROLS/ INDICATOR FUNCTION NAV VOL Provides power to set when turned from OFF. Turning knob clock-wise increases volume. VOR/MB TEST Switch spring-loaded from TEST position. At TEST, the MKR BCN lights illuminate. Megahertz Select Knob Selects last two digits of frequency in 50 -kilohertz increments. MB VOL Adjusts audio output lev- el of marker beacon re- ceiver. Provides power to the marker beacon when turned from OFF.