TM 1-1510-218-10 3A-23 1.  Decision Height Annunciator 2.  Pointer Mask 3.  Failure Warning Flag 4.  Decision Height Marker 5.  Altitude Pointer 6.  Decision Height Set Knob 7.  Test Switch 8.  Altitude Scale Figure 3A-13.  Radio Altimeter (RA-315) (2)   Pointer Mask.          The     pointer     mask, placarded  ABS  ALT,  covers  the  pointer  for  altitudes above 2500 feet. (3)   Failure  Warning  Flag.    A  flag,  placarded OFF,   will   be   in   view   whenever   the   radio   altimeter system information is unreliable. (4)   Decision   Height   Marker.      The   decision height marker is set to the desired decision height by the DH SET knob. (5)   Altitude   Pointer.        The    altitude    pointer indicates altitude above ground or surface level. (6)   Decision    Height    Set    Knob.      A   knob, placarded DH SET, located on the lower right corner of the radar altimeter indicator, is used to set the orange decision height marker to the desired DH. (7)   Test  Switch.    A  momentary  push-button switch,   placarded   TEST,   located   on   the   lower   left corner   of   the   radio   altimeter   indicator,   is   used   to activate the unit's self test function.  When the switch is pressed, the   OFF  warning  flag  will  come  into  view and the altitude pointer will indicate approximately 100 +20 feet.  Releasing the switch will cause the altitude pointer to return to existing altitude, and  OFF warning flag to retract from view. (8)   Altitude    Scale.        Indicates    altitude    of aircraft. 3A-21.  GYROMAGNETIC COMPASS SYSTEMS. a.   Description. Dual identical compass systems  provide  accurate  directional  information  for the aircraft at all latitudes of the earth.  As a heading reference, two modes of operation are used, directional   gyro   (FREE)   mode   or   slaved   (SLAVE) mode.    In  Polar  Regions  of  the  earth  where magnetic heading   references   are   not   reliable,   the   system   is operated in the FREE mode.  In this mode, the system furnishes  an  inertial  heading  reference,  with  latitude corrections    introduced    manually.        In    areas    where magnetic heading references are reliable, the system is  operated  in  the  SLAVE  mode.    In  this  mode,  the directional   gyro   is   slaved   to   the   magnetic   azimuth detector, which supplies long-term magnetic reference for    correction    of    the    apparent    drift    of    the    gyro.   Magnetic   heading   information   from  both   systems   is applied  to  various  aircraft  systems  through  pilot  and copilot's   COMPASS   switches.      There   are   no   front panel  fuses  or  circuit  breakers  for  the  gyromagnetic compass systems. b.  Controls  and  Functions.    Refer  to  Figure 2-17, Sheet 1. (1)   Pilot's   COMPASS   #   1   /    #   2   Switch.   Selects desired source for magnetic heading information for display on pilot's HSI copilot's RMI. (a)   # 1.  Selects compass system No. 1 for display. (b)   # 2.   Selects compass system No. 2 for display. (2)   Copilot's   COMPASS   #   1  /   #  2   Switch.   Selects desired source for magnetic heading information for display on copilot's HSI and pilots RMI. (a)   # 1.   Selects compass system No. 1 for display. (b)   # 2.  Selects compass system No. 2 for display. (3)   COMPASS SLAVE Annunciator.   Presents  a  visual  indication  of  system synchronization operation.