TM 55-1510-222-10 b. GPS Controls, Indicators, and Functions. (1)   GPS control/display unit (fig.  3-23A). (a)   Line selection keys.  Four line selection keys, located to the left of the CDU display screen, are used to initiate and terminate data entries, and to select various system options. (b)   Display  screen.    System  information  is shown  on  the  cathode  ray  tube  display  screen.    The display   screen   can   show   four   lines   of   text   with   13 alphanumeric characters on each. (c)   Mode selector switch.  The four-position mode selector switch, placarded PULL OFF, INIT, NAV, and PULL TEST, is used to select the operating mode of the GPS system. (d)   Display   brightness   control.      A   control knob     placarded     BRT     is     provided     to     control     the brightness   of   the   cathode   ray   tube   display   screen. Clockwise rotation of the control increases brightness. (e)   Data entry keys (0 through 9).  The data entry keys are used to enter alphanumeric data. (f) USE   LTR   key.      The   use   letter   key, placarded USE LTR, is used to select alphabetic prompt in free format data entry.  The USE LTR key terminates alphabetic entry when pressed. (g)   Clear   key.      The   clear   key,   placarded CLR, is used to clear erroneous data entry and message displays. (h)   Slew   key.      The   slew   key   is   used   to access  additional  pages  within  a  data  display  selected by  the  data  select  switch.    If  more  than  one  page  is available  a  double  arrow  is  displayed  in  the  lower  right corner of the display.  Pressing the slew key will access the  next  page.    Repeated  pressing  of  the  slew  key  will return the display to the first page after the last page has been accessed. (i) Data    selector    switch.        For    all    data selector    switch    positions    there    are    two    modes    of displayed data: 1. Destination  mode  (active  waypoint as destination) 2. Waypoint (WP) examine mode (any waypoint) Pressing the WP key switches the CDU between the two modes. The 10 position data select switch is used to select the type of information to be displayed on the CDU: POS. Position data is displayed. MSN. Mission data is displayed. OPT. Option data is displayed.  Six pages of information pertaining to the GPS receiver  are  made  available  when the OPT position is selected. STAT. Status data is displayed. VAR-DTM. Magnetic  variation  and  map  datum data is displayed. ERR. Error data is displayed. WIND. Wind data is displayed. DIS-TG. Distance   and   time   to   go   data   is displayed. TRK-GS. Track   and   ground   speed   data   is displayed. DTK-VA. Desired    track    and    vertical    angle data is displayed. (j) Waypoint    key.        The    waypoint    key, placarded  WP,  is  used  to  enter  and  examine  waypoint data. (k)   Mark  key.    The  MARK  key  is  used  for MARK and FREEZE functions. (2)   Observer    Headset    and    GPS    Key    Panel Controls, Indicators, and Functions (fig.  3-28B). (a)   Interphone hot microphone, normal, key radio switch.  This switch, placarded INTPH HOT MIC - NORM - KEY RADIO allows selection of hot microphone intercom, normal, and key radio positions. (b)   Observer   headset   connector.      Allows connection of observer headset. (c)   GPS    Zeroize    switch.        Actuating    the guarded switch, placarded ZEROIZE GPS, will declassify the GPS receiver. 3-69