because  the  system  is  disabled  by  the  weight- on-wheels   switch. RECEIVER-TRANSMITTER.—    This  system uses    the    RT-1042/APN-194(V)    as    its    receiver- transmitter (fig. 2-10, view B). It is an airtight unit that  contains  all  the  electronic  components  for  the generation,   detection,   and   time   difference computations of the radar pulses. There  are  two  altitude  modes  of  operation-one for  low  level  and  one  for  high  level.  The  low-level mode  is  for  altitudes  less  than  1,000  feet.  In  this mode, the RT transmits a very narrow, low-powered pulse  to  get  maximum  range  resolution.  The high-level mode is for altitudes above 1,000 feet. In this mode, the output pulse is a wider, high-powered pulse, which ensures sufficient ground return energy for tracking. There are two ranging modes of operation—one for search and one for tracking. In the search mode, the   system   successively   examines   increments   of range with each cycle of operation until the complete altitude range is searched for ground return. When the range is found, the system switches to the track mode. In the track mode, the system locks onto and tracks the leading edges of the ground return pulses. It then sends continuous altitude information to the indicator. In  most  aircraft  with  this  system,  there  is  an Interference   Blanker,   MX-9132A/APN-194(V), located  next  to  the  RT  (fig.  2-10,  view  B).  This blanker   attenuates   any   RF   from   direct   antenna leakage  and  provides  isolation  of  the  receiver  from the transmit antenna. LOW ALTITUDE AUDIBLE ALARM.— Some of the aircraft with the APN-194(V) system have the BZ-157A  low  altitude  audible  alarm  installed  (fig. 2-10, view C). In the EA-6A aircraft, this box is on the ICS relay box behind the EWO’s seat. This alarm will apply a tone to the intercommunication system when the aircraft falls below the altitude that the limit bug  is  set.  The  tone  is  a  2-second  tone  alternating between  700  and  1700  Hz  at  2-Hz  intervals.  This alarm  will  also  sound  when  there  is  an  unreliable condition  in  the  system.  This  tone  alternates  between 700 and 1700 Hz at 8-Hz intervals. The alarm will also send a signal to the indicator to cause the OFF flag  to  appear. The  unreliable  condition  warning signal  takes  precedence  over  the  low  altitude  warning signal. The BZ-157A has three switches on it. The first one is the volume control, which controls the volume of the alarm. The other two are press-to-test switches. Depressing the LOW ALT switch will cause the alarm to  be  heard.  Depressing  the  UNREL  switch  will cause  the  alarm  to  be  heard  and  the  OFF  flag  to appear. LOW  ALTITUDE  WARNING  LIGHT.—  The low altitude warning light is mounted on the pilot’s instrument  panel.  This  light  will  illuminate  whenever the aircraft falls below the altitude that the limit bug is set. RADAR ALTIMETER WARNING SET The radar altimeter warning set (RAWS) works in conjunction with the radar altimeter systems that do not have the BZ-157A alarm unit. The P3-C aircraft uses the AN/APQ-107 system. This system provides the pilot and copilot with warning signals whenever any of the following conditions exists: . Aircraft flies below preselected altitudes. .  Input  power  to  radar  altimeter  fails. . RAWS warning circuit indicates unreliability. When  the  aircraft  descends  to  380  (±20)  feet (high-altitude  index),  the  radar  altimeter  signal  to RAWS is interrupted. This causes the AUTOPILOT/ RADAR ALTM warning lights to flash and a 1-kHz interrupted  tone  to  be  heard  over  the  ICS.  Both signals occur at a rate of two pulses-per-second for a 3-second duration. This also happens as the aircraft descends through 170 feet (low-altitude index). The warnings will continue as long as the aircraft remains below the low index. When the nosegear is down or the  flaps  are  in  the  approach,  takeoff,  or  landing position, the warning signals are disabled. If the radar altimeter receiver signal is too weak to provide  reliable  altitude  information  or  the  altimeter malfunctions,  the  RAWS  will  give  both  warnings. This  occurs  unless  the  signals  are  disabled  by  the nosegear  or  flap  position. There  are  two  RAWS  press-to-test  switches located in the aircraft. One on the RAWS itself, and one  on  the  forward  load  center.  If  you  depress  the switch  on  the  RAWS,  it  will  actuate  both  signals, unless the nosewheel is down and locked-then only the aural warning occurs. Depressing the switch on the forward load center results in both the visual and the  aural  warnings. 2-11