DOPPLER In  the  following  text  we  will  discuss  the AN/APN-153(V)  navigation  set.  For  more  informa- tion  on  the  basics  of  the  Doppler  theory,  refer  to Aviation  Electronics  Technician  2  (Organizational), NAVEDTRA 12330. The   AN/APN-153(V)   navigation   set   is   a self-contained,   airborne,   pulsed   Doppler   radar system. It is designed to provide navigation data to the  navigation  computers  onboard  various  aircraft. This  system  automatically  and  continuously  provides ground  speed  and  drift  angle  information  to  the computer. This information is then used for the dead reckoning  of  the  aircraft  without  the  aid  of  wind estimates or true airspeed. The system operates without the use of ground installations over an unlimited geographical range. Weather  does  not  affect  the  system  performance.  The navigation set is accurate from 40 feet to 50,000 feet over land and water that has a sea state of 1 or greater. Ground speed is accurate from 80 to 800 knots, with the drift being accurate to 40 degree right or left. This system  is  difficult  to  jam  because  of  the  directivity and   narrow   width   of   the   microwave   beam transmission and the variation of PRF. Theory of Operation The  Doppler  system  determines  the  ground  speed and   drift   angle   components   by   measuring   the frequency  shift  in  received  echoes.  In  other  words, the system beams signals to the ground, receives the return  echoes,  and  then  measures  the  frequency  shift produced by the relative motion between the aircraft and the earth. Since the aircraft moves both along its length and across its length, more than one beam is required. The AN/APN-153(V) uses four. These four beams strike the ground at the corners of a rectangle. The  system  is  pulsed  so  that  only  two  beams (diagonally  opposite)  are  transmitted  or  received  at  a time.  Of  these,  aircraft  motion  shifts  the  forward beam  up  in  frequency  and  the  rearward  down  in frequency.  These  two  shifts  are  compared,  and  a difference  signal  for  the  pair  is  formed.  Then  the other  two  beams  are  used.  The  two  difference  signals are then compared, and an azimuth motor rotates the antenna to make and keep them equal. This keeps the antenna aligned with the ground track. Since the drift angle is derived directly from antenna position, drift angle  accuracy  is  not  affected  by  signal  quality, terrain,  or  sea  state,  as  long  as  any  echo  at  all  is received. Once the antenna is aligned with the ground track the measured frequency shift is used to derive ground speed. The drift angle and the ground speed information is then sent to the navigation computer as inputs  for  dead  reckoning  navigation. Major  Components There  are  three  major  components  in  the AN/APN-153(V)   system.   They   are   the   Receiver- Transmitter   RT-680A/APN-153(V),   Antenna AS-1350/APN-153(V),   and   the   Control   Indicator C-4418A/APN-153(V). RECEIVER-TRANSMITTER.—  The RT-680A/ APN-153(V) contains the transmitter, receiver, and the frequency tracker circuits (fig. 2-16, view A). It is essentially a conventional radar system that uses a magnetron  power  oscillator  whose  PRF  is  varied  by  a sawtooth voltage. The receiver is a superheterodyne receiver, where the two signals are amplified, mixed, and detected. The resultant detector signal is a single audio   signal.   This   audio   signal   is   then   filtered, amplified, and sent to the frequency tracking circuits. Here, the signal is mixed with the output frequency generator in the main tracking loop. Any difference between  the  two  is  amplified  and  phase  detected.  The resulting voltage is fed to the frequency generator, which  makes  its  frequency  equal  to  the  received audio.  The  received  audio  is  a  function  of  the Doppler  shift,  and,  therefore,  is  the  ground  speed. ANTENNA.—   The    AS-1350/APN-153(V) contains  the  microwave  plumbing,  pitch  and  roll rotary couplers, antenna arrays, and the pitch and roll servo  networks  that  maintain  the  arrays  in  level position  during  aircraft  motion  (fig.  2-16,  view  B). The  antenna  pitch  and  roll  data  used  for  leveling  are obtained from the course attitude data transmitter group of the aircraft. The antenna takes the RF pulses from the RT and radiates them in two patterns emitted alternately at half-second  intervals.  It  then  receives  the  return echoes and feeds them to the RT. The antenna also takes the signal from the detector in the main tracking loop  to  position  the  antenna  arrays  parallel  to  the aircraft’s ground track. The  array  position,  which now represents aircraft drift angle, is fed to a servo follow-up  in  the  control  panel. CONTROL  INDICATOR.—  The   C-4418A/ APN-153(V)  contains  the  controls  and  indicators required  for  system  operation  (fig.  2-16,  view  C). The control indicator applies system power, selects 2-18