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Pulse-Doppler   Radar   System
Basic   Continuous-Wave   Radar   System

Fire Controlman Volume 02-Fire Control Radar Fundamentals
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nal,  a  basic  pulse  radar  can  detect  a  weak  signal  from a  moving  target  in  the  presence  of  strong  signals  from large  targets,  such  as  landmasses  and  heavy  seas. Pulse-Doppler  radars  use  the  Doppler  shift  signal. These  radars  can  detect  an  aircraft  flying  close  to  a hill   or   mountain   where   the   strong   landmass   echo would  block  detection  with  a  basic  pulse  radar.  Cir- cuitry   in   the   pulse-Doppler   radar   normally   would reject  the  stationary  target,  allowing  easy  detection  of the  weak  signal  from  the  moving  target. The   ranging   system   of   a   pulse-Doppler   radar   is more   complex   than   that   of   a   pulse   or   a   frequency- modulated,    continuous-wave    (FM-CW)    radar.    A pulse-Doppler  radar  senses  both  range  and  velocity  by time-sharing  its  waveform  between  these  functions. To  detect  a  Doppler  frequency  from  the  target  echo, most  pulse-  Doppler  radars  use  a  much  higher  pulse- repetition   frequency   (PRF)   than   basic   pulse   radars. Higher   PRF   decreases   the   pulse-repetition   time (PRT)   between   pulses,   resulting   in   the   possibility   of a  target  echo  returning  at  the  time  of  the  next  trans- mission.  This,  in  turn,  results  in  a  blind  spot  in  the range.  If  the  echo  from  the  first  pulse  returns  after  the second   pulse   is   transmitted,   then   a   range   ambiguity occurs. The  range  blind  spot  and  ambiguities  can  be  com- pensated  for  by  changing  the  PRF  over  a  wide  range. For   example,   the   fire-control   computer   could   adjust the   radar   PRF   based   on   the   expected   range   of   a designated  target.  If  the  designation  were  for  a  target at 50 nmi (100 kyd), the PRF could be changed so that the  second  pulse  would  not  occur  until  enough  time had  elapsed  for  the  target  echo  to  return  for  that  range, plus   an   additional   range   interval   for   the   acquisition and   tracking   gates.   Varying   the   PRF   over   a   wide range   by   computer   control   can   resolve   range   ambi- guities  and  blind  ranges. DOPPLER  SHIFT  THEORY.—   A  Doppler  shift allows  distinguishing  between  the  target  and  the  trans- mitter  leakage.  The  amount  of  Doppler  shift  is  deter- mined   by   the   radial   velocity   of   the   target   since   the radial  velocity  is  the  apparent  speed  that  the  target  is closing  on  or  going  away  from  the  radar. A  target  can  move  in  any  direction  and  in  a  wide range   of   speed;   therefore,   the   radial   velocity   can change  considerably.  If  the  target  is  moving  at  a  900 angle  to  the  radar,  then  no  Doppler  shift  is  produced. However,  if  the  target  moves  straight  at  or  away  from the  radar,  radial  velocity  will  equal  the  actual  target speed. The  amount  of  Doppler  shift  is  also  dependent  on the   wavelength   resulting   from   the   transmitter   fre- quency.  A  target  radial  velocity  that  produces  a  spe- cific  Doppler  shift  at  5,000  MHz  would  produce  twice as  much  at  10,000  MHz. DOPPLER   SHIFT   DETECTION.—   Pulse-Dop- pier  radars  can  detect  moving  targets  by  the  Doppler shift.   Moving-target   indication   (MTI)   is   used   pri- marily   to   detect   moving   targets   with   pulse-Doppler search   radars. Since  stationary  targets  produce  no  Doppler  shifts, the  return  signal  echo  has  the  same  frequency  and phase  as  the  transmitted  pulse.  However,  moving  tar- gets   do   produce   Doppler   shifts;   therefore,   the   return signal  echo  has  a  different  phase  from  that  of  the transmitted    pulse. To   use   this   principle,   pulse-Doppler   radars   must be  able  to  compare  the  echo  signal  with  a  reference signal  that  is  in  phase  (coherent)  with  the  transmitted signal.   A   means   of   storing   or   controlling   the   trans- mitted  phase  is  required  to  provide  coherent  detection. One  method  uses  a  magnetron  for  the  transmitter. This  requires  that  the  local  oscillator  be  stable  for  a small  fraction  of  a  cycle  during  one  pulse  period.  A sample   of   the   transmitted   and   stable   local   oscillator (STALO)  signals  are  fed  to  the  coherent  oscillator (COHO).  This  locks  in  the  phase  of  the  COHO  until the  next  transmitter  pulse.  Figure  2-2  is  a  diagram  of coherent   MTI   with   a   phase-locked   COHO   oscillator. 2-3






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