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Monopulse    Scanning

 
  
 
compared.   Phasing   scanning   causes   an   actual   move- ment   of   the   radar   beam   with   respect   to   the   antenna axis.  In  addition,  electronic  scanning  includes  conical- scan-on-receive-only    (COSRO)    scanning    techniques. Monopulse    Scanning With   monopulse   (simultaneous   lobing)   scanning, range,   bearing,   and   elevation   angle   information   of   a target  is  obtained  from,  as  the  name  implies,  a  single pulse.  This  type  of  tracking  radar  normally  produces a  narrow  circular  beam  of  pulsed  RF  energy  at  a  high pulse-repetition  rate  (PRR).  Each  pulse  is  divided  into four   signals   that   are   equal   in   amplitude   and   phase. The  four  signals  are  radiated  at  the  same  time  from each  of  four  feed  horns  that  are  grouped  in  a  cluster. The  radiated  energy  is  focused  into  a  beam  by  a microwave  lens.  In  turn,  energy  reflected  from  the target  is  refocused  by  the  lens  into  the  feed  horns.  The amount   of   the   total   energy   received   by   each   horn varies,  depending  on  the  position  of  the  target  relative to  the  beam  axis.  The  four  targets  are  at  different  posi- tions  with  respect  to  the  beam  axis.  A  phase  inversion takes  place  at  the  microwave  lens  similar  to  the  image inversion  in  an  optical  system. The   amplitude   of   returned   signals   received   by each   horn   is   continuously   compared   with   those   re- ceived  in  the  other  horns,  and  error  signals  are  gen- erated  that  indicate  the  relative  position  of  the  target with  respect  to  the  axis  of  the  beam.  Angle  servo  cir- cuits  receive  these  error  signals  and  correct  the  posi- tion  of  the  radar  antenna  and  the  director  to  keep  the beam  axis  on  target. An   important   advantage   of   a   monopulse-tracking radar  over  a  radar  using  conical  scan  is  that  the  instan- taneous   angular   measurements   are   not   subject   to   er- rors  caused  by  target  scintillation.  Scintillation  is  the rapid  fluctuation  of  the  echo  signal  amplitude  as  the target  maneuvers  or  moves,  resulting  in  radar  beams bouncing  off  different  areas  of  the  target  and  causing random   reflectivity,   which   may   lead   to   tracking   er- rors.  A  monopulse-tracking  radar  is  not  subject  to  this error   because   each   pulse   provides   an   angular   meas- urement  without  regard  to  the  rest  of  the  pulse  train; therefore,  scintillation  does  not  affect  the  measure- ment. An  additional  advantage  of  monopulse  tracking  is that   no   mechanical   action   is   required,   such   as   a scanner.   Figure   2-6   (on   page   2-8)   shows   monopulse variations   of   received   energy   with   target   positions. Phasing   Scanning Phased   array   antennas   use   the   phasing   scanning method.   This   method   controls   the   phase   of   the   RF signals  fed  to  multiple  feed  horns,  dipoles,  or  radia- tors.  The  angular  position  of  the  beam  is  determined by  the  relative  phase  of  the  signals  at  each  element. When  the  phase  of  the  signals  applied  is  changed,  the beam  can  be  steered  without  moving  the  antenna. Another   method   of   phasing   uses   changes   in   the transmitter    frequency. Changing   the   frequency changes  the  wavelength;  thus,  with  a  fixed  length  of waveguide   between   the   elements,   the   phase   relation- ship  changes  as  the  frequency  changes. Phased   arrays   can   be   used   for   tracking   by   a monopulse-receiving   technique.   The   array   is   divided into  quadrants,  with  each  quadrant  equivalent  to  one of  the  four  horns.  The  sum  of  all  four  quadrants  is compared   to   the   sums   and   differences   of   different quadrants,  just  as  in  monopulse  scanning.  This  tech- nique   is   also   adaptable   to   receive-only   antenna   sys- tems. COSRO  Scanning Scanning   the   received   signal   by   electronically switching  between  the  antenna  elements  (feed  horns) to  produce  a  conical  antenna  receive  pattern  is  called conical-scan-on-receive-only     (COSRO).   COSRO scanning   is   used   with   monopulse   transmissions   and with  single-beam  transmissions. Angle  errors  are  produced  in  much  the  same  way as   mechanical   conical   scanning.   However,   COSRO scanning   is   less   effective   than   monopulse   scanning, 2-7

   


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