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Radar reference coordinates
RADAR  DETECTING  METHODS

Fire Controlman Volume 02-Fire Control Radar Fundamentals
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RANGE/BEARING/ALTITUDE Radar  systems  use  the  reference  coordinates  to provide   early   detection   of   surface   and   air   targets, resulting   in   extremely   accurate   information   on   di- stance,  direction,  height,  and  speed  of  the  targets.  The visual  radar  data  required  to  determine  a  target’s  posi- tion  and  to  track  the  target  is  usually  displayed  on  a specially   designed   cathode-ray   tube   (CRT)   installed in  a  unit  known  as  a  plan  position  indicator  (PPI). Some  types  of  radar  are  also  used  to  guide  mis- siles  to  targets  and  to  direct  the  firing  of  gun  systems; other  types  of  radar  provide  long-distance  surveillance and   navigation   information. Range  and  bearing  (and  in  the  case  of  aircraft,  alti- tude)  are  necessary  to  determine  target  movement.  It is  very  important  that  you  understand  the  limitations of  your  radar  system  in  the  areas  of  range,  bearing, and   altitude. Range The   radar   measurement   of   range   (or   distance)   is possible  due  to  the  properties  of  radiated  electromag- netic  energy.  This  energy  normally  travels  through space  in  a  straight  line,  at  a  constant  speed,  and  varies only  slightly  due  to  atmospheric  and  weather  condi- tions. MINIMUM  RANGE.—   Radar   duplexers   alter- nately  switch  the  antenna  between  the  transmitter  and the  receiver  so  that  one  antenna  can  be  used  for  both functions.  The  timing  of  this  switching  is  critical  to the   operation   of   the   radar   and   directly   affects   the minimum  range  of  the  radar  system.  A  reflected  pulse will  not  be  received  during  the  transmit  pulse  and subsequent   receiver   recovery   time.   Therefore,   any reflected   pulses   from   close   targets   that   return   before the  receiver  is  connected  to  the  antenna  will  be  un- detected. MAXIMUM  RANGE.—  The  maximum  range  of a  pulse-radar  system  depends  on  carrier  frequency, peak   power   of   the   transmitted   pulse,   pulse-repetition frequency   (PRF)   or,   and   receiver   sensitivity   pulse- repetition   rate   (PRR). The   peak   power   of   the   pulse   determines   what maximum  range  the  pulse  can  travel  to  a  target  and still   return   a   usable   echo.   A   usable   echo   is   the smallest   signal   detectable   by   a   receiver   that   can   be processed  and  presented  on  an  indicator. The  PRR  will  determine  the  frequency  that  the indicator  is  reset  to  the  zero  range.  With  the  leading edge  of  each  transmitted  pulse,  the  indicator  time  base used   to   measure   the   returned   echoes   is   reset,   and   a new   sweep   appears   on   the   screen.   If   the   transmitted pulse  is  shorter  than  the  time  required  for  an  echo  to return,  that  target  will  be  indicated  at  a  false  range  in a  different  sweep.  For  example,  the  interval  between pulses   is   610   µsec   with   a   repetition   rate   of   1,640 pulses   per   second.   Within   this   time,   the   radar   pulse can   go   out   and   come   back   a   distance   equal   to   610 µsec  x  164  yards  per  µsec,  or  100,000  yards,  which becomes  the  scope’s  sweep  limit.  Echoes  from  targets beyond  this  distance  appear  at  a  false  range.  Whether an  echo  is  a  true  target  or  a  false  target  can  be  deter- mined  by  simply  changing  the  PRR. RANGE   ACCURACY.—   The  shape  and  width  of the   radio-frequency   (RF)   pulse   influences   minimum range,  range  accuracy,  and  maximum  range.  The  ideal pulse  shape  is  a  square  wave  that  has  vertical  leading and  trailing  edges.  A  sloping  trailing  edge  lengthens the   pulsewidth.   A   sloping   leading   edge   provides   no definite  point  from  which  to  measure  elapsed  time  on the  indicator  time  base. Other   factors   affecting   range   are   the   antenna height,  the  antenna  beamwidth,  and  the  antenna  rota- tion  rate.  A  higher  antenna  will  create  a  longer  radar horizon,   which   allows   a   greater   range   of   detection. Likewise,   a   more-concentrated   beam   has   a   greater range   capability,   since   it   provides   higher   energy density  per  unit  area.  Also,  because  the  energy  beam strikes  each  target  more  times,  a  slower  antenna  rota- tion  provides  stronger  echo  returns  and  a  greater  de- tection  range  for  the  radar. 1-3






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