RADAR MEASUREMENTS We stated earlier that radar is used to determine the distance   and   direction   to   and   the   height   of   distant objects.   These three pieces of information are known, respectively, by the standard terms range, bearing, and altitude.  The use of these standard terms allows anyone interested  in  a  specific  target  to  establish  its  position quickly  and  accurately.    Radar  operators  determine  a target’s range, bearing, and altitude by interpreting its position displayed on a specially designed cathode-ray tube (CRT) installed in a unit known as a plan position indicator (PPI). While most radars are used to detect targets, some types are used to guide missiles to targets and to direct the   firing   of   gun   systems;   other   types   provide long-distance surveillance and navigation information. Range  and  bearing  (and  in  the  case  of  aircraft, altitude) are necessary to determine target movement. To be a successful radar operator, you must understand the capabilities and limitations of your radar system in determining range, bearing, and altitude. Range The  radar  measurement  of  range  (or  distance)  is p o s s i b l e    d u e    t o    t h e    p r o p e r t i e s    o f    r a d i a t e d electromagnetic energy.   This energy normally travels through space in a straight line, at a constant speed, and varies  only  slightly  due  to  atmospheric  and  weather conditions. The   frequency   of   the   radiated   energy causes   the   radar   system   to   have   both   a   minimum effective range and a maximum effective range. M I N I M U M    R A N G E . — R a d a r    d u p l e x e r s alternately 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. The   minimum range  of  a  radar,  therefore,  is  the  minimum  distance between  the  radar’s  antenna  and  a  target  at  which  a radar   pulse   can   be   transmitted,   reflected   from   the target,   and   received   by   the   radar   receiver. If   the antenna is closer to the target than the radar’s minimum range, any pulse reflected from the target will return before the receiver is connected to the antenna and will not be 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 pulse-repetition rate (PRR) (PRF and   PRR   are   synonymous   terms.);   and   receiver sensitivity,   with   PRF/PRR   as   the   primary   limiting factor. The peak power of a pulse determines how far the pulse can travel to a target and still return a usable echo. A usable echo is the weakest signal that a receiver can detect, process, and present on a display. The  PRR  determines  the  rate  at  which  the  range indicator is reset to zero.   As the leading edge of each pulse  is  transmitted,  the  indicator  time  base  used  to measure the returned echo is reset, and a new sweep appears on the screen. 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. The   vertical   edge   provides   a definite point from which to measure elapsed time on the   indicator   time   base. 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’s height, beamwidth, and rotation rate. A higher antenna will create a longer radar horizon, allowing a greater range   of   detection. An   antenna   with   a   narrow beamwidth, provides a greater range capability, since it provides more concentrated beam with a higher energy density per unit area.   A slower antenna rotation rate, providing more transmitted pulses during the sweep, allows   the   energy   beam   to   strike   each   target   more times,  providing  stronger  echo  returns  and  a  greater detection range. From  the  range  information,  the  operator  knows the   distance   to   an   object. He   now   needs   bearing information   to   determine   where   the   target   is   in reference to the ship. Bearing Radar bearing is determined by the echo’s signal strength  as  the  radiated  energy  lobe  moves  past  the t a r g e t . S i n c e    s e a r c h    r a d a r    a n t e n n a s    m o v e continuously,  the  point  of  maximum  echo  return  is determined   either   by   the   detection   circuitry   as   the beam  passes  the  target  or  visually  by  the  operator. Weapons   control   and   guidance   radar   antennas   are positioned to the point of maximum signal return and 1-3