between  the  horizontal  plane  and  the  LOS  is  the ELEVATION   ANGLE.   The   angle   measured clockwise from true north in the horizontal plane is called the  TRUE  BEARING  or AZIMUTH  angle. Information  based  on  these  terms  describes  the  location of an object with respect to the antenna, giving the operator data on range, bearing, and altitude. RANGE/BEARING/ALTITUDE Using  the  coordinate  system  discussed  above,  radar systems provide early detection of surface or air objects, giving  extremely  accurate  information  on  distance, direction,  height,  and  speed  of  the  objects.  The  visual radar data required to determine a target’s position and to track the target is usually displayed on a specially designed cathode-ray tube (crt) installed in a unit known as  a  planned  position  indicator  (ppi). Radar is also used to guide missiles to targets and to direct the firing of gun systems. Other types of radar provide  long-distance  surveillance  and  navigation information. Bearing  and  range  (and  in  the  case  of  aircraft, altitude) are necessary to determine target movement. It is very important that you understand the limitations of your radar system in the areas of range, hewing, and altitude. Range Radar measurement of range (or distance) is made possible   because   of   the   properties   of   radiated electromagnetic  energy.  This  energy  normally  travels through space in a straight line, at a constant speed, and will  vary  only  slightly  because  of  atmospheric  and weather conditions. The range to an object, in nautical miles, can be determined by measuring the elapsed time (in microseconds) during the round trip of a radar pulse and  dividing  this  quantity  by  the  number  of microseconds required for a radar pulse to travel 2 nautical miles (12.36). In equation form this is: elapsed  time range  (nautical  miles)  = 12.36 MINIMUM   RANGE.—   Radar   duplexers alternately  switch  the  antenna  between  the  transmitter and 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  undetected. MAXIMUM RANGE.— The maximum range of a pulse radar system depends upon carrier frequency peak power  of  the  transmitted  pulse,  pulse  repetition frequency  (prf),  or  pulse  repetition  rate  (prr),  and receiver  sensitivity. 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 sec with a repetition rate of 1640 pulses per second. Within this time the radar pulse can go out and come back a distance equal to 610 sec ’ 164 yards per sec, 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 determined by simply changing the prr. RANGE ACCURACY.—  The shape and width of the 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  pulse  width.  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, antenna beam width, and antenna rotation 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 would strike each target more times, a slower antenna rotation provides stronger echo returns and a greater detection range for the radar. Given the range information, the operator knows the distance to an object, but information on bearing is still required to determine in which direction from the ship the target lies. 1-2