are maintained at that position either manually or by automatic tracking circuits. You need to be familiar with two types of bearing: true and relative. TRUE   BEARING.—True   bearing   is   the   angle between  true  north  and  a  line  pointed  directly  at  the target.   This angle is measured in the horizontal plane and in a clockwise direction from true north. RELATIVE BEARING.—Relative bearing is the angle  between  the  centerline  of  the  ship  and  a  line pointed directly at the target.  This angle is measured in a   c l o c k w i s e   d i r e c t i o n   f r o m   t h e   b ow. M o s t surface-search radars provide only range and bearing information.  Both true and relative bearing angles are illustrated in figure 1-2. Altitude Altitude or height-finding radars use a very narrow beam  in  the  vertical  plane.    This  beam  is  scanned  in elevation,   either   mechanically   or   electronically,   to pinpoint targets.   Tracking and weapons-control radar systems  in  current  use  scan  the  beam  by  moving  the antenna   mechanically   or   the   radiation   source electronically. Most   air-search   radars   use   electronic   elevation scanning   techniques. Some   older   air-search   radar systems use a mechanical elevation scanning device; but these are being replaced by electronically scanning radar systems. RADAR TRANSMISSION METHODS Radar   systems   are   normally   divided   into   two operational   categories   (purposes)   based   on   their method  of  transmitting  energy.     The  most  common method, used for applications from navigation to fire control,  is  the   pulse-modulation   method.     The  other method  of  transmitting  is   continuous-wave   (CW). CW  radars  are  used  almost  exclusively  for  missile guidance. Pulse Modulation In the pulse method, the radar transmits the RF in a short, powerful pulse and then stops and waits for the return echo.   By measuring the elapsed time between the end of the transmitted pulse and the received echo, the radar can calculate a range.   Pulse radars use one antenna for both transmitting and receiving.  While the transmitter is sending out its high-power RF pulse, the antenna   is   connected   to   the   transmitter   through   a special   switch   called   a    duplexer. As   soon   as   the transmitted   pulse   stops,   the   duplexer   switches   the antenna  to  the  receiver. The  time  interval  between transmission and reception is computed and converted into  a  visual  indication  of  range  in  miles  or  yards. Pulse-radar  systems  can  also  be  modified  to  use  the Doppler effect to detect a moving object.    The Navy uses pulse radars to a great extent. Continuous Wave In   a   CW   radar   the   transmitter   sends   out   a “continuous wave” of RF energy.   Since this beam of RF   energy   is   “always   on”,   the   receiver   requires   a separate antenna.   One disadvantage of this method is that   an   accurate   range   measurement   is   impossible because there is no specific “stop time”.   This can be overcome, however, by modulating the frequency.   A frequency-modulated   continuous   wave   (FM-CW) radar  can  detect  range  by  measuring  the  difference between  the  transmitted  frequency  and  the  received frequency.  This is known as the “Doppler effect”.  The c o n t i n u o u s - wave    m e t h o d    i s    u s u a l l y    u s e d    b y fire-control  systems  to  illuminate  targets  for  missile systems. RADAR SYSTEM ACCURACY To   be   effective,   a   radar   system   must   provide accurate   indications. That   is,   it   must   be   able   to determine and present the correct range, bearing, and, in  some  cases,  altitude  of  an  object.    The  degree  of accuracy  is  primarily  determined  by  two  factors:  the r e s o l u t i o n    o f    t h e    r a d a r    s y s t e m    a n d    ex i s t i n g atmospheric conditions. 1-4 Figure 1-2.—True and relative bearings.