Pulse Modulation With  the  pulse  modulation  method,  depending  on the type of radar, energy is transmitted in pulses that vary from less than 1 microsecond to 200 microseconds. 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 modulation radars to a great  extent. FACTORS AFFECTING RADAR PERFORMANCE Radar accuracy is a measure of the ability of a radar system to determine the correct range, bearing, and in some  cases,  altitude  of  an  object.  The  degree  of accuracy is primarily determined by the resolution of the radar  system  and  atmospheric  conditions. Range Resolution Range resolution is the ability of a radar to resolve between two targets on the same bearing, but at slightly different  ranges. The  degree  of  range  resolution depends on the width of the transmitted pulse, the types and  sizes  of  targets,  and  the  efficiency  of  the  receiver and indicator. Bearing Resolution Bearing, or azimuth, resolution is the ability of a radar system to separate objects at the same range but at slightly  different  bearings.  The  degree  of  bearing resolution depends on radar beamwidth and the range of the  targets.  The  physical  size  and  shape  of  the  antenna determines beamwidth. Two targets at the same range must be separated by at least one beamwidth to be distinguished  as  two  objects. Earlier in this chapter, we talked about other internal characteristics of radar equipment that affect range performance.  But  there  are  also  external  factors  that effect radar performance. Some of those are the skill of the operator; size, composition, angle, and altitude of the target;   possible   electronic-countermeasure   (ECM) activity;  readiness  of  equipment  (completed  PMS requirements); and weather conditions Atmospheric  Conditions Several  conditions  within  the  atmosphere  can  have an adverse effect on radar performance. A few of these are  temperature  inversion,  moisture  lapse,  water droplets, and dust particles. Either temperature inversion or moisture lapse, alone or in combination, can cause a huge change in the refraction  index  of  the  lowest  few-hundred  feet  of atmosphere. The result is a greater bending of the radar waves passing through the abnormal condition. The increased bending in such a situation is referred to as DUCTING,  and  may  greatly  affect  radar  performance. The  radar  horizon  may  be  extended  or  reduced, depending on the direction in which the radar waves are bent. The effect of ducting is illustrated in figure 1-3. Water droplets and dust particles diffuse radar energy  through  absorption,  reflection,  and  scattering. This leaves less energy to strike the target so the return echo is smaller. The overall effect is a reduction in usable range. Usable range varies widely with weather conditions. The  higher  the  frequency  of  the  radar system, the more it is affected by weather conditions such as rain or clouds. All radar systems perform the same basic functions of detection, so, logically, they all have the same basic equipment requirements. Next, we will talk about that basic  radar  system. BASIC RADAR SYSTEMS Radar  systems,  like  other  complex  electronics systems, are composed of several major subsystems and many  individual  circuits. Although modern radar systems   are   quite   complicated,   you   can   easily understand  their  operation  by  using  a  basic  block diagram of a pulsed radar system. FUNDAMENTAL  RADAR  SYSTEM Since most radars used today are some variation of the pulse radar system, the units we discuss in this section will be those used in a pulse radar. All other Figure 1-3.—Ducting effect on the radar wave. 1-4


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