FACTORS AFFECTING RADAR PERFORMANCE

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