Atmospheric Conditions

Range Resolution Range resolution is the ability of a radar to distinguish 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 the targets, and the efficiency of the receiver and the 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 the radar’s 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. 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. The temperature and moisture content of the atmosphere normally decrease uniformly with an increase in altitude. However, under certain conditions the temperature may first increase with height and then begin to decrease. Such a situation is called a temperature inversion. An even more important deviation from normal may exist over the ocean. Since the atmosphere close to the surface over large bodies of water may contain more than a normal amount of moisture, the moisture content may decrease more rapidly at heights just above the sea. This effect is referred to as moisture lapse. Either temperature inversion or moisture lapse, alone or in combination, can cause a large change in the refraction index of the lowest few-hundred feet of the atmosphere. The result is a greater bending of the radar waves passing through the abnormal condition. This increase in bending, referred to as ducting, 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 such weather conditions. The higher the frequency of the radar system, the more it is affected by weather conditions, such as rain or clouds. Other Factors Some other factors that affect radar performance are operator skill; size, composition, angle, and altitude of the target; possible Electronic Attack (EA) activity; readiness of equipment (completed planned maintenance system requirements); and weather conditions. Q1. For radar surface angular measurements, what is considered to be at the center of the coordinate system? Q2. What determines radar bearing? Q3. What is the most common method of radar transmission? Q4. What two factors determine radar accuracy? 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 pulse-radar system. FUNDAMENTAL (PULSE) RADAR SYSTEM Since most radars used today are some variation of the pulse-radar system, this section discusses components used in a pulse radar. All other types of radars use some variation of these units. Refer to the block diagram in figure 1-4. 1-5 Figure 1-3.—Ducting effect on the radar wave.