Bearing Resolution

Continuous Wave The continuous-wave (CW) method uses the Dop- pler effect to detect the presence and speed of an object moving toward or away from the radar. The system is unable to determine the range of the object or to differentiate between objects that lie in the same direction and are traveling at the same speed. It is usually used by fire-control systems to track fast- moving targets at close range. Frequency Modulation In the frequency-modulation (FM) method, energy is transmitted as RF waves that continuously vary, increasing and decreasing, from a fixed-reference fre- quency. Measuring the difference between the fre- quency of the returned signal and the frequency of the radiated signal gives an indication of range. This system works well with stationary or slowly moving targets, but it is not satisfactory for locating fast- moving objects. It is used in aircraft altimeters that give a continuous reading of how high the aircraft is above the Earth. Pulse Modulation In the pulse-modulation method, depending on the type of radar, energy is transmitted in pulses that vary from less than 1µ to 200µ. 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 resolu- tion 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 radar beamwidth and the range of the targets. The physical size and shape of the an- tenna determines beamwidth. Two targets at the same range must be separated by at least one beamwidth to be distinguished as two objects. Some external factors that affect radar perfor- mance 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 con- ditions. 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 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. The increased bending in such a situation is referred to as ducting, and may greatly affect radar perfor- mance. 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. 1-5