RADAR DETECTING METHODS

Bearing Radar bearing is determined by the echo signal strength as the radiated energy lobe moves past the target. Since search radar antennas move continuously, the point of maximum echo return is determined either by the detection circuitry as the beam passes the target or visually by the operator. Weapons control and guidance radar systems are positioned to the point of maximum signal return and maintained at that position either manually or by automatic tracking circuits. TRUE BEARING.— The angle between true north and a line pointed directly at a target is called the true bearing (referenced to true north) of a radar target. This angle is measured in the horizontal plane and in a clockwise direction from true north. RELATIVE BEARING.— The angle between the centerline of your own ship or aircraft and a line pointed directly at a target is called the relative bearing of the radar target. This angle is measured in a clockwise direction from the centerline. Both true and relative bearing angles are illustrated in figure 1-2. Most surface search radars will provide only range and bearing information. If the operator had a need to direct air traffic or to track incoming missiles, the radar would also have to provide altitude. Altitude An operator can determine the altitude of a target by adjusting a movable height line on a height indicator to Figure 1-2.—True and relative bearings. the point where it bisects the center of the target. The altitude is then displayed by an altitude dial or digital readout. A search radar system that detects altitude as well as range and bearing is called a three-dimensional (3D) radar. 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 commonly use mechanical elevation scanning techniques. This requires moving the antenna or radiation source mechanically. Most air search radars use electronic elevation scanning techniques. Some older air search radar systems use a mechanical elevation scanning device; however, these are being replaced by electronically-scanned radar systems. RADAR DETECTING METHODS Radar systems are normally divided into operational categories based on energy transmission methods. Although the pulse methcd is the most common method of transmitting radar energy, two other methods are sometimes used in special applications. These are the continuous wave (cw) method and the frequency modulation (fm) method. Continuous Wave The continuous wave (cw) method uses the Doppler 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 With the frequency modulation (fm) method, energy is transmitted as radio frequency (rf) waves that continuously vary, increasing and decreasing, from a fixed reference frequency. Measuring the difference between the frequency of the returned signal and the frequency of the radiated signal will give an indication of range. This system works well with stationary or slowly-moving targets, but it is not satisfactory for locating moving objects. It is used in aircraft altimeters that give a continuous reading of how high the aircraft is above the earth. 1-3