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SCANNING Radar systems are often identified by the type of SCANNING the system uses. Scanning is the systematic movement of a radar beam in a definite pattern while searching for or tracking a target. The type and method of scanning used depends on the purpose and type of radar and on the antenna size and design. In some cases, the type of scan will change with the particular system mode of operation. For example, in a particular radar system, the search mode scan may be quite different from that of the track mode scan. Stationary-Lobe Scanning A SINGLE STATIONARY-LOBE SCANNING SYSTEM is the simplest type of scanning. This method produces a single beam that is stationary in relation to the antenna. The antenna is then mechanically rotated continuously to obtain complete 360-degree azimuth coverage. A stationary lobe, however, cannot satisfactorily track a moving object because it does not provide enough information about the object's movement to operate automatic tracking circuits, such as those in fire-control tracking radar. A two-dimensional search radar, however, does use a single-lobe that is scanned in a 360-degree pattern because automatic tracking circuits are not normally used in 2D radars. Single-lobe scanning is unsuitable for use as a tracking radar for several reasons. For example, let's assume that a target is somewhere on the lobe axis and the receiver is detecting signals reflected from the target. If these reflected signals begin to decrease in strength, the target likely has flown off the lobe axis. In this case, the beam must be moved to continue tracking. The beam might be moved by an operator tracking the target with an optical sight, but such tracking is slow, inaccurate, and limited by conditions of visibility. An automatic tracking system would require that the beam SCAN, or search, the target area in such a case. Again, assume that a missile is riding (following) the axis of a single beam. The strength of the signals it receives (by means of a radar receiver in the missile) will gradually decrease as its distance from the transmitter increases. If the signal strength decreases suddenly, the missile will know, from built-in detection circuitry, that it is no longer on the axis of the lobe. But it will not know which way to turn to get back on the axis. A simple beam does not contain enough information for missile guidance. Methods of Beam Scanning The two basic methods of beam scanning are MECHANICAL and ELECTRONIC. In mechanical scanning, the beam can be moved in various ways: (1) The entire antenna can be moved in the desired pattern; (2) the energy feed source can be moved relative to a fixed reflector; or (3) the reflector can be moved relative to a fixed source. In electronic scanning, the beam is effectively moved by such means as (1) switching between a set of feeder sources, (2) varying the phasing between elements in a multielement array, or (3) comparing the amplitude and phase differences between signals received by a multielement array. A combination of mechanical and electronic scanning is also used in some antenna systems. MECHANICAL SCANNING. - The most common type of mechanical scanning is the rotation of the antenna through 360 degrees to obtain azimuth coverage. Most search radar sets use this method. A common form of scanning for target tracking or missile beam-rider systems is CONICAL (cone-like) SCANNING. This is generally accomplished mechanically by NUTATING the rf feed point. Nutation is difficult to describe in words but easy to demonstrate. Hold a pencil in two hands. While holding the eraser end as still as possible, swing the point in a circular motion. This motion of the pencil is referred to as nutation; the pencil point corresponds to the open, or transmitting, end of the waveguide antenna. The important fact to remember is that polarization of the beam is not changed during the scanning cycle. This means that the axis of the moving feeder does not change either horizontal or vertical orientation while the feeder is moving. You might compare the feeder movement to that of a ferris wheel; that is, the vertical orientation of each seat remains the same regardless of the position of the wheel. Recall that a waveguide is a metal pipe, usually rectangular in cross section, used to conduct the rf energy from the transmitter to the antenna. The open end of the waveguide faces the concave side of the reflector and the rf energy it emits is bounced from the reflector surface. A conical scan can be generated by nutation of the waveguide. In this process the axis of the waveguide itself is moved through a small conical pattern. In an actual installation of a nutating waveguide, the three-dimensional movement is fast and of small amplitude. To an observer, the waveguide appears merely to be vibrating slightly. By movement of either the waveguide or the antenna, you can generate a conical scan pattern, as shown in figure 1-17. The axis of the radar lobe is made to sweep out a cone in space; the apex of this cone is, of course, at the radar transmitter antenna or reflector. At any given distance from the antenna, the path of the lobe axis is a circle. Within the useful range of the beam, the inner edge of the lobe always overlaps the axis of scan. Figure 1-17. - Conical scanning.
Now assume that we use a conically scanned beam for target tracking. If the target is on the scan axis, the strength of the reflected signals remains constant (or changes gradually as the range changes). But if the target is slightly off the axis, the amplitude of the reflected signals will change at the scan rate. For example, if the target is to the left of the scan axis, as shown in figure 1-18, the reflected signals will be of maximum strength as the lobe sweeps through the left part of its cone; the signals will quickly decrease to a minimum as the lobe sweeps through the right part. Information on the instantaneous position of the beam, relative to the scan axis, and on the strength of the reflected signals is fed to a computer. Such a computer in the radar system is referred to as the angle-tracking or angle-servo circuit (also angle-error detector). If the target moves off the scan axis, the computer instantly determines the direction and amount of antenna movement required to continue tracking. The computer output is used to control servomechanisms that move the antenna. In this way, the target is tracked accurately and automatically. Figure 1-18. - Reflected signal strength.
Q.20 What is the simplest type of scanning? |
 
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