SCANNING METHODS

The simple CW radar has the same basic com- ponents found in pulse radars; the main difference is the use of a separate antenna to receive and the use of filters to detect a Doppler shift. The filters are nor- mally designed for the IF range, since working at the transmitter frequencies is more critical in the con- struction of circuits. The filters are set up to detect a particular narrow frequency band. The bands are set so they are adjacent to each other and cover the expected Doppler frequencies above and below the zero shift. The narrower the filter bandwidth, the more filters are required and the more discrete velocities can be determined within the receiver bandwidth. With the basic CW radar, target range cannot be determined. However, a target can be tracked by the angle method. Angle resolution is determined the same way for both CW and pulse radars. CW radars have an advantage over pulse radars when detecting moving targets in clutter. This is very useful when the clutter is caused by chaff, since detec- tion is based on the Doppler frequency and not on a return pulse. With a simple CW radar, it is almost im- possible to detect a stationary target because of clutter and leakage at the transmitter frequency. Moving tar- gets are easily detected, but range determination is more difficult. A diagram of a basic CW radar is shown in figure 2-4. Figure 2-4.—Basic CW radar. FM-CW Radar System The limitation of being unable to determine target range with C W radars can be overcome by frequency modulating the transmitted signal. The resulting radar is the frequency-modulated, continuous-wave (FM- CW) radar. The transmitter still transmits continuously, but the frequency is changed in a predetermined fashion. The modulation can be in the form of a sawtooth, tri- angular, sinusoidal, or other shape as long as it pro- duces a frequency change of known rate. SCANNING METHODS For a radar to track a target, some means of keep- ing the radar beam pointed at the target is required. The radar system must be able to determine in which direction the radar beam must be moved so that the target remains in the center of the beam. A visual indication on the CRT can depict range and angle of the target in respect to the beam center. The operator can then move the beam by positioning the antenna center on the target. Today’s radar systems use com- puter-aided automatic tracking systems. Their two basic scanning methods are mechanical and electronic scanning. MECHANICAL SCANNING Mechanical scanning can be flexible in that the antenna can be moved in one of two desired patterns: (1) The feed horn can be moved relative to a fixed reflector, or (2) the reflector can be moved relative to a fixed feed horn. The most common mechanical scan technique used by fire-control radars is a movable feed horn relative to a fixed reflector, which is called conical scanning. Included in mechanical scanning are nutation, nu- tating waveguide, and angle tracking. 2-5