Component parts of a radar system

Basic parts of a radar system.

Shows the basic parts of a typical radar system. The transmitter generates the high-power signal that is radiated by the antenna. The antenna is often in the shape of a parabolic reflector, similar in concept to an automobile headlight but much different in construction and size. It also might consist of a collection of individual antennas operating together as a phased-array antenna. In a sense, an antenna acts as a "transducer" to couple electromagnetic energy from the transmission line to radiation in space, and vice versa. The duplexer permits alternate transmission and reception with the same antenna; in effect, it is a fast-acting switch that protects the sensitive receiver from the high power of the transmitter.

The receiver selects and amplifies the weak radar echoes so that they can be displayed on a television-like screen for the human operator or be processed by a computer. The signal processor separates the signals reflected by the target (e.g., echoes from an aircraft) from unwanted echo signals (the clutter from land, sea, rain, etc.). It is not unusual for these undesired reflections to be much larger than desired target echoes, in some cases more than one million times larger. Large clutter echoes from stationary objects can be differentiated from small echoes from a moving target by noting the shift in the observed frequency produced by the moving target. This phenomenon is called the Doppler frequency shift.

At the output of the receiver a decision is made (either by the human operator or automatically by a computer circuit) as to whether or not a target echo is present. If the output of the receiver is larger than a predetermined value, a target is assumed to be present.
 

Once it has been decided that a target is present and its location (in range and angle) has been determined, the track of the target can be obtained by measuring the target location at different times. During the early days of radar, target tracking was performed by an operator marking the location of the target "blip" on the face of a cathode-ray tube (CRT) display with a grease pencil. Manual tracking has been largely replaced by automatic electronic tracking, which can process a much greater number of target tracks (many hundreds or even thousands) than can an operator, who can handle only a few simultaneous tracks. Automatic tracking is an example of an operation performed by a data processor.
 

The type of signal waveform transmitted and the associated received-signal processing in a radar system might be different depending on the type of target involved and the environment in which it is located. An operator can select the parameters of the radar to maximize performance in a particular environment. Alternatively, electronic circuitry in the radar system can automatically analyze the environment (determine which portions are land, sea, or rain) and select the proper transmitted signal, signal processing, and other radar parameters to optimize performance. The box labeled "system control" in Figure 3 is intended to represent this function. The system control also can provide the timing and reference signals needed to permit the various parts of the radar to operate effectively as an integrated system.