The transmitter of a radar system must be efficient, reliable, not too large in size and weight, and easily maintained, as well as have the wide bandwidth and high power that are characteristic of radar applications. In general, the transmitter must generate low-noise, stable transmissions so that extraneous (unwanted) signals from the transmitter do not interfere with the detection of the small Doppler frequency shift produced by weak moving targets.
It is observed in the section History of radar that the invention of the magnetron transmitter in the late 1930s resulted in radar systems that could operate at the higher frequencies known as microwaves. The magnetron transmitter has certain limitations, but it continues to be used, for example, in low-average-power applications such as ship navigation radar and airborne weather-avoidance radar. The magnetron is a power oscillator in that it self-oscillates (i.e., generates microwave energy) when voltage is applied. Other radar transmitters usually are power amplifiers in that they take low-power signals at the input and amplify them to high power at the output. This provides stable high-power signals, as the signals to be radiated can be generated with precision at low power.
The klystron amplifier is capable of some of the highest power levels used in radar (many hundreds of kilowatts of average power). It has good efficiency and good stability. The disadvantages of the klystron are that it is usually large and it requires high voltages (e.g., about 90 kilovolts for one megawatt of peak power). At low power the instantaneous bandwidth of the klystron is small, but the klystron is capable of large bandwidth at high peak powers of a few megawatts.
The traveling-wave tube (TWT) is related to the klystron. It has very wide bandwidths at low peak power, but, as the peak power levels are increased to those needed for pulse radar, its bandwidth decreases. As peak power increases, the bandwidths of the TWT and the klystron approach one another.
Solid-state transmitters, such as the transistor, are attractive because of their potential for long life, ease of maintenance, and relatively wide bandwidth. An individual solid-state device generates relatively low power and can be used only when the radar application can be accomplished with low power (as in short-range applications or in the radar altimeter). High power can be achieved, however, by combining the outputs of many individual solid-state devices.
While the solid-state transmitter is easy to maintain and is capable of wide-band operation, it has certain disadvantages. It is much better suited for long pulses (milliseconds) than for short pulses (microseconds). Long pulses can complicate radar operation because signal processing (such as pulse compression) is needed to achieve the desired range resolution. Furthermore, a long-pulse radar generally requires several different pulse widths: a long pulse for long range and one or more shorter, high-energy pulses with less energy to observe targets at the ranges masked when the long pulse is transmitting. (A one-millisecond pulse, for example, masks echoes from 0 to about 80 nautical miles, or 150 km.)
Every kind of transmitter has its disadvantages as well as advantages. In any particular application, the radar engineer must continually search for compromises that give the results desired without too many negative effects that cannot be adequately accommodated.
Receivers
Like most other receivers, the radar receiver is a classic superheterodyne. It has to filter the desired echo signals from clutter and receiver noise that interfere with detection. It also must amplify the weak received signals to a level where the receiver output is large enough to actuate a display or a computer. The technology of the radar receiver is well established and seldom sets a limit on radar performance.
The receiver must have a large dynamic range in situations where it is necessary to detect weak signals in the presence of very large clutter echoes by recognizing the Doppler frequency shift of the desired moving targets. Dynamic range can be loosely described as the ratio of the strongest to the weakest signals that can be handled without significant distortion by a receiver. A radar receiver might be required to detect signals that vary in power by a million to one—and sometimes much more.
In most cases the sensitivity of a radar receiver is determined by the noise generated internally at its input. Because it does not generate much noise of its own, a transistor is usually used as the first stage of a receiver.
Signal and data processors
The signal processor is the part of the receiver that extracts the desired target signal from unwanted clutter. 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 separated from small moving target echoes by noting the Doppler frequency shift produced by the moving targets. Most signal processing is performed digitally with computer technology. Digital processing has significant capabilities in signal processing not previously available with analog methods.
Pulse compression is sometimes included under signal processing. It too benefits from digital technology, but analog processors (e.g., surface acoustic wave delay lines) are used rather than digital methods when pulse compression must achieve resolutions of a few feet or less.
Displays
Although it has its limitations, the cathode-ray tube (CRT) has been the preferred technology for displaying information ever since the early days of radar. There have been, however, considerable improvements in flat-panel displays because of the demands of computers and television. Flat-panel displays occupy less volume and require less power than CRTs, but they also have their limitations. Radar has taken advantage of flat-panel displays and has become increasingly important as a display.
In the early days of radar, an operator decided whether a target was present on the basis of what raw data were displayed. Modern radars, however, present processed information to the operator. Detections are made automatically in the receiver without operator involvement and are then presented on the display to the operator for further action.
A commonly used radar display is the plan position indicator (PPI), which provides a maplike presentation in polar coordinates of range and angle. The display is “dark” except when echo signals are present.
All practical radar displays have been two-dimensional, yet many radars provide more information than can be displayed on the two coordinates of a flat screen. Colour coding of the signal indicated on the PPI is sometimes used to provide additional information about the echo signal. Colour has been employed, for example, to indicate the strength of the echo. Doppler weather radars make good use of colour coding to indicate on a two-dimensional display the levels of rain intensity associated with each echo shown. They also utilize colour to indicate the radial speed of the wind, the wind shear, and other information relating to severe storms.
Factors affecting radar performance
The performance of a radar system can be judged by the following: (1) the maximum range at which it can see a target of a specified size, (2) the accuracy of its measurement of target location in range and angle, (3) its ability to distinguish one target from another, (4) its ability to detect the desired target echo when masked by large clutter echoes, unintentional interfering signals from other “friendly” transmitters, or intentional radiation from hostile jamming (if a military radar), (5) its ability to recognize the type of target, and (6) its availability (ability to operate when needed), reliability, and maintainability. Some of the major factors that affect performance are discussed in this section.
Transmitter power and antenna size
The maximum range of a radar system depends in large part on the average power of its transmitter and the physical size of its antenna. (In technical terms, this is called the power-aperture product.) There are practical limits to each. As noted before, some radar systems have an average power of roughly one megawatt. Phased-array radars about 100 feet (30 metres) in diameter are not uncommon; some are much larger. There are specialized radars with (fixed) antennas, such as some HF over-the-horizon radars and the U.S. Space Surveillance System (SPASUR), that extend more than one mile (1.6 km).
Receiver noise
The sensitivity of a radar receiver is determined by the unavoidable noise that appears at its input. At microwave radar frequencies, the noise that limits detectability is usually generated by the receiver itself (i.e., by the random motion of electrons at the input of the receiver) rather than by external noise that enters the receiver via the antenna. A radar engineer often employs a transistor amplifier as the first stage of the receiver even though lower noise can be obtained with more sophisticated (and more complex) devices. This is an example of the application of the basic engineering principle that the “best” performance that can be obtained might not necessarily be the solution that best meets the needs of the user.
The receiver is designed to enhance the desired signals and to reduce the noise and other undesired signals that interfere with detection. A designer attempts to maximize the detectability of weak signals by using what radar engineers call a “matched filter,” which is a filter that maximizes the signal-to-noise ratio at the receiver output. The matched filter has a precise mathematical formulation that depends on the shape of the input signal and the character of the receiver noise. A suitable approximation to the matched filter for the ordinary pulse radar, however, is one whose bandwidth in hertz is the reciprocal of the pulse width in seconds.
Target size
The size of a target as “seen” by radar is not always related to the physical size of the object. The measure of the target size as observed by radar is called the radar cross section and is given in units of area (square metres). It is possible for two targets with the same physical cross-sectional area to differ considerably in radar size, or radar cross section. For example, a flat plate 1 square metre in area will produce a radar cross section of about 1,000 square metres at a frequency of 3 GHz when viewed perpendicular to the surface. A cone-sphere (an object resembling an ice-cream cone) when viewed in the direction of the cone rather than the sphere could have a radar cross section of about 0.001 square metre even though its projected area is also 1 square metre. In theory, the radar cross section has little to do with the size of the cone or the cone angle. Thus, the flat plate and the cone-sphere can have radar cross sections that differ by a million to one even though their physical projected areas are the same.
The sphere is an unusual target in that its radar cross section is the same as its physical cross-sectional area (when its circumference is large compared with the radar wavelength). That is to say, a sphere with a projected area of 1 square metre has a radar cross section of 1 square metre.
Commercial aircraft might have radar cross sections from about 10 to 100 square metres, except when viewed broadside, where the cross sections are much larger. Most air-traffic-control radars are required to detect aircraft with a radar cross section as low as 2 square metres, since some small general-aviation aircraft can be of this value. For comparison, the radar cross section of a man has been measured at microwave frequencies to be about 1 square metre. A bird can have a cross section of 0.01 to 0.001 square metre. Although this is a small value, a bird can be readily detected at ranges of several tens of kilometres by long-range radar. In general, many birds can be detected by radar, so special measures must usually be taken to ensure that their echoes do not interfere with the detection of desired targets.
The radar cross section of an aircraft and that of most other targets of practical interest fluctuate rapidly as the aspect of the target changes with respect to the radar unit. It would not be unusual for a slight change in aspect to cause the radar cross section to change by a factor of 10 to 1,000.
Clutter
Echoes from land, sea, rain, snow, hail, birds, insects, auroras, and meteors are of interest to those who observe and study the environment, but they are a nuisance to those who want to detect aircraft, ships, missiles, or other similar targets. Clutter echoes can seriously limit the capability of a radar system; thus, a significant part of radar design is devoted to minimizing the effects of clutter without reducing the echoes from desired targets. The Doppler frequency shift is the usual means by which moving targets are distinguished from the clutter of stationary objects. Detection of targets in rain is less of a problem at the lower frequencies, since the radar echo from rain decreases rapidly with decreasing frequency and the average cross section of aircraft is relatively independent of frequency in the microwave region. Because raindrops are more or less spherical (symmetrical) and aircraft are asymmetrical, the use of circular polarization can enhance the detection of aircraft in rain. With circular polarization, the electric field rotates at the radar frequency. Because of this, the electromagnetic energy reflected by the rain and the aircraft will be affected differently, which thereby makes it easier to distinguish between the two. (In fair weather most radars use linear polarization; i.e., the direction of the electric field is fixed.)