What is RADAR?
Radar, electromagnetic sensor used for detecting, locating, tracking, and recognizing objects of various kinds at considerable distances. It operates by transmitting electromagnetic energy toward objects, commonly referred to as targets, and observing the echoes returned from them. The targets may be aircraft, ships, spacecraft, automotive vehicles, and astronomical bodies, or even birds, insects, and rain. Besides determining the presence, location, and velocity of such objects, radar can sometimes obtain their size and shape as well. What distinguishes radar from optical and infrared sensing devices is its ability to detect faraway objects under adverse weather conditions and to determine their range, or distance, with precision.
Radar is an “active” sensing device in that it has its own source of illumination (a transmitter) for locating targets. It typically operates in the microwave region of the electromagnetic spectrum—measured in hertz (cycles per second), at frequencies extending from about 400 megahertz (MHz) to 40 gigahertz (GHz). It has, however, been used at lower frequencies for long-range applications (frequencies as low as several megahertz, which is the HF [high-frequency], or shortwave, band) and at optical and infrared frequencies (those of laser radar, or lidar). The circuit components and other hardware of radar systems vary with the frequency used, and systems range in size from those small enough to fit in the palm of the hand to those so enormous that they would fill several football fields.
Radar underwent rapid development during the 1930s and ’40s to meet the needs of the military. It is still widely employed by the armed forces, where many technological advances have originated. At the same time, radar has found an increasing number of important civilian applications, notably air traffic control, weather observation, remote sensing of the environment, aircraft and ship navigation, speed measurement for industrial applications and for law enforcement, space surveillance, and planetary observation.
Fundamentals of radar
Radar typically involves the radiating of a narrow beam of electromagnetic energy into space from an antenna (see the figure). The narrow antenna beam scans a region where targets are expected. When a target is illuminated by the beam, it intercepts some of the radiated energy and reflects a portion back toward the radar system. Since most radar systems do not transmit and receive at the same time, a single antenna is often used on a time-shared basis for both transmitting and receiving.
A receiver attached to the output element of the antenna extracts the desired reflected signals and (ideally) rejects those that are of no interest. For example, a signal of interest might be the echo from an aircraft. Signals that are not of interest might be echoes from the ground or rain, which can mask and interfere with the detection of the desired echo from the aircraft. The radar measures the location of the target in range and angular direction. Range, or distance, is determined by measuring the total time it takes for the radar signal to make the round trip to the target and back (see below). The angular direction of a target is found from the direction in which the antenna points at the time the echo signal is received. Through measurement of the location of a target at successive instants of time, the target’s recent track can be determined. Once this information has been established, the target’s future path can be predicted. In many surveillance radar applications, the target is not considered to be “detected” until its track has been established.
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Pulse radar
The most common type of radar signal consists of a repetitive train of short-duration pulses. The figure shows a simple representation of a sine-wave pulse that might be generated by the transmitter of a medium-range radar designed for aircraft detection. The sine wave in the figure represents the variation with time of the output voltage of the transmitter. The numbers given in parentheses in the figure are meant only to be illustrative and are not necessarily those of any particular radar. They are, however, similar to what might be expected for a ground-based radar system with a range of about 50 to 60 nautical miles (90 to 110 km), such as the kind used for air traffic control at airports. The pulse width is given in the figure as 1 microsecond (10?6 second). It should be noted that the pulse is shown as containing only a few cycles of the sine wave; however, in a radar system having the values indicated, there would be 1,000 cycles within the pulse. In the figure the time between successive pulses is given as 1 millisecond (10?3 second), which corresponds to a pulse repetition frequency of 1 kilohertz (kHz). The power of the pulse, called the peak power, is taken here to be 1 megawatt. Since a pulse radar does not radiate continually, the average power is much less than the peak power. In this example, the average power is 1 kilowatt. The average power, rather than the peak power, is the measure of the capability of a radar system. Radars have average powers from a few milliwatts to as much as one or more megawatts, depending on the application.
A weak echo signal from a target might be as low as 1 picowatt (10?12 watt). In short, the power levels in a radar system can be very large (at the transmitter) and very small (at the receiver).
Another example of the extremes encountered in a radar system is the timing. An air-surveillance radar (one that is used to search for aircraft) might scan its antenna 360 degrees in azimuth in a few seconds, but the pulse width might be about one microsecond in duration. Some radar pulse widths are even of nanosecond (10?9 second) duration.
Radar waves travel through the atmosphere at roughly 300,000 km per second (the speed of light). The range to a target is determined by measuring the time that a radar signal takes to travel out to the target and back. The range to the target is equal to cT/2, where c = velocity of propagation of radar energy, and T = round-trip time as measured by the radar. From this expression, the round-trip travel of the radar signal through air is at a rate of 150,000 km per second. For example, if the time that it takes the signal to travel out to the target and back was measured by the radar to be 0.0006 second (600 microseconds), then the range of the target would be 90 km. The ability to measure the range to a target accurately at long distances and under adverse weather conditions is radar’s most distinctive attribute. There are no other devices that can compete with radar in the measurement of range.
The range accuracy of a simple pulse radar depends on the width of the pulse: the shorter the pulse, the better the accuracy. Short pulses, however, require wide bandwidths in the receiver and transmitter (since bandwidth is equal to the reciprocal of the pulse width). A radar with a pulse width of one microsecond can measure the range to an accuracy of a few tens of metres or better. Some special radars can measure to an accuracy of a few centimetres. The ultimate range accuracy of the best radars is limited by the known accuracy of the velocity at which electromagnetic waves travel.
Directive antennas and target direction
Almost all radars use a directive antenna—i.e., one that directs its energy in a narrow beam. (The beamwidth of an antenna of fixed size is inversely proportional to the radar frequency.) The direction of a target can be found from the direction in which the antenna is pointing when the received echo is at a maximum. A precise means for determining the direction of a target is the monopulse method—in which information about the angle of a target is obtained by comparing the amplitudes of signals received from two or more simultaneous receiving beams, each slightly offset (squinted) from the antenna’s central axis. A dedicated tracking radar—one that follows automatically a single target so as to determine its trajectory—generally has a narrow, symmetrical “pencil” beam. (A typical beamwidth might be about 1 degree.) Such a radar system can determine the location of the target in both azimuth angle and elevation angle. An aircraft-surveillance radar generally employs an antenna that radiates a “fan” beam, one that is narrow in azimuth (about 1 or 2 degrees) and broad in elevation (elevation beamwidths of from 20 to 40 degrees or more). A fan beam allows only the measurement of the azimuth angle.