A radar display. The light radial band shows the azimuth direction in which the microwave beam is currently transmitted and received. (Not all radar displays show this band.)
The term radar is derived from the words radio detection and ranging. Electromagnetic (EM) waves having certain frequencies reflect from various objects, especially if those objects contain metals or other good electrical conductors. By ascertaining the direction(s) from which radio signals are returned, and by measuring the time it takes for a pulsed beam of EM energy to travel from the transmitter location to a target and back again, it is possible to pinpoint the geographic positions of distant objects. During the Second World War in the 1940s, this property of radio waves was put to use for the purpose of locating aircraft.
In the years following the war, it was discovered that radar can be useful in a variety of applications, such as measurement of automobile speed (by the police), weather forecasting (rain and snow reflect radar signals), and even the mapping of the moon and the planet Venus. Radar is extensively used in aviation, both commercial and military. In recent years, radar has also found uses in robot guidance systems.
A complete radar set consists of a transmitter, a directional antenna with a narrow main lobe and high gain, a receiver, and an indicator or display. The transmitter produces intense pulses of radio microwaves at short intervals. The pulses are propagated outward in a narrow beam from the antenna, and they strike objects at various distances. The reflected signals, or echoes, are picked up by the antenna shortly after the pulse is transmitted. The farther away the reflecting object, or target, the longer the time before the echo is received. The transmitting antenna is rotated so that all azimuth bearings (compass directions) can be observed.
A typical circular radar display consists of a CRT or LCD. Above figure shows the basic display configuration. The observing station is at the center of the display. Azimuth bearings are indicated in degrees clockwise from true north, and are marked around the perimeter of the screen. The distance, or range, is indicated by the radial displacement of the echo; the farther away the target, the farther from the display center the echo or blip. The radar display is, therefore, a set of polar coordinates. In the drawing, a target is shown at an azimuth of about 125° (east-southeast). Its range is near the maximum for the display.
The maximum range of a radar system depends on the height of the antenna above the ground, the nature of the terrain in the area, the transmitter output power and antenna gain, the receiver sensitivity, and the weather conditions in the vicinity. Airborne long-range radar can detect echoes from several hundred kilometers (km) away under ideal conditions. A low-power radar system, with the antenna at a low height above the ground, might receive echoes from only 50 to 70 km.
The fact that precipitation reflects radar echoes is a nuisance to aviation personnel, but it is invaluable for weather forecasting and observation. Radar has made it possible to detect and track severe thunderstorms and hurricanes. A mesocyclone, which is a severe thunderstorm likely to produce tornadoes, causes a hook-shaped echo on radar. The eye of a hurricane, and the eyewall and rainbands surrounding it, all show up clearly on a radar display.
Some radar sets can detect changes in the frequency of the returned pulse, thereby allowing measurement of wind speeds in hurricanes and tornadoes. This is called Doppler radar. This type of radar is also employed to measure the speeds of approaching or receding targets.