A method of detecting the presence, position, and direction of motion of distant objects (such as ships and aircraft) by means of their ability to reflect a beam of electromagnetic radiation of centimetric wavelengths. It is also used for navigation and guidance. It consists of a transmitter producing radio-frequency radiation, often pulsed, which is fed to a movable aerial from which it is transmitted as a beam. If the beam is interrupted by a solid object, a part of the energy of the radiation is reflected back to the aerial. Signals received by the aerial are passed to the receiver, where they are amplified and detected. An echo from a reflection of a solid object is indicated by a sudden rise in the detector output. The time taken for a pulse to reach the object and be reflected back (t) enables the distance away (d) of the target to be calculated from the equation d = ct/2, where c is the speed of light. In some systems the speed of the object can be measured using the Doppler effect. The output of the detector is usually displayed on a cathode-ray tube in a variety of different formats.
Radar. Types of cathode-ray tube radar display.
Acronym for radio direction and ranging. A system that locates distant objects using reflected radiowaves of microwave frequencies. Modern radar systems are highly sophisticated and can produce detailed information about both stationary and moving objects and can be used for navigation and guidance of ships, aircraft, and other vehicles and systems.
A complete radar system contains a source of microwave power, such as a magnetron, a modulator to produce pulses of microwave energy where necessary, transmitting and receiving antennas, a receiver that detects the echo, and a cathode-ray tube (CRT) that displays the output in a suitable form. Several types of radar system are in common use.
• Pulse radar systems transmit short bursts of high-frequency radiowaves and the reflected pulse is received during the time interval between the transmitted pulses.
• Continuous-wave systems transmit energy continuously and a small proportion is reflected by the target and returned to the transmitter.
• Doppler radar utilizes the Doppler effect in order to distinguish between stationary and moving objects. The change in frequency between the transmitted and received waves is measured and hence the velocity of moving targets deduced.
• Frequency-modulated radar is a system that transmits a frequency-modulated radar wave. The reflected echo beats with the transmitted wave and the range of the target is deduced from the beat frequency produced.
• Volumetric radar systems can produce three-dimensional positional information about one or more targets. Two transmitters used simultaneously are commonly employed. V-beam radar is a volumetric system using two fan-shaped beams.
In any of the above systems the direction and distance of the target is given by the direction of the receiving antenna and the time interval between transmission of the radar signal and reception of the echo.
The direction of the transmitting and receiving antennas can be periodically varied in order to scan a given area. Coarse scanning is often used to obtain an approximate target location before repeating the scan more accurately. A common arrangement is to rotate the antennas in a horizontal plane, and produce a synchronous circular scan on the CRT, in order to display any targets within the vicinity of the transmitter. Such a presentation is termed a plan position indicator (PPI). Scanning in a vertical plane can also be used.
Pulsed radar systems frequently use the same antenna as both a transmitting and receiving antenna. The appropriate transmitting and receiver circuits are connected to it using a transmit-receive (TR) switch. The pulse repetition frequency of the transmitted signals is determined by a multivibrator known as the master trigger.
The radar range is the maximum distance at which a particular radar system is effective in detecting a target. It is usually defined as the distance at which a designated target is distinguished for at least 50% of the transmitted pulses. The range is dependent on the minimum discernible signal that the radar receiver can accept, i.e. the minimum power input to the receiver that produces a discernible signal on the radar indicator. The power in the return echo is dependent on the peak power of the transmitted pulse. In general the larger the output power of the transmitter, the greater the range of the system. A given radar system is characterized by the performance figure; this is the ratio of the peak power of the transmitted pulse to the minimum discernible signal of the receiver. The ability of a radar system to differentiate objects along the same bearing is usually defined as the minimum radial distance separating targets at which they can be separately resolved.
Radar systems are used for the detection and control of aircraft (see precision approach radar), guiding of ships in fog, and for locating distant storm centres when an echo is produced by the associated heavy rainfall. Radar is used in astronomy and also has an extremely wide range of military applications.
https://www.ll.mit.edu/workshops/education/videocourses/introradar/index.html A series of lectures on radar systems, from the Lincoln Laboratory
Acronym for radio detection and ranging; the use of electromagnetic energy for the detection of objects which are capable of reflecting it. For example, cloud-detection radars are extensively used in meteorological forecasting and rainfall measurement, while sideways-looking radars are used for topographic mapping. Radar has been used to probe through ice sheets in order to determine ice thickness and to detect internal reflection events; in polar regions the radar equipment has usually been airborne, but the instruments can be mounted on sledges for surface use. Radar can also be used in arid environments to probe through sand in search of water. Ground radar is being developed for use in engineering site investigations, but has very limited depth penetration where the moisture content is high because of the high dielectric loss associated with water.
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