RADAR
	Today sea Captains can guide their ships safely through a crowded harbor in dense fog, and pilots can land their planes through a thick overcast. An electronic system called radar makes this possible. A radar unit can pierce darkness and weather conditions in which human eyesight is obstructed. Within its range it can show an observer ships, planes, storm clouds, small islands, coastlines, and prominent landmarks. It can also be used to measure the distance to an object and the speed at which the object is moving toward or away from the observer. Radar was developed from the work of many scientists. Discoveries made by Heinrich Hertz, Karl F. Braun, and Christian Hulsmeyer of Germany, Guglielmo Marconi of Italy, and Lee De Forest of the United States laid some of the foundations. Robert Watson-Watt of Scotland patented a radar system in 1935. British and American scientists, working together, perfected radar during World War II. Radar uses electromagnetic waves usually short-wavelength waves called microwaves. These waves bounce back from solid objects in their path, the way sound waves bounce back from an object and produce an echo. Scientists learned to use reflected microwaves for detection and measurement. The name radar was coined from the first letters of the words "radio detecting and ranging." A radar set with its antenna both transmits and receives. In the most widely used type of system, wave bursts, or pulses, are spaced so that the echo from one pulse is received before the next pulse goes out. For example, a set may be designed for a range of 93 miles. It sends out pulses at intervals of a thousandth of a second, and these travel at a speed of 186,000 miles (300,000 kilometers) per second. In a thousandth of a second a pulse has time to travel to the edge of the search area, strike an object, and return a round trip of 186 miles (300 kilometers) before the next pulse of radio energy goes out.
	Design and Operation of a Radar System
	The details of a particular type of radar system depend primarily upon the use for which it is intended. However, the basic principles are the same for all systems. An example is the type of unit used for search and fire control against aircraft. The complete system includes an antenna with parabolic reflector, a transmitter and a modulator, a receiver and its indicator, and a power supply unit. High-frequency electrical impulses are created in a pulse generator. They move to a modulator where they are amplified. The pulse produced in the modulator has a voltage greater than 10,000 volts and lasts about 1 microsecond. Pulses from the modulator provide power for the oscillator in the transmitter. The oscillator generates an alternating electrical pulse with a tremendously high frequency. It is commonly as high as 100 million cycles a second. In some instances, it may be as high as 10 billion cycles a second. The oscillator used in most radar transmitters is a special kind of vacuum tube called a resonant-cavity magnetron. A magnetron consists of a block of copper with holes or cavities inside. The structure is enclosed in a vacuum. A cathode mounted in the center provides a supply of free electrons. Under the influence of a magnet, these electrons are whirled in a spiral path at very great speeds. As the electrons whirl past the cavities, they produce electrical vibrations in the cavities. The vibrations form an alternating current of extremely high frequency that is carried to the antenna for transmission into space. A radar transmitter produces a large amount of power. For most systems, it exceeds 10,000 watts. Powerful pulses must be sent out so that the reflected waves will have sufficient power to be detected clearly by the receiver.
	Transmission and Reception of Radar Signals
	The radar antenna does two jobs. First, it receives the high-frequency pulses from the transmitter, focuses them into a beam, and sends them into space. Second, it picks up reflected pulses coming from objects that have been struck by the beam. The antenna focuses the reflected waves and sends them to the receiver. In operation the antenna rotates continuously. Its angle of elevation may also be adjusted. The field being observed is scanned by the antenna and its signal just as it would be scanned by a light beam for visual searching. The operation of the antenna is controlled by an electronic switch located between the transmitter and the receiver. When the switch is in one position, the antenna transmits. It does this for about 1/1000 of the time it operates. At the end of this brief transmitting period the switch automatically shifts to a second position. In this position it receives and focuses reflected pulses. Thus, though the transmitter produces a large amount of power, it uses this power for only about 1/1000 of the total time of operation. The radar pulses sent out by the antenna travel through the air at a speed of 186,000 miles a second until they strike a solid. Upon striking an object, a portion of the energy of the pulses is reflected back to the antenna in the form of waves. All objects do not reflect these microwaves equally well. The strength of the reflection depends upon the size, shape, and composition of the object. Metal objects are the best reflectors of microwaves. Wood and plastic produce weaker reflections than does metal. Sea water is a good reflector; open countryside is a poor one. The reflected microwaves form a pattern which is an image of the object that was struck by the transmitted waves. When reflected pulses return to the antenna, they are directed to the receiver. In the receiver their voltage is amplified by a multiple of several million. This alternating voltage is then converted to direct current and impressed upon a cathoderay oscilloscope.
	Making the Radar Image Visible
	There are a number of electronic methods for converting reflected pulses into visible symbols on a display system composed of advanced data processors and symbol generators. They may be divided into range indicators and plan-position indicators. Some radar systems use a combination of both types of indicators. One type of indicator, the A-scope, has an electron beam which sweeps across the oscilloscope screen once in the interval between pulses. In other words, this sweep is made when the antenna is receiving reflected waves. The line of light formed by the sweep is called a time base. The length of the time base corresponds to the range of the radar system. Thus, if pulses are emitted 1/1000 of a second apart, the time base corresponds to a range of 93 miles. Repeated sweeps of the electron beam maintain the straight line on the screen. A reflected wave causes the line to spurt upward in a narrow peak called a pip. The pip occurs at a point that corresponds to the distance of the reflected object. Thus, with a range of 93 miles, an object 31 miles away produces a pip one third of the distance along the line. In a plan-position indicator system (PPI), the antenna's movement is tracked by the trace of an oscilloscope tube. The position of the trace on the scope corresponds to the direction of the beam from the antenna. A reflection appears as a bright spot on the oscilloscope. The scanning is radial. A sweep starts from the center of the oscilloscope screen and radiates outward at a constant rate. When the beam reaches its maximum radial length, it quickly returns to the center. The direction of the line on the screen matches that of the antenna's radio beam. The position of the spot on the screen bears a direct relation to the distance and direction of the object. A B-scope produces an enlarged image of a part of a PPI picture and projects it on a screen bisected by a horizontal range line. The PPI system is accurate in the measurement of the direction of objects. For exact measurement of distance, however, an A-scope or a B-scope is needed.
	Other Types of Radar Systems
	There are several types of radar systems in addition to those that use radar pulses. Continuous-wave radar systems send out continuous radio signals rather than short bursts of radio waves. A signal of a particular frequency is radiated into space. When the transmitted signal strikes a radially moving object, the reflected signal is altered in frequency. The frequency change is used to calculate the object's speed. Simple continuous-wave radar cannot measure the distance of an object, but frequency-modulated radar can. The frequency of the transmitted radio signal is altered continuously. If the rate of frequency change is known, the difference in frequency of the reflected signal can be interpreted as the distance to the object. Another form of radar is optical radar, or lidar, which transmits very narrow signal beams of laser light instead of radio waves. The narrowness of the beam permits sharp definition of objects. The distance to an object is measured in the same way as with pulse radar.
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