RADARToday sea Captains can guide their shipssafely through a crowded harbor in
densefog, and pilots can land their
planesthrough a thick overcast. An electronic system calledradar makes this possible.
A radar unit can pierce darkness and
weatherconditions in which human eyesight is obstructed. Withinits range it can show an observer ships,
planes, storm clouds, smallislands,
coastlines, andprominent landmarks. It can also be used to measure thedistance to an object and the speed at which the objectis moving toward or away from the observer.
Radar was developed from the work of many scientists.Discoveries made by Heinrich Hertz, Karl F. Braun, andChristian Hulsmeyer of Germany,
GuglielmoMarconi of Italy, and Lee De Forest of the UnitedStates laid some of the foundations. Robert Watson-Wattof Scotland patented a radar system in 1935. British andAmerican scientists, working together, perfected radarduring World War II.
Radar uses electromagnetic waves usually short-wavelengthwaves called microwaves. These waves bounce back fromsolid objects in their path, the way sound waves bounceback from an object and produce an echo. Scientistslearned to use reflected microwaves for detection andmeasurement. The name radar was coined from the firstletters of the words "
radio detecting andranging."
A radar set with its antenna both transmits and receives.In the most widely used type of system, wave bursts, orpulses, are spaced so that the echo from one pulse isreceived before the next pulse goes out. For example, aset may be designed for a range of 93 miles. It sends outpulses at intervals of a thousandth of a second, andthese travel at a speed of 186,000 miles (300,000kilometers) per second. In a thousandth of a second apulse 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 energygoes out.
Design and Operation of a Radar System The details of a particular type of radar systemdepend primarily upon the use for which it is intended.However, the basic principles are the same for allsystems. An example is the type of unit used for searchand fire control against
aircraft.The complete system includes an antenna with parabolicreflector, a transmitter and a modulator, a receiver andits indicator, and a power supply unit. High-frequencyelectrical impulses are created in a pulse generator.They move to a modulator where they are amplified. Thepulse produced in the modulator has a voltage greaterthan 10,000 volts and lasts about 1 microsecond. Pulsesfrom the modulator provide power for the oscillator inthe transmitter.
The oscillator generates an alternating electrical pulsewith a tremendously high frequency. It is commonly ashigh as 100 million cycles a second. In some instances,it may be as high as 10 billion cycles a second. Theoscillator used in most radar transmitters is a specialkind of vacuum tube called a resonant-cavity magnetron. Amagnetron consists of a block of copper with holes orcavities inside. The structure is enclosed in a vacuum. Acathode mounted in the center provides a supply of freeelectrons. Under the influence of a magnet, theseelectrons are whirled in a spiral path at very greatspeeds. As the electrons whirl past the cavities, theyproduce electrical vibrations in the cavities. Thevibrations form an alternating current of extremely highfrequency that is carried to the antenna for transmissioninto space. A radar transmitter produces a large amountof power. For most systems, it exceeds 10,000 watts.Powerful pulses must be sent out so that the reflectedwaves will have sufficient power to be detected clearlyby the receiver.
Transmission and Reception of RadarSignals The radar antenna does two jobs. First, it receivesthe high-frequency pulses from the transmitter, focusesthem into a beam, and sends them into space. Second, itpicks up reflected pulses coming from objects that havebeen struck by the beam. The antenna focuses thereflected waves and sends them to the receiver. Inoperation the antenna rotates continuously. Its angle ofelevation may also be adjusted. The field being observedis scanned by the antenna and its signal just as it wouldbe scanned by a light beam for visual searching.
The operation of the antenna is controlled by anelectronic switch located between the transmitter and thereceiver. When the switch is in one position, the antennatransmits. It does this for about 1/1000 of the time itoperates. At the end of this brief transmitting periodthe switch automatically shifts to a second position. Inthis position it receives and focuses reflected pulses.Thus, though the transmitter produces a large amount ofpower, it uses this power for only about 1/1000 of thetotal time of operation.
The radar pulses sent out by the antenna travel throughthe air at a speed of 186,000 miles a second until theystrike a solid. Upon striking an object, a portion of theenergy of the pulses is reflected back to the antenna inthe form of waves. All objects do not reflect thesemicrowaves equally well. The strength of the reflectiondepends upon the size, shape, and composition of theobject. Metal objects are the best reflectors ofmicrowaves. Wood and plastic produce weaker reflectionsthan does metal. Sea water is a good reflector; opencountryside is a poor one.
The reflected microwaves form a pattern which is an imageof the object that was struck by the transmitted waves.When reflected pulses return to the antenna, they aredirected to the receiver. In the receiver their voltageis amplified by a multiple of several million. Thisalternating voltage is then converted to direct currentand impressed upon a cathoderay oscilloscope.
Makingthe Radar Image Visible There are a number of electronic methods forconverting reflected pulses into visible symbols on adisplay system composed of advanced data processors andsymbol generators. They may be divided into rangeindicators and plan-position indicators. Some radarsystems use a combination of both types of indicators.
One type of indicator, the A-scope, has an electron beamwhich sweeps across the oscilloscope screen once in theinterval between pulses. In other words, this sweep ismade when the antenna is receiving reflected waves. Theline of light formed by the sweep is called a time base.The length of the time base corresponds to the range ofthe radar system. Thus, if pulses are emitted 1/1000 of asecond apart, the time base corresponds to a range of 93miles.
Repeated sweeps of the electron beam maintain thestraight line on the screen. A reflected wave causes theline to spurt upward in a narrow peak called a pip. Thepip occurs at a point that corresponds to the distance ofthe reflected object. Thus, with a range of 93 miles, anobject 31 miles away produces a pip one third of thedistance along the line.
In a plan-position indicator system (PPI), the antenna'smovement is tracked by the trace of an oscilloscope tube.The position of the trace on the scope corresponds to thedirection of the beam from the antenna. A reflectionappears as a bright spot on the oscilloscope.
The scanning is radial. A sweep starts from the center ofthe oscilloscope screen and radiates outward at aconstant rate. When the beam reaches its maximum radiallength, it quickly returns to the center. The directionof the line on the screen matches that of the antenna'sradio beam. The position of the spot on the screen bearsa direct relation to the distance and direction of theobject.
A B-scope produces an enlarged image of a part of a PPIpicture and projects it on a screen bisected by ahorizontal range line. The PPI system is accurate in themeasurement of the direction of objects. For exactmeasurement of distance, however, an A-scope or a B-scopeis needed.
OtherTypes of Radar Systems There are several types of radar systems in additionto those that use radar pulses. Continuous-wave radarsystems send out continuous radio signals rather thanshort bursts of radio waves. A signal of a particularfrequency is radiated into space. When the transmittedsignal strikes a radially moving object, the reflectedsignal is altered in frequency. The frequency change isused to calculate the object's speed.
Simple continuous-wave radar cannot measure the distanceof an object, but frequency-modulated radar can. Thefrequency of the transmitted radio signal is alteredcontinuously. If the rate of frequency change is known,the difference in frequency of the reflected signal canbe interpreted as the distance to the object.
Another form of radar is optical radar, or lidar, whichtransmits very narrow signal beams of laser light insteadof radio waves. The narrowness of the beam permits sharpdefinition of objects. The distance to an object ismeasured in the same way as with pulse radar.
