RADAR MEASUREMENTSWe stated earlier that radar is used to determine thedistance and direction to and the height of distantobjects. These three pieces of information are known,respectively, by the standard terms range, bearing, andaltitude. The use of these standard terms allows anyoneinterested in a specific target to establish its positionquickly and accurately. Radar operators determine atarget’s range, bearing, and altitude by interpreting itsposition displayed on a specially designed cathode-raytube (CRT) installed in a unit known as a plan positionindicator (PPI).While most radars are used to detect targets, sometypes are used to guide missiles to targets and to directthe firing of gun systems; other types providelong-distance surveillance and navigation information.Range and bearing (and in the case of aircraft,altitude) are necessary to determine target movement.To be a successful radar operator, you must understandthe capabilities and limitations of your radar system indetermining range, bearing, and altitude.RangeThe radar measurement of range (or distance) isp o s s i b l e d u e t o t h e p r o p e r t i e s o f r a d i a t e delectromagnetic energy. This energy normally travelsthrough space in a straight line, at a constant speed, andvaries only slightly due to atmospheric and weatherconditions. The frequency of the radiated energycauses the radar system to have both a minimumeffective range and a maximum effective range.M I N I M U M R A N G E . — R a d a r d u p l e x e r salternately switch the antenna between the transmitterand the receiver so that one antenna can be used forboth functions. The timing of this switching is criticalto the operation of the radar and directly affects theminimum range of the radar system. A reflected pulsewill not be received during the transmit pulse andsubsequent receiver recovery time. The minimumrange of a radar, therefore, is the minimum distancebetween the radar’s antenna and a target at which aradar pulse can be transmitted, reflected from thetarget, and received by the radar receiver. If theantenna is closer to the target than the radar’s minimumrange, any pulse reflected from the target will returnbefore the receiver is connected to the antenna and willnot be detected.MAXIMUM RANGE.—The maximum range ofa pulse-radar system depends on carrier frequency;peak power of the transmitted pulse; pulse-repetitionfrequency (PRF) or pulse-repetition rate (PRR) (PRFand PRR are synonymous terms.); and receiversensitivity, with PRF/PRR as the primary limitingfactor.The peak power of a pulse determines how far thepulse can travel to a target and still return a usable echo.A usable echo is the weakest signal that a receiver candetect, process, and present on a display.The PRR determines the rate at which the rangeindicator is reset to zero. As the leading edge of eachpulse is transmitted, the indicator time base used tomeasure the returned echo is reset, and a new sweepappears on the screen.RANGE ACCURACY.—The shape and width ofthe radio-frequency (RF) pulse influences minimumrange, range accuracy, and maximum range. The idealpulse shape is a square wave that has vertical leadingand trailing edges. The vertical edge provides adefinite point from which to measure elapsed time onthe indicator time base. A sloping trailing edgelengthens the pulsewidth. A sloping leading edgeprovides no definite point from which to measureelapsed time on the indicator time base.Other factors affecting range are the antenna’sheight, beamwidth, and rotation rate. A higher antennawill create a longer radar horizon, allowing a greaterrange of detection. An antenna with a narrowbeamwidth, provides a greater range capability, since itprovides more concentrated beam with a higher energydensity per unit area. A slower antenna rotation rate,providing more transmitted pulses during the sweep,allows the energy beam to strike each target moretimes, providing stronger echo returns and a greaterdetection range.From the range information, the operator knowsthe distance to an object. He now needs bearinginformation to determine where the target is inreference to the ship.BearingRadar bearing is determined by the echo’s signalstrength as the radiated energy lobe moves past thet a r g e t . S i n c e s e a r c h r a d a r a n t e n n a s m o v econtinuously, the point of maximum echo return isdetermined either by the detection circuitry as thebeam passes the target or visually by the operator.Weapons control and guidance radar antennas arepositioned to the point of maximum signal return and1-3
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