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www.jntuworld.com RADAR     Questions & Answers 1.What are the desirable pulse characteristics and the factors that govern them in a Radar system? Radar is an electromagnetic system for the detection and location of objects. It operates by transmitting a particular type of waveform, a pulse-modulated sine wave for example, and detects the nature of the echo signal. Radar is used to extend the capability of one's senses for observing the environment, especially the sense of vision. The value o
  RADAR Questions & AnswersGRIET-ECE 1 1.What are the desirable pulse characteristics and the factors that governthem in a Radar system? Radar is an electromagnetic system for the detection and location of objects. It operates bytransmitting a particular type of waveform, a pulse-modulated sine wave for example, anddetects the nature of the echo signal. Radar is used to extend the capability of one's senses for observing the environment, especially the sense of vision. The value of radar lies not in being asubstitute for the eye, but in doing what the eye cannot do Radar cannot resolve detail as wellthe eye, nor is it capable of recognizing the color of objects to the degree of sophisticationwhich the eye is capable. However, radar can be designed to see through those conditionsimpervious to normal human vision, such as darkness, haze, fog, rain, and snow. In addition,radar has the advantage of being able to measure the distance or range to the object. This is probably its most important attribute.An elementary form of radar consists of a transmitting antenna emitting electromagneticradiation generated by an oscillator of some sort, a receiving antenna, and an energy-detectingdevice or receiver. A portion of the transmitted signal is intercepted by a reflecting object(target) and is reradiated in all directions. It is the energy reradiated in the back direction that isof prime interest to the radar. The receiving antenna collects the returned energy and delivers itto a receiver, where it is processed to detect the presence of the target and to extract its locationand relative velocity. The distance to the target is determined by measuring the time taken for the radar signal to travel to the target and back. The direction, or angular position, of thetarget may be determined from the direction of arrival of the reflected wave front. The usualmethod of measuring the direction of arrival is with narrow antenna beams. If relative motionexists between target and radar, the shift in the carrier frequency of the reflected wave(doppler effect) is a measure of the target's relative (radial) velocity and may be used todistinguish moving targets from stationary objects. In radars which continuously track themovement of a target, a continuous indication of the rate of change of target position isalso available.The name radar reflects the emphasis placed by the early experimenters on adevice to detect the presence of a target and measure its range. Radar is a contraction of thewords radio detection and ranging. It was first developed as a detection device to warn of theapproach of hostile aircraft and for directing antiaircraft weapons. Although a well-designedmodern radar can usually extract more information from the target signal than merely range,the measurement of range is still one of radar's most important functions. There seem to beno other competitive techniques which can measure range as well or as rapidly as can a radar.The most common radar waveform is a train of narrow, rectangular-shape pulsesmodulating a sinewave carrier. The distance, or range, to the target is determined by measuring www.jntuworld.com www. ntuworld.com  RADAR Questions & AnswersGRIET-ECE 2 the time T R  taken by the pulse to travel to the target and return. Since electromagneticenergy propagates at the speed of light c = 3 x 10 8 m/s, the range R isThe factor 2 appears in the denominator because of the two-way propagation of radar. With therange in kilometers or nautical miles, and T R  in microseconds, Eq. above becomesEach microsecond of round-trip travel time corresponds to a distance of 0.081 nautical mile,0.093 statute mile, 150 meters, 164 yards, or 492 feet. Once the transmitted pulse is emitted bythe radar, a sufficient length of time must elapse to allow any echo signals to return and bedetected before the next pulse may be transmitted. Therefore the rate at which the pulses may be transmitted is determined by the longest range at which targets are expected. If the pulserepetition frequency is too high, echo signals from some targets might arrive after thetransmission of the next pulse, and ambiguities in measuring range might result. www.jntuworld.com www. ntuworld.com  RADAR Questions & AnswersGRIET-ECE 3Fig.1 Plot of maximum unambiguous range as a function of the pulse repetition frequency .Echoes that arrive after the transmission of the next pulse are called second-time-around(or multiple-time-around) echoes. Such an echo would appear to be at a much shorter rangethan the actual and could be misleading if it were not known to be a second-time-aroundecho. The range beyond which targets appear as second-time-around echoes is called themaximum unambiguous range and isWhere f   p = pulse repetition frequency, in Hz. A plot of the maximum unambiguous range as afunction of pulse repetition frequency is shown in Fig.1. 2. Derive the maximum range for a radar system from first principles. The radar equation relates the range of a radar to the characteristics of the transmitter,receiver, antenna, target, and environment. It is useful not just as a means for determining themaximum distance from the radar to the target, but it can serve both as a tool for under-standing radar operation and as a basis for radar design.If the power of the radar transmitter is denoted by P t , and if anisotropic antenna is used (one which radiates uniformly in all directions), the power density(watts per unit area) at a distance R from the radar is equal to the transmitter power divided by the surface area 4 ∏ R  2 of an imaginary sphere of radius R, or Radars employ directive antennas to channel, or direct, the radiated power P t into some particular direction. The gain G of an antenna is a measure of the increased power radiated in thedirection of the target as compared with the power that would have been radiated from anisotropic antenna. It may be defined as the ratio of the maximum radiation intensity from thesubject antenna to the radiation intensity from a lossless, isotropic antenna with the same www.jntuworld.com www. ntuworld.com  RADAR Questions & AnswersGRIET-ECE 4  power input. (The radiation intensity is the power radiated per unit solid angle in a givendirection.) The power density at the target from an antenna with a transmitting gain G isThe target intercepts a portion of the incident power and reradiates it in various directions.The measure of the amount of incident power intercepted by the target and reradiated back in thedirection of the radar is denoted as the radar cross section σ , and is defined by the relationThe radar cross section σ has units of area. It is a characteristic of the particular target and is ameasure of its size as seen by the radar. The radar antenna captures a portion of the echo power.If the effective area of the receiving antenna is denoted A e , the power P r, received by the radar isThe maximum radar range R  max is the distance beyond which the target cannot be detected. Itoccurs when the received echo signal power P, just equals the minimum detectable signal S min,  ThereforeThis is the fundamental form of the radar equation. Note that the important antenna parametersare the transmitting gain and the receiving effective area.Antenna theory gives the relationship between the transmitting gain andthe receiving effective area of an antenna asSince radars generally use the same antenna for both transmission and reception, Eq. can besubstituted into Eq. above, first for A e , then for G, to give two other forms of the radar equation www.jntuworld.com www. ntuworld.com
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