Radar Systems PDF

Chapter 7 RADAR SYSTEMS Learning Outcomes  Operating Principle of Radar Transmitter and Receiver  Detection Methods of Radar  Radar Equation  Functional Blocks of Radar System  Pulse Techniques for Radar Signals AVIONIC SYSTEMS RADAR SYSTEM 7.1 Introduction The acronym RA...

Chapter 7 RADAR SYSTEMS Learning Outcomes  Operating Principle of Radar Transmitter and Receiver  Detection Methods of Radar  Radar Equation  Functional Blocks of Radar System  Pulse Techniques for Radar Signals AVIONIC SYSTEMS RADAR SYSTEM 7.1 Introduction The acronym RADAR comes from ‘Radio Detection and Ranging’, which suggests its military origin. 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 a substitute for the eye, but in doing what the eye cannot do. Radar cannot resolve details as well as the eye, nor is it capable of recognising the colour of objects to the degree of sophistication of which the eye is capable. However, Radar is an electromagnetic device designed, which can be used to detect the presence of objects such as airplanes or ships even in darkness, haze, fog, rain and snow. In addition, radar has the advantage of being able to measure the distance and bearing of the detected objects. 7.2 Principle of RADAR The radar principle works on the same theory of sound wave reflection, if a person shouts at a cliff or mountain he can hear back his own return echo from the cliff (Figure 7-1). Which means sound waves generated from the person travels through the air until they strike the cliff and, they are reflected and some are returned to the person and heard as echo. There will be some time difference between the shout and echo because sound waves travels at a speed of 333 meters per second in air. As an example, if a person is at a distance of 333 meters from the cliff and shouts, about 2 seconds lapses before he hears the echo, means 1 second for sound waves to reach cliff and 1 second for them to return to the person. For calculation of distance and direction of the cliff, the sound transmitter and receiver is placed at same point as in the radar system. The distance of cliff (target in radar) is computed by multiplying half of the lapsed time by the velocity of sound. The direction of the cliff will be at an angle where the receiver will swing for maximum echo. The height of the cliff (XY) can be determined by tilting the transmitter and receiver to an angle where the echo will disappear completely. At this angle sound wave passes over the cliff and there is no chance for reflection. So, the height of the cliff can be calculated from the angle and the distance of the cliff. ET0171/Chaganti Page 7-1 AVIONIC SYSTEMS Y CLIFF SOUND SHOUT TRANSMITTER/ ECHO RECEIVER X θ Figure 7-1 Cliff Distance and Bearing Calculation ET0171/Chaganti Page 7-2 AVIONIC SYSTEMS 7.2.1 Radar Transmitter and Receiver At this point of time it is easier to say that radar system works on the same principle as the sound system which described above, but in radar system sound waves are replaced by High Frequency radio waves. An elementary form of radar consists of an antenna emitting radio frequency signals generated by a transmitter, a receiver to detect the signals received by the antenna (Figure 7-2). 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 is of prime interest of the radar. The receiving antenna collects the returned energy and delivers it to a receiver, where it is processed and displayed to detect the presence of the target and to extract its distance and position. TRANSMITTER ANTENNA DISPLAY DUPLEXER RECEIVER TARGET Figure 7-2 Radar Transmitter and Receiver The reflectivity of an object depends on the shape it presents to the signal beam, and also depends to a considerable extent on its size relative to the wavelength of the radio signal. Ships, for example, are large and can reflect quite long wavelength (low frequency) signals. Raindrops however, only reflect very short wavelength (very high frequency) signals. If a relative motion exits between target and radar the shift in the carrier frequency of the reflected wave is a measure of the target’s relative velocity and may be used to distinguish moving targets from stationary objects as shown in Figure 7-3. ET0171/Chaganti Page 7-3 AVIONIC SYSTEMS TARGET RANGE MOVING TARGET RADAR PERMANENT STRUCTURES Figure 7-3 Moving Target and Permanent Structures 7.2.2 Target Range Calculation The most common radar waveform is a train of narrow rectangular pulses (Figure 7-4) modulating the sine wave carrier. Range of the target is determined by measuring time td taken for the pulses to travel to the target and return. Since radio frequency waves are nothing but electromagnetic energy propagates at the speed of light i.e. c = 3 x 108 m/s. TRANSMIT RECEIVE PULSE PULSE td Figure 7-4 Radar Transmit Pulse and Receive Pulse ET0171/Chaganti Page 7-4 AVIONIC SYSTEMS So, range of the target (R) is given by c td R meters 2  310 t8 d 10 6  150t meters d 2 Where td is in µs, factor 2 appears in the denominator because of the two way propagation of radar signals. Example 7-1: In a radar system, if a pulse travel to a target and return in 50µs, what is target range? 7.3 Pulse Modulation Detection Method of Radar Pulse Modulation (PM) detection is most common method used in today’s radar operation to detect the targets. Pulse Modulation method transmits the power in short pulses with a time delay which can vary from 0.1µs to 50µs. If the transmitter is switched off before reflected power returns from the object, receiver can distinguish between the transmitted pulse and the received pulse (Figure 7-5). Once all reflections have returned, the transmitter switched on again and the process is repeated. The received signal is applied to an indicator that measures the time interval between the transmitted pulse and received pulse. Since radiofrequency waves travels at the speed of light i.e. c = 3 x 108 m/s, half the time interval between the transmitted and its return pulse becomes a measure of the distance to the target. This method does not depend on the motion of the target. ET0171/Chaganti Page 7-5 AVIONIC SYSTEMS ft PULSE CW ft MODULATOR OSCILLATOR RECEIVER fd INDICATOR ft ± fd Figure 7-5 Pulse Modulation Radar ET0171/Chaganti Page 7-6 AVIONIC SYSTEMS 7.4 Radar Frequency Bands Conventional radar generally has been operated at frequencies extending from about 22 MHz to 35 GHz (Table 7-1), a spread of more than seven octaves. Table 7-1: Radar Frequency Band Nomenclature Band Nominal Designation Frequency Range HF 3-30 MHz VHF 30-300 MHz UHF 300-1000 MHz L 1000-2000 MHz S 2000-4000 MHz C 4000-8000 MHz X 8000-12000 MHz Ku 12-18 GHz K 18-27 GHz Ka 27-40 GHz mm 40-300 GHz ET0171/Chaganti Page 7-7 AVIONIC SYSTEMS 7.5 Simplified Radar Equation The radar equation relates the range of radar to the characteristics of the transmitter, receiver, antenna, target and environment. It is useful not just as a means for determining the maximum distance from the radar to the target, but it can serve both as a tool for understanding radar operation and as a basis for radar design. In this section, the simple form of the radar equation is derived. If the power of the radar transmitted is denoted by Pt, and if an isotropic antenna is used (one which radiates uniformly in all directions), the power density (watts/unit area) at a distance R from the radar is equal to the transmitter power divided by the surface area 4R 2 (Figure 7- 6) of an imaginary sphere of radius R, or R 4R 2 Pt Figure 7-6 Power Density Sphere Pt Power density from isotropic antenna = 4R 2 Radar employs directive antennas to channel, or direct, the radiated power Pt into some particular direction. The gain G of an antenna is a measure of the increased power radiated in the direction of the target as compared with the power that would have been radiated from an isotropic antenna. So, ET0171/Chaganti Page 7-8 AVIONIC SYSTEMS Pt G Power density from directive antenna = 4R 2 The 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 the direction of the radar is denoted as the Radar Cross Section (RCS), and is defined by, Pt G RCS Power density of echo signal at radar = 4R 2 4R 2 The Radar Cross Section (RCS) has units of area. It is a characteristic of the particular target and is a measure 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 Ae, the power Pr received by the radar is Pt G RCS Pr  Ae 4R 2 4R 2 Since radars generally use same antenna for transmitting and receiving, the relation between transmitting gain and receiving effective area of an antenna is 2 G Ae  4  is the wavelength of transmitted signal Pt G RCS 2 G Pt G 2 2 RCS  Pr   4R 2 4R 2 4 4 3 R 4 This is the fundamental form of the radar equation. ET0171/Chaganti Page 7-9 AVIONIC SYSTEMS Example 7-2: Calculate the signal to a receiver from a radar system that has a 36 dB antenna gain, a transmitter power of 1000 watts, and an operating frequency of 5.65 GHz, from a business jet target with an RCS of 1 m2 at a distance of 75 nm. c 3  108 Calculate λ from the frequency =λ =   0.053 m f 5.65  10 9 Convert 75 nm to meters= R = 75  1852  1.39  105 m Converting antenna gain in to a number G = 103.6  3981 Pt G 2 2 RCS So, signal power received by radar system = Pr = 4 3 R4 = 6.0 x 10-17 watts A signal of 6.0 x 10-17 watts returns from 1000 watt transmitter 7.6 Functional Blocks of Radar System The block diagram of a typical pulse radar system is shown in Figure 7-7, consists of different parts listed below:  Timer  Pulse Modulator  Transmitter  Duplexer  Antenna  Low-Noise RF Amplifier  Mixer-Local Oscillator  IF Amplifier  Detector  Video Amplifier  Display System Functional operation of these blocks are described below ET0171/Chaganti Page 7-10 AVIONIC SYSTEMS DUPLEXER ANTENNA PULSE TRANSMITTER MODULATOR TIMER LOW-NOISE MIXER IF DETECTOR VIDEO RF AMPLIFIER AMPLIFIER AMPLIFIER DISPLAY SYSTEM LO Figure 7-7 Typical Pulse Radar System ET0171/Chaganti Page 7-11 AVIONIC SYSTEMS 7.6.1 Timer The timer is a trigger generator, generates timing pulses at a fixed rate. These pulses switch on the pulse modulator which pulses the transmitter. Timing signals are also applied to the display system to synchronise range sweep cycles. 7.6.2 Pulse Modulator Pulse modulator pulses the high-power transmitter on reception of the timing pulses. Maximum time duration that the transmitter is kept ON is controlled by the output pulse duration from the pulse modulator. A common Pulse Forming Network is shown in Figure 7- 8. CHARGING INDUCTANCE PULSE FORMING NETWORK V SWITCH LOAD OUTPUT Figure 7-8 Pulse Forming Network 7.6.3 Transmitter The transmitter generates high power RF signals, using Magnetron like power oscillator, whenever it is turned ON. 7.6.4 Duplexer A duplexer is a circuit designed to allow the use of the same antenna for both transmission and reception, with minimal interference between the transmitter and receiver. Duplexer uses a switching technique for pulsed transmission in radars, typical branch-type duplexer is shown in Figure 7-9. ET0171/Chaganti Page 7-12 AVIONIC SYSTEMS Basic concept of the duplexer is to switch between transmitter and receiver depending on pulse timing. It has two switches, TR and ATR (Anti-TR), arranged in such a manner that the receiver and the transmitter are alternately connected to the antenna, without ever being connected to each other. When the transmitter produces an RF impulse both switches become short-circuited. The ATR switch reflects an open circuit across the main waveguide, through the quarter-wave section connected to it, and so does the TR switch, for the same reason. Therefore, neither of them affects the transmission. At the termination of the transmitted pulse, both switches are open circuited. The ATR switch now throws a short circuit across the wave guide leading to transmitter, so received signal cannot enter into transmitter. At the input to the guide joining the TR branch of the main waveguide, this short circuit has now become an open circuit and hence has no effect. Meanwhile, the guide leading through the TR switch is now continuous and correctly matched. The signal from the antenna can thus go directly to the receiver. WAVEGUIDE λ/4 TO TRANSMITTER ANTENNA λ/4 λ/4 ATR TR SWITCH SWITCH RECEIVER Figure 7-9 Duplexer Concept for Radar ET0171/Chaganti Page 7-13 AVIONIC SYSTEMS 7.6.5 Antenna It is a highly directional antenna (parabolic reflector) which is made to scan a given area of the surroundings space. The scan information is synchronised with the angular position of the indicator. 7.6.6 Low-Noise RF Amplifier The receiver is of the superheterodyne type. To reduce the noise contribution of the received signal before it is applied to the mixer, low noise transistor amplifier (GaAs FET or TWT) is used, which also increases dynamic range of the radar receivers. 7.6.7 Mixer-Local Oscillator (LO) The mixer and the local oscillator (LO) convert the RF signal to an intermediate frequency (IF) signal, since it is easier to build amplifiers and detectors at low frequencies. LO is a commonly reflex klystron, non-linear diodes can be used in mixers. 7.6.8 IF Amplifier The receiver gain is provided by an IF amplifier. Basically, IF amplifier is broadband, permit the use of fairly narrow pulse streams. A practical IF amplifier will have centre frequency at 30 MHz or 60 MHz and bandwidth of 1 MHz or 2 MHz. 7.6.9 Detector Pulse information is extracted from the IF signal, A diode detector may be used for the purpose. However, since the shape of the pulse is important, to reduce distortion in the pulse waveform detector load is compensated by inductance. 7.6.10 Video Amplifier Any non-sinusoidal waveform such as square or pulse consists of a fundamental frequency and a number of harmonics, these harmonics determine the shape of the composite waveform. ET0171/Chaganti Page 7-14 AVIONIC SYSTEMS To amplify non-sinusoidal signals without harmonic distortion, amplifier must provide uniform amplification to signals ranging from very low frequencies (10 Hz) to very high frequencies (4 MHz). An amplifier capable of handling such a range of frequencies is called a wideband amplifier. When a wide band amplifier is used with a display system (Cathode Ray Tube) to provide signal visible information is known as video amplifier. The output of the detector is radar echoes in amplitude and time, which are amplified by the video amplifier to a level, where it can drive the display system. 7.6.11 Display System Radar echoes are normally displayed in two ways,  A-Scope  Plan Position Indicator (PPI) A-scope: The operation of this display system is rather similar to that of an ordinary oscilloscope. A sweep waveform is applied to the horizontal deflection plates (X PLATES) of the Cathode Ray Tube (CRT) and moves the beam slowly from left to right across the face of the tube, and then back to the stating point. In the absence of any received signal the display is simply a horizontal straight line. The detected and amplified signal is applied to the vertical deflection plates (Y PLATES) and causes the departures from the horizontal line, as seen in Figure 7-10. The horizontal deflection sweep waveform is synchronised with the transmitted pulses, so that the width of the CRT screen corresponds to the time interval between successive pulses. Displacement from the left hand side of the CRT corresponds to the range of the target. As indicated, the first blip is due to the transmitted pulse for reference, and then comes to various blips due to reflection from ground, nearby permanent objects followed by noise. ET0171/Chaganti Page 7-15 AVIONIC SYSTEMS The various targets then show up as large blips, the height of each blip corresponds to the strength of the returned echo, while its distance from the reference blip is a measure of its range. Perhaps the most important thing in A-scope is range calibration, which always shows horizontally across the tube. By its very nature, A-scope presentation is more suitable for use with tracking than with search antennas, since the echoes returned from one direction only are displayed, the antenna direction is generally indicated elsewhere. Y PLATE A M Transmit P Pulse L Permanent Targe Distant I X PLATE X PLATE Objects Noise t Target T U T E 0 RANGE MARKERS Y PLATE Figure 7-10 A-Scope Display Plan Position Indicator: The antenna can be moved horizontally, in a circular sweep. As the antenna rotates slowly, the beam of radiation also rotates. While the target is being illuminated by the beam, the reflection will be received. As shown in Figure 7-11, PPI display is intensity modulated CRT. In essence, the time base sweep rotates around the centre of the tube, and each received reflection paints in the same position on the display, for as long as the signal is being reflected. When the reflection in no longer received, there is no more painting and target fades. The fade is slow enough to show the target faintly long after the next set of paints has arrived with the rotating time base. A moving target will leave a trail of these fading paints to indicate its past movement as well as its current position. ET0171/Chaganti Page 7-16 AVIONIC SYSTEMS Long persistence phosphors are normally used to ensure that the face of the PPI screen does not flicker and also applicable for low scanning speeds (less than 60Hz). The resolution on the screen depends on the beam width of the antenna, pulse length and even the diameter of the CRT beam. Circular screens are used, of course, with diameters ranging up to 40 cm, but 30 cm is more common. As can be appreciated, the PPI display lends itself to use with search radars more effectively. N 00 OBJECTS SWEEP LINE 2700 900 W 20 15 10 5 5 10 15 20 E RANGE LAND MARKERS MASS 1800 S Figure 7-11 Plan Position Indicator (PPI) Display ET0171/Chaganti Page 7-17 AVIONIC SYSTEMS 7.7 Pulse Techniques in Radar System Every radar system associated with certain specifications related to following parameters. Choice of these parameters determined by, practical use, accuracy required, range coverage and physical size. Carrier Frequency Pulse Modulation Pulse Repetition Frequency Receiver Bandwidth Pulse Width Thermal Noise Power Relation Range Resolution Pulse Characteristics 7.7.1 Carrier Frequency The carrier frequency (f0) is the frequency (Figure 7-12) at which RF signal is generated and transmitted through antenna. Selection of carrier frequency in radar system depends on the antenna size, transmission and reception. To concentrate more transmitter energy in a given direction requires directive antenna. Higher the carrier frequency, shorter the wavelength will be. Hence the antenna size is smaller to get sharp radiation beam. The problem of generating and amplifying reasonable amounts of RF energy at extremely high frequencies is complicated by physical construction of transistors to be used, common BJT becomes impractical. At high frequency, modifications are done to compensate inter electrode capacitances, transit time, stray capacitance and stray inductance. At the receiver side it is very complicated to amplify RF signals, instead, in coming RF signal frequency is mixed with local oscillator frequency to produce a difference frequency called Intermediate Frequency (IF). At IF frequency, signal can be processed easily with normal conventional detectors and amplifiers. ET0171/Chaganti Page 7-18 AVIONIC SYSTEMS Figure 7-12 Carrier Frequency Signal 7.7.2 Pulse Repetition Frequency (PRF) The number of Pulses per Second is called Pulse Repetition Frequency (PRF) or sometimes the pulse rate (Figure 7-13). Sufficient time must be allowed between each transmitted pulse for an echo to return from any target located within the maximum workable range of the system. Else, the reception of the echoes from the more distant targets would be obscured by succeeding transmitted pulses. So, range (maximum unambiguous range) of a radar system depends upon the pulse rate provided the power is sufficient. Pulse 0 Time Pulse Period (T) Figure 7-13 Pulse Repetition Frequency (PRF) ET0171/Chaganti Page 7-19 AVIONIC SYSTEMS Example 7-3: Calculate the range at 1μs/meter, a radar system transmitting 100 pulses per second. 1 Pulse period (T) = =10ms 100 10  10 3 Range at 1μs/meter = meters = 10k meters 1  10 6 When the antenna system is rotated at a constant speed, the beam of energy strikes a target for a relatively short time. During this time, a sufficient number of pulses of energy must be transmitted in order to return a signal that will produce the necessary indication on the display. 7.7.3 Pulse Width The length of time that a radar pulse is transmitted is called the Pulse Width (τ) as shown in Figure 7-14. The minimum range at which target can be detected is determined by width of the transmitted pulse. If a target is so close to the transmitter that the echo is returned to the receiver before the transmitter is turned off, the reception of the echo obviously will be masked by the transmitted pulse. Example 7-4: Calculate the minimum range for a pulse width of 1μs. Minimum range = 3  10 1 10  8 6 meters 2 = 150 meters As shown in example, if a target range is less than 150 meters, will be blocked out on the display system. In this respect, radars for close in ranging or navigation work use pulses of the order 0.1μs. For long range radars the pulse width is normally from 1μs ET0171/Chaganti Page 7-20 AVIONIC SYSTEMS Pulse Width τ 0 Time Pulse Period (T) Figure 7-14 Pulse Width 7.7.4 Power Relation Radar transmits RF energy in the form of short pulses and is switched off between pulses for comparative long intervals. The useful power of the transmitter is that contained in the radiated pulses and is termed as Peak Power of the system, power is generally measured as an average value over a period of time. Because the radar transmitter is resting for a time that is long with respect to the operating time, the average power delivered during one cycle of operation is relatively low compared with the peak power available during the pulse time. A standard relation exists between the average power dissipated over a period of time and the peak power developed during the pulse time. The time taken for one cycle of operation (pulse period) is reciprocal of the Pulse Repetition Frequency (PRF). Other parameters remaining constant, greater the pulse width the higher will be the average power, and longer the pulse repetition time, the lower will be the average power. Thus, ET0171/Chaganti Page 7-21 AVIONIC SYSTEMS Average power Pulse width  Peak power Pulse period These general relationships are shown in Figure 7-15. Pulse Width Peak Power τ Equal Average Areas Power 0 Resting Time Time Pulse Period (T) Figure 7-15 Relationship of Peak and Average Power The operating cycle of the radar transmitter can be described in terms of the fraction of the total time that RF energy is radiated. This time relationship is called the Duty Cycle and may be represented as Pulse width  Average power Duty Cycle   Pulse period T  Peak power High peak power is desirable in order to produce a strong echo over the maximum range of the equipment. Low average power enables the transmitter tubes and circuit components to be made smaller and more compact. ET0171/Chaganti Page 7-22 AVIONIC SYSTEMS Example 7-5: Find duty cycle of a radar system having pulse width, 1μs and pulse repetition frequency of 100 Hz. 1 Pulse period   10 ms 100 1  10 6 Duty cycle  3  1  10  4  0.01% 10  10 Example 7-6: If a peak power of 100kw is supplied to an antenna for 1μs, what is its average power for 10ms? 7.7.5 Pulse Characteristics The time required for the pulse to increase from 10 percent to 90 percent of its normal amplitude is called the raise time (tr) as shown in Figure 7-16. Similarly, the time required for the pulse to decrease from 90 percent to 10 percent of its maximum amplitude is called the fall time (tf). When the initial amplitude rise exceeds the correct value, overshoot occurs, this overshoot is nothing but ringing. When the maximum amplitude of the pulse is not constant but decreases slowly, the pulse is said to sag. Pulse width is measured at the 50% amplitude levels. ET0171/Chaganti Page 7-23 AVIONIC SYSTEMS Width Overshoot Ringing Sag 90% Amplitude 50% 10% Rise Time Fall Time Pulse period (T) Figure 7-16 Pulse Characteristics 7.7.6 Pulse Modulation In radar transmitter, the pulse modulation technique is most commonly used, where continuous carrier wave being switched on for short time to produce RF pulses as shown in Figure 7-17. These RF pulses are amplified and fed to high gain antenna to produce directional radiation beam. ET0171/Chaganti Page 7-24 AVIONIC SYSTEMS RF PULSES CONTINUOUS CARRIER WAVE TIMER PULSES Figure 7-17 Concept of Pulse Modulation ET0171/Chaganti Page 7-25 AVIONIC SYSTEMS 7.7.7 Receiver Bandwidth Radar receiver frequency response is governed by the width of the pulses which it is desired to receive. Narrower the pulses, greater is the Intermediate Frequency (IF) bandwidth required, where, RF bandwidth is normally much greater. With a given pulse period T, the receiver bandwidth may still vary, depending on how many harmonics of the pulse repetition frequency are needed to provide a received pulse having a suitable shape. It is seen that the bandwidth must be increased if more information about the target is required, but too large bandwidth will reduce the maximum range by admitting more noise. The IF bandwidth of a radar receiver is made n/T where T is the pulse period and n is a number whose value ranges from under 1 to over 10, depending on the circumstances. Values of n from 1 to about 1.4 are the most common. Because pulse width normally ranges from 0.1 to 10 μs, it is seen that the radar receiver bandwidth may lie in the range from about 200 kHz to over 10 MHz. Bandwidths from 1 to 2 MHz are the most common. 7.7.8 Thermal Noise The main contributor of noise in radar receivers is Thermal Noise. It is due to rapid and random motion of electrons across the finite resistance of the conductor. Since the random motion of electrons increases with temperature, then noise power Pn to be proportional to temperature, T. Studies have shown that thermal noise has uniform power spectrum density, i.e. P(f) is a constant equal to kT W/Hz as shown in Figure 7-20. From the Figure 9-18, it can be deduced that the thermal noise power Pn over a bandwidth, B is given by Pn  kTBWatts ET0171/Chaganti Page 7-26 AVIONIC SYSTEMS Where k = Boltzman’s constant (1.38 x 1023 J/K) T = Temperature, K = 273 + oC B = Bandwidth (Hz) P(f) W/Hz kT f Figure 7-18 Power Spectral Density of Thermal Noise Common noise expression is Signal to Noise ratio SNR Signal Power SNR  Noise Power SNRdB 10 log10 SNR) ET0171/Chaganti Page 7-27 AVIONIC SYSTEMS Example 7-7: Find the Signal to Noise ratio, signal power of 30 mW and channel contributing a noise of 3 mW. + = Signal Power Noise Power = 30 mW = 3 mW SNR = 10 (10dB) 7.7.9 Range Resolution Range resolution is the radar’s ability to display in- line targets separately. Targets must be more than one-half pulse width apart or they occupy the pulse together, their returned energy is merged making it impossible for the radar to see their separation. Targets too close appear as one and are displayed as stretched. Range resolution is solely a function of pulse width, pulse width is unaffected by distance, therefore separation criteria remains constant. In Figure 7-19, a radar pulse is approaching two targets, separated apart by one-half pulse width (view A). In view (B), the pulse has hit the first target and some of the energy is reflected back to the radar. In view (C), the pulse has just reached the second target and more energy is reflected back to the radar from the first target. In view (D), the pulse strikes the second target and energy is now reflected back from that target. In view (E), reflected energy from the first target continues to reflect towards the radar along with the second target, which is now one-half pulse width long. From this, it can be seen, it’s impossible for the radar to tell where one pulse ends and another begins. The radar sees one continuous signal. The slightest increase in target separation will overcome this limitation and enable the radar to display both targets correctly. ET0171/Chaganti Page 7-28 AVIONIC SYSTEMS Target 1 Target 2 Target 1 Target 2 Radar Signal A B Reflection Target 1 Target 2 Target 1 Target 2 Reflection C D Target 1 Target 2 E Figure 7-19 Pulse Width versus Range Resolution ET0171/Chaganti Page 7-29 AVIONIC SYSTEMS Notes ET0171/Chaganti Page 7-30 AVIONIC SYSTEMS Notes ET0171/Chaganti Page 7-31