Avionics Chpt 7

• RADAR: Radio Detection & Ranging:

  • It does what the eye cannot do
  • An electromagnetic device, that can detect the presence of objects
  • Able to measure the distance & bearing of the detected objects
• Principle of RADAR:

  • Works the same theory of sound reflection - if a person shouts at a cliff, he can hear back his return echo from the cliff

  * There will be some time difference between the shout & echo because sound waves travel at a speed of 333 meters
• E.g: If a person is at a distance of 333 meters from the cliff & shouts, about 2 seconds lapses before he hears the echo, which means 1 second for sound waves to reach cliff & 1 second for them to return to the person
• Principle of RADAR - Calculation of distance & direction of the cliff:

  • 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 & receiver to an angle where the echo will disappear completely
• Radar transmitter & receiver:

  • At this point, it is easier to say that radar system works on the same principle as the sound system
  • But in radar system sound waves are replaced by HF 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
• Radar transmitter & receiver:

  • A portion of the transmitted signal is intercepted by a reflecting object (target) & is reradiated in all directions
  • It is the energy reradiated in the back direction that is of prime interest of the radar
• Radar transmitter & receiver:

  • The reflectivity of an object depends on the shape it presents to the signal & also depends to a considerable extent on its size relative to the wavelength of the radio signal
• Radar transmitter & receiver:

  • Ships, for example, are large & can reflect quite long wavelength (low frequency) signals
  • Raindrops, however, only reflect very short wavelength (very high frequency) signals
• Radar transmitter & receiver:

  • If a relative motion exits between target & radar the shift in the carrier frequency of the reflected wave is a measure of the target’s relative velocity & may be used to distinguish moving targets from stationary objects
• Target range calculation:

  • The most common radar waveform is a train of narrow rectangular pulses 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 10^8 m/s
• Target range calculation: So, range of the target (R) is given by:

  R = ct2/2 meters

  = (3x10^8)(tdx10^-6)/2

  = 150td meters

  Where td is in µs, factor 2 appears in the denominator because of the two way propagation of radar signals
• E.g 7-1: In a radar system, if a pulse travel to a target & return in 50 μs, what is target range?

  
  

  Ans: R = 150 x 50 = 7500 meters
• Pulse modulation (PM) detection method of radar:

  • Most common method used in today's radar operation
  • Transmits the power in short pulses with a time delay that can vary from 0.1 µs to 50 µs
  • If the transmitter is switched off before reflected power returns from the object, receiver can distinguish between the transmitted pulse & the received pulse
  • Once all reflections have returned, the transmitter switched on again and the process is repeated
  • This method does not depend on the motion of the target
• Radar Frequency Band Nomenclature:

  Band Designation | Nominal 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
• Simplified radar equation:

  • The radar equation relates the range of radar to the characteristics of the transmitter, receiver, antenna, target & 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 & as a basis for radar design
• • Power density from isotropic antenna = Pt/4(pi)R^2
  • Power density from directive antenna = PtG/4(pi)R^2
  • Power density from echo signal at radar = (PtG/4(pi)R^2)*(RCS/4(pi)R^2)
  • Pr = (PtG/4(pi)R^2)*(RCS/4(pi)R^2)Ae
  • Ae = λ^2/4(pi)
  • Pr = PtG^2λ^2RCS/(4(pi))^3R^4
• E.g 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 nautical miles

  
  

  Ans:

  Calculate λ from the frequency = λ = c/f = 3x10^8/5.65x10^9 = 0.053 m

  Convert 75 nm to meters= R = 75 x 1852 = 1.39x10^5

  Converting antenna gain into a number G = 10^3.6 = 3981

  So, signal power received by radar system = Pr = Pt G^2 λ^2 RCS/(4(PI))^3 R^4 = 6.0x10^-17 watts

  A signal of 6.0x10^-17 watts returns from 1000 watt transmitter
• Functional blocks of radar system:

  • Timer
  • Pulse Modulator
  • Transmitter
  • Duplexer
  • Antenna
  • Low-Noise RF Amplifier
  • Mixer-Local Oscillator
  • IF Amplifier
  • Detector
  • Video Amplifier
  • Display System
• Timer:

  • Is a trigger generator, that 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
• Pulse modulator:

  • Pulse 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
• Transmitter:

  • Generates high power RF signals, using magnetron like power oscillator, whenever it is turned ON
• Duplexer:

  • Is a circuit designed to allow the use of the same antenna for both transmission & reception
  • It has 2 switches, TR & ATR (Anti-TR), arranged in such a manner that the receiver & the transmitter are alternately connected to the antenna, without ever being connected
• Antenna:

  • It is a highly directional antenna (parabolic reflector)which is made to scan a given area of the surroundings space
• 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
• Mixer-local oscillator (LO):

  • Convert the RF signal to an intermediate frequency (IF) signal, since it is easier to build amplifiers & detectors at low frequencies
  • LO is a commonly reflex klystron, non-linear diodes can be used in mixers
• IF amplifier:

  • The receiver gain is provided by an IF amplifier. 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
• 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, toreduce distortion in the pulse waveform detector load is compensated by inductance
• Video amplifier:

  • Any non-sinusoidal waveform such as square or pulse consists of a fundamental frequency & several harmonics, these harmonics determine the shape of the composite waveform
• Video amplifier:

  • 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)
• Video amplifier:

  • An amplifier capable of handling such a range of frequencies is called a wide band amplifier
• Video 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
• Video amplifier:

  • The output of the detector is radar echoes in amplitude & time, which are amplified by the video amplifier to a level, where it can drive the display system
• Radar display system:

  • A-scope (Cathode Ray Tube)
  • Plan Position Indicator (PPI)
• A-scope:

  • The operation is similar to an ordinary oscilloscope
  • A sweep waveform is applied to the horizontal deflection plates (X PLATES) of the Cathode Ray Tube (CRT) & moves the beam slowly from left to right across the face of the tube, & back to the starting point
  • Absence of any received signal - horizontal straight line
• A-scope:

  • The detected & amplified signal is applied to the vertical deflection plates (Y PLATES) & causes the departures from the horizontal lines
  • 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
• A-scope:

  • The first blip (bright light flashing on the screen) is due to the transmitted pulse for reference, & then comes to various blips due to reflection from ground, nearby permanent objects followed by noise
• A-scope:

  • 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
• Plan position indicator (PPI):

  • 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
• Plan position indicator (PPI):

  • PPI display is intensity-modulated CRT. In essence, the time base sweep rotates around the centre of the tube, & each received reflection paints in the same position on the display, for as long as the signal is being reflected
• Plan position indicator (PPI):

  • When the reflection is no longer received, there is no more painting & 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