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 multiplyinghalf 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 notdepend 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 harmonicdistortion, 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 widebandamplifier
Video amplifier:
When a wide band amplifier is used with a display system (Cathode Ray Tube) to provide signal visibleinformation 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 & targetfades
The fade is slow enough to show the target faintly long after the next set of paints has arrived with the rotatingtimebase