RADAD ENGINEERING M.VAMSHI KRISHNA ASST PROF DEPT OF ECE
RADAR ENGINEERING
• Radar is an electromagnetic device and it is a power powerfu full electr electroni onic c eye. eye. • RADAR represents means Detection And Ranging.
RAdio, RAdio,
Radar can see the objects in • day or night • rain or shine • land or air • cloud or clutter • fog or frost • earth or planets • stationary or moving and • good or bad weather.
In brief, Radar can see the objects hidden any where in the globe or planets except hidden behind good conductors.
INFORMATION GIVEN BY THE RADAR Radar gives the following information : • • • • • • • • • • •
The position of the object The distance of objects from the location of radar The size of the object Whether the object is stationary or moving Velocity of the object Distinguish friendly and enemy aircrafts The images of scenes at long range in good and adverse weather conditions Target recognition Weather target is moving towards the radar or moving away The direction of movement of targets Classification of materials
APPLICATIONS OF RADARS Radars have a number of applications for domestic, civilian and military purposes. In particular, radar is used • To indicate speed of the automobiles, cricket and tennis balls etc. • To control guided missiles and weapons • To provide early warning of enemy • To aircrafts, ships, submarines and spacecrafts for defence purposes • For weather forecast • For remote sensing • For ground mapping • For airport control • For airport surveillance
• For precise measurement of distances for land surveying • To detect and measure objects under the earth • For navigating aircrafts and ships and submarines in all the weather conditions and night. • To detect and locate ships, land features and sea conditions to avoid collision • To map the land and sea from aircrafts and spacecrafts • To study the nature of stars and planets • To measure altitude from the earth for aircrafts and missile navigation etc. • For searching of submarines, land masses etc. • For bombing aircrafts, ships and cities in all weather conditions • To aim at enemy air crafts, ships and locations.
NATURE AND TYPES OF RADARS
RADAR FREQUENCY BANDS The IEEE standard radar frequency bands are given in table Band Name
Frequency Range (GHz)
Wavelength
Applications Radar experiments
mm
40 – 300
7.5 – 1 mm
ka
27 – 40
1.11 – 7.5 mm
k
18 – 27
1.67 – 1.11 cm
ku
12 – 18
2.5 – 1.67 cm
X
8 – 12
3.75 – 2.5 cm
C
4–8
7.5 – 3.75 cm
S
2–4
15 – 7.5 cm
L
1–2
30 – 15 cm
UHF
0.3 – 1
1 – 30 cm
VHF
0.03 – 0.3
10 – 1 m
HF
0.003 – 0.03
100 – 10 m
Satellite communication, radars microwave labs etc
Television, satellite, navigation aids Television, satellite communication, FM broadcast police radio telephone
LIMITATIONS • Radar can not recognize the color of the targets. • It can not resolve the targets at short distances like human eye. • It can not see targets placed behind the conducting sheets. • It can not see targets hidden in water at long ranges. • It is difficult to identify short range objects.
• The duplexer in radar provide switching between the transmitter and receiver alternatively when a common antenna is used for transmission and reception. • The switching time of duplexer is critical in the operation of radar and it affects the minimum range. A reflected pulse is not received during • the transmit pulse • subsequent receiver recovery time • The reflected pulses from close targets are not detected as they return before the receiver is connected to the antenna by the duplexer.
RANGE EQUATION OF BASIC RADAR • Radar range equation gives a relation for the maximum radar range in terms of transmitter power, effective area of the antenna, radar cross-section, wavelength, minimum detectable signal, and gain of the antenna. • Radar range equation is R max
ptA 2 4 s min 2 e
1
4
ptG 3 2 4 s min 2
R max
2
1
4
In the above equations,
p t = transmitter power (watts) G = maximum gain of the antenna (no units) A e = effective area of the receiving antenna m 2
= Radar cross-section of the target = Maximum m 2 range of the radar (m) R max = Minimum detectable signal
s min = Minimum detectable signal
TYPES OF BASIC RADARS • Monostatic and Bistatic • CW • FM-CW • Pulsed radar
MONOSTATIC RADARS • Monostatic radar uses the antenna for transmit and receive. • Its typical geometry is shown in the below fig. Target
Antenna Fig. Monostatic radar
Monostatic Radar Equation • The monostatic radar equation is given by
pR
p t G 2 2 M 4 3 d 4 L t L r L m
pR
p t G t 2 M 4 3 d 4
If L t represents transmitter losses
L r represents receiver losses
L m represents medium losses
BISTATIC RADAR • Bistatic radars use transmitting and receiving antennas placed in different locations. • CW radars in which the two antennas are used, are not considered to be bistatic radars as the distance between the antennas is not considerable. • The bistatic radar geometry is shown in below fig.
Target
Antenna
Antenna
Fig. Bistatic radar geometry
Bistatic Radar Equation pR
p t G t G r 2 B 43 d 2t d r2 L t L r L m
• If L t represents L r transmitter L m losses, represents receiver losses and represents medium losses.
THE PULSED RADAR • A simple pulsed radar is shown in below fig.
RF Pulse
Fig. Simple pulsed radar
Pulsed Radar Equation R max
Here,
p t G t G r 2 C 4 2 4 k T V C L n o Bf
1 4
C Bf
= bandwidth correction factor.
Tn
= noise temperature
The Block Diagram of Pulsed Radar • The diagram of pulsed radar is shown in below fig. Synchronizer
Modulator
Duplexer
Local Oscillator
Display Unit Video Amplifier
High Frequency Oscillator
Detector
IF Amplifier
Mixer
Local Noise RF Amplifier
Fig. Block diagram of pulsed radar
MEASUREMENT OF RANGE WITH PULSED RADAR • The measurement of range on the CRT by pulsed radar is made from the leading edge of the transmitted pulse to the leading edge of the received echo. (below Fig.).
Range
Fig. Measurement of range
• The Measurement of range by pulsed radar involves the measurement of time taken for an electromagnetic wave to travel towards a target and back to the radar. • Veloc locity of ele electro ctroma magn gnet etiic wave = 3 108 m /s • or velocity of electromagnetic wave = 30 3000 m /s • Velocity of electromagnetic wave = 0.3 km/s • It is obvious from the above data, there exists a time interval of 2 × 3.333 = 6.666 s between the pulse leaving the transmitter towards a target and echo arriving back to the radar for every kilometer. • The range is therefore given by • Range (in km) = (0.15) × time interval between the transmission and return of echo in microseconds
APPLICATION OF PULSED RADAR The pulsed radar is used to find the target’s • range • bearing and elevation angle • height
CONTINUOUS WAVE (CW) RADAR • CW radar detects objects and measures velocity from Doppler shift. • It can not measure range. • It can be monostatic or bistatic.
The Doppler Effect • The Doppler Effect was discovered by Doppler. • Doppler is Austrian mathematician.
Principle of Doppler Effect • The radars radiate electromagnetic waves towards the targets for detection and also to obtain details of the target. • When the target is stationary, the frequency of the received echoes is constant. • However, when the target is moving, the frequency of the received echoes are found to be different from transmitted frequency. • If the target approaches the radar, the frequency is increased and if the target moves away from the radar, the frequency is decreased. • That is, in the moving targets, there exists a frequency shift in the received echo signals.
• The presence of frequency shift in the received echo signals in the radar due to moving targets is known as Doppler effect. • The frequency shift is known as Doppler frequency shift and it is given by
fd Here,
fd fo
2 t
o
fo
= Doppler shift frequency, Hz = transmitter frequency, Hz
t
= velocity of the target, m/s
o
= velocity of electromagnetic waves in free
space
• The Doppler Effect is shown in below fig. f o f r
f o f d
CW Radar
Aircraft moving towards the radar radar
f o f r
f o f d
CW Radar
Aircraft moving away from the radar
Fig. Doppler Effect If t is expressed in knots, the Doppler shift frequency is given by
1.03 t kts t kts fd Hz m m
• A simple CW radar is shown in below fig. CW Radar Transmitter
Mixer
Accurate Frequency Measuring Device Display Unit
Fig. CW radar using Doppler Effect • The CW radar consists of a transmitter, mixer, accurate frequency measuring device and display unit.
Transmitter • The transmitter emits continuous electromagnetic waves towards the targets. • A single antenna is used for transmission and reception. The duplexer is used to isolate the receiver from high transmitter power. • For radar approaching targets, the reflected signal frequency is high than the transmitter frequency. for moving away targets from radar, the reflected signal frequency is lower than the transmitter frequency.
• That is, fr ft fd for incoming targets fr ft fd for moving away targets Here,
fr = frequency of reflected signal ft = frequency of transmitted signal fd = Doppler shift frequency
Mixer • The transmitted signal of frequency reflected echo signal of frequency given as input to the mixer.
and are
• The output of the mixer is Doppler frequency signal.
Accurate Frequency Measuring Device • The output of the mixer is given to an accurate frequency measuring device to find out the radial velocity of the target.
Display Unit • The output of the mixer is given to the display unit. • This indicates the presence of moving target. • In the case of stationary target, the Doppler shift frequency is zero. • That is, the transmitted frequency and reflected echo signal frequency are the same. • In the case of moving targets, the Doppler shift frequency is very small compared to transmitter frequency. • Sometimes, it is difficult to recognize this frequency. however, such as small frequency is meas measur ured ed usin using g supe superh rhet etro rody dyne ne prin princi cipl ple. e.
BLOCK DIAGRAM OF CW DOPPLER RADAR • The detailed block diagram of CW Doppler Radar is shown in below fig. Transmitting Antenna
Receiving Antenna
Transmitter
Mixer 1
Local Oscillator
Mixer 2
IF Amplifier
Mixer 3
IF Amplifier 2
Frequency Discriminator
Display
Fig. Detailed block diagram of CW Doppler radar
MEASUREMENT OF VELOCITY OF TARGET • The velocity of the moving object is determined by fd o
t ft 2
Here,
t
= velocity of the target
f d = Doppler shift frequency
f t = transmitter frequency
o
= free space velocity of EM wave
MEASUREMENT OF BEARING AND ELEVATION ANGLES OF THE TARGET • The transmitting antenna focuses the radar waves and radiates them in the shape of the beam. • The beam is pointed directly at the target in free space. The receiver antenna picks up the maximum signal when it is pointed directly at the reflecting target. • The received echo signal is maximum when both the transmitting and receiving antennas are pointed directly at the target. • The position of the radar antenna corresponding to the maximum received echo signal represent bearing and elevation angles of the target which is in the path of the beam.
• A typical example is shown in below fig. Range
N Azimuth Angle, W
Elevation Angle,
E
S
Fig. Measurement of bearing and elevation of a target
APPLICATIONS OF CW RADAR The CW radar is used to find the targets • • • • • •
bearing angle elevation angle velocity and to indicate the presence of moving targets radial velocity of moving targets whether an object is approaching or moving away
DISADVANTAGES OF CW RADAR • The CW radar does not give range information
CW RADAR EQUATION • The range equation of CW radar is given by
p CW Td G 2 2 SNR 43 R 4 k Te F L L W Here,
p av = CW average transmitted power over the dwell interval = p CW (say) Ti = Target illumination time G = antenna gain R = Range of target from radar k = Boltzman constant = 1.38 10 23 J / k Te = Effective noise temperature F = Noise figure L = Radar losses
FMCW RADAR • FMCW radar detects, measures range and radial velocity of objects. • An FM CW Radar is a Frequency Modulated Continuous Wave radar in which the frequency of continuously transmitted wave is varied at a known rate and the frequency of reflected signals is compared with the frequency of the transmitted signal.
• A simple FMCW radar is shown in below fig.
Fig. Frequency modulated CW radar
BLOCK DIAGRAM OF FMCW RADAR • The block diagram of FMCW radar is shown in below fig. Frequency Generator
Frequency Modulator
FM Transmitter
Limiter
Amplifier
Mixer
Frequency Clutter
Display
Fig. Block diagram of FMCW radar
APPLICATIONS FMCW radar is used to measure • Slant range of the target • Bearing and elevation angles of target • Height of the target
PULSED DOPPLER RADAR • Radar with high PRFs is called pulsed Doppler radar. • It contains pulse and CW radars. • It operates at high PRF to avoid the problems of blind speeds.
TYPES OF PULSED DOPPLER RADAR They are • MTI with many Doppler ambiguities and without no range ambiguities. • The pulsed Doppler radar with high PRF, many range ambiguities and without Doppler ambiguities. • The pulsed Doppler radar with some range ambiguities and Doppler ambiguities.
BLOCK DIAGRAM OF PULSED DOPPLER RADAR • It is shown in below fig. Locking Mixer
Transmitter
Doppler
COHO
STALO
Receiver Mixer
Processor
Phase Detector
IF Amplifier
Display
Fig. Block diagram of pulsed Doppler radar
APPLICATIONS • Weather warning • Detection of the target and estimation of target motion.
ADVANTAGES OF PULSED DOPPLER RADAR These are • It is able to reject unwanted echoes with the help of Doppler filters. • It is able to measure the range and velocity even in the presence of multiple targets. • Signal-to-noise ratio is high.
NAVIGATION RADARS • Navigation radars are also in the category of surface search radars. • Helps pilots in the navigation of aircrafts and ships. • Its operating range is small • It has high resolution than surface search radars.
SURVEILLANCE (SEARCH) RADAR • The search radars scan the radiation beam continuously over a specified volume in space for searching the targets. • The search radars determines range, angular position and target velocity.
SEARCH RADAR EQUATION The search radar equation is given by
SNR
p av A e Ts 4 R 4 k Te LF
Here, pav = Average power = pt PW PRF = pt d c dc = Duty cycle PW = Pulse width PRF = Pulse repetition frequency A = Aperture area = D4 4 D = Aperture diameter = Radar cross-section = Scan time = Search volume k = Boltz man constant = = Effective noise temperature, Kelvin F = Noise figure
MTI RADAR Meaning of MTI Radar •
MTI radar means Moving Target Indication radar.
•
This is one form of pulsed radar.
•
MTI radar is characterized by its very low pulse repetition frequency and hence there is no range ambiguity in MTI radar.
•
The unambiguous range is given by
R un
o f p
Here, f p= pulse repetition frequency
o = velocity of electromagnetic wave in free space
• At the same time, MTI radar has many ambiguities in the Doppler domain. •
It determines target velocity and distinguishes moving targets from stationary targets.
BLOCK DIAGRAM OF MTI RADAR • The block diagram of MTI radar is shown in below fig. Modulator
f
f c
Mixer
Microwave Signal Amplifier
f
f c
Duplexer
f STALO
f COHO
f c f d
Mixer 2
f c
f d
IF Amplifier
f c
Phase Detection Amplifier 1
Subtractor
MTI Output
Amplifier 2
f d Delay Line Cancellation
T 1/ f p
Display Unit
Fig. Block diagram of MTI radar
BLIND SPEEDS Definitions • Definition 1 : Blind speed is defined as the radial velocity of the target at which the MTI response is zero. • Definition 2 : It is also defined as the radial velocity of the target which results in a phase difference of exactly 2 radians between successive pulses. • Definition 3 : Blind speed is defined as the radial velocity of the target at which no shift appears making the target appears stationary and echoes from the target are cancelled.
Definition 4 : The blind speed of the target is defined as b
f p
n 2
n 2 T p
Here, b = blind speed f p = pulse repetition frequency n = any integer = 0, 1, 2, 3, . . . = wavelength T p= pulse repetition interval
The first blind speed in knots is given by
b1 knots 0.97m f p Hz m f p Hz The other blind speeds are integer multiples of . The blind speeds are serious limitation in MTI radar.
METHODS OF REDUCTION OF EFFECT OF BLIND SPEEDS There are four methods to reduce the effect of blind speeds by operating the radar at • long wavelengths • high pulse repetition frequency • more than one pulse repetition frequency • more than one wavelength
MST RADAR Meaning of MST Radar • MST radar represents Mesosphere, Stratosphere and Troposphere radar. • The MST radar is one type of wind profiler designed to measure winds and other atmospheric parameters up to altitudes of 100 km or more. • Mesosphere is the atmospheric region between 50 – 100 km above the earth. • Stratosphere is the atmospheric region between 10 – 50 km above the earth. • Troposphere is the atmospheric region between 0 – 10 km above the earth.
SYNTHETIC APERTURE RADAR (SAR) • SAR is a radar which moves the antenna beam across an area to synthesize a very large aperture. • It provides excellent angle and cross range resolution. • SAR uses a technique which synthesizes a large antenna with a small antenna by examining the volume of interest sequentially. • The length of the synthetic antenna aperture is given by
L off
R D
Here, D is horizontal dimension of physical antenna R is maximum length of synthetic aperture
is the operating wavelength
Salient Features of Synthetic Aperture Radars • It synthesizes very large apertures. • It provides excellent angle and cross range resolutions. • In these systems, radars moves rapidly and the targets are stationary. • It is also useful where the radar is stationary and the targets move rapidly. • It synthesizes a large antenna with a small real antenna systematically examining a large volume. • If the radar is stationary and the targets move rapidly, the above system is known as inverse Synthetic Aperture Radar (SAR). • ISAR is used to analyze formatting of aircraft from ground base or shipborn radars. • ISAR is used to find how many aircrafts are in the formation. • ISAR is also diagnostic radar which analyzes the scattering of targets to reduce their radar reflectivity
• SAR is used in remote sensing and mapping. • SAR is also used to obtain a map like display from the image of earth’s surface. • The imaging map by SAR is useful for military reconnaissance. • It is used for weapon targeting. • SAR is also used for geological and mineral explorations. • SAR was first used by NASA, USA. • SAR mapping is similar to that the Doppler Beam Sharpening (DBS). • SAR provides two-dimensional image of a target in range and cross range. • SAR produces images scenes at a ling range and in adverse weather. • SAR has a theoretical cross range equal to , being the horizontal dimension of the antenna. • SAR does not provide images of moving targets accurately. • SAR images of moving targets are distorted and displaced from the pitch.
• The concept of synthetic aperture radar is shown in below fig. Target x Effective length of real antenna n
Target y Target z
Target x
Effective length of SAR antenna
Target y Target z
Fig. Concept of SAR
• The design of SAR waveforms is made by satisfying the following inequality. o 2 PRF d 2R
Here, is velocity of the source R is the range of the target PRF is pulse repetition frequency d is the aperture of the incremental radiator o is free space velocity of electromagnetic wave This condition avoids range and velocity ambiguity.
SYNTHETIC APERTURE RADAR EQUATION • The single pulsed radar equation is given by pt G 2 2 SNR 43 R i4 k Te B L
Here,
p t= Peak transmitter power G = Antenna gain = Wavelength = Radar cross-section R i= Slant range of ith bin k = Boltzman’s constant = 1.38 1023 J /k B = Receiver bandwidth L = Radar losses Te = Effective noise temperature
APPLICATIONS OF SAR • SAR is used for remote sensing and ground mapping purposes. • It is used for military reconnaissance. • It is used for determining sea state and ocean wave conditions. • It is used for geological and mineral explorations. • It is used to obtain two dimensional image of targets. • It is used to produce images of scenes at ling ranges and in adverse weather. • It is used to obtain excellent angle and cross range resolutions. • SAR images provide information about ice, floods, earth contents, resource prospects, land use, crop quality, snow fields, inventory, industrial distributions, forestry, deserts, buildings and hills etc.
DISADVANTAGES OF SAR • It does not provide the images of moving targets. • SAR images of moving targets are distorted and displaced from the pitch.
MONOPULSE TRACKING RADAR • Monopulse tracking radar is a radar in which the information about angle error is obtained on a single pulse. • This is also called as simultaneous lobing. • The monopulse angle measurement is done by several methods. • The amplitude comparison monopulse method is most popular.
Amplitude Comparison Monopulse Tracking Radar • The block diagram of amplitude comparison monopulse tracking radar is shown in below fig. • This is used for the measurement of single angular coordinate of the target. Sum Channel Transmitter
TR
Mixer 1
IF Amplifier
Amplitude Detector
Hybrid Junction
LO
Phase Detector
Mixer 2 Difference Channel
IF Amplifier 2
Range Signal Display
Angle Error Signal
Fig. Block diagram of amplitude comparison monopulse tracking radar for a single angle coordinate measurement
PHASE COMPARISON MONOPULSE RADAR SYSTEM • The phase comparison monopulse radar is also called Interferometer radar. In this method, two antenna beams looking in the same direction are used. • Here, the amplitudes of the signals are the same with different phases. The phase difference in the two signals received by the two antennas is given by 2 d sin .
• Here, is wavelength, d is the spacing between the two antennas, is the direction of arrival of signal with respect to normal to the baseline. The pulse comparison method used in one angle coordinate is shown in below fig. It consists of two antennas producing identical beams.
Bore site
2
1 d
Fig. Phase comparison method
ADVANTAGES OF PHASE COMPARISON MONOPULSE RADAR
• The scanning of radiation beams and beam shaping are very fast.
DISADVANTAGES • It is less efficient than the amplitude comparison method. • It has the effect of grating lobes due to spacing of the two antennas. • It is less popular method. • Only one-fourth of the available antenna area is used for transmitting and only one-half the area is used while receiving, to obtain each angle coordinate. • When the spacing between the antennas is greater than the antenna diameter, the sidelobes in the radiation patterns are high and EMI is produced.
SEQUENTIAL LOBING RADAR • In sequential lobing, only one beam is switched between two squinted sequential angular positions for target-angle measurement. • This method is called sequential lobing. • It is also called sequential switching or lobe switching. • Here, time sharing is done in using single antenna beam. • The method is simple and requires less equipment and cost effective. • But it is not very accurate.
• An antenna and its lobe which is switched sequentially between X and Y directions is shown in below fig.. Target X
Y
Fig. Sequential lobing in polar coordinates
ADVANTAGES OF SEQUENTIAL LOBING • It requires only one antenna • Operation is simple • It requires less equipment • It is cost affective.
DISADVANTAGES • It is not very accurate.
CONICAL SCAN TRACKING RADAR • The conical scan tracking radar is a radar in which the squinted beam is continuously rotated to obtain angle measurements in two coordinates for tracking the target. • The conical scan is also simply called con-scan.
MAIN FACTORS AFFECTING RADAR OPERATION The radar operation is affected by several factors. These are • the external man-made EMI • the electromagnetic interference coming from other transmitters • EMI generated within the receiver • signals reflected by natural phenomenon like rain, fog, and cloud etc. • the electromagnetic interference due to natural sources like lightening, solar and cosmic radiations. • signals reflected by clutter land masses, buildings and hills. • the curvature of the earth • noise produced within the receiver • the peak transmitter power
• • • • • • • • • • • • • • • • • •
sensitivity of the receiver antenna efficiency antenna beam shape sidelobes of radiation pattern beamwidth of antenna pattern radar cross-section of the target ambient temperature radar location type of earth at the location of the radar size of the target shape of the target polarization of the radar antenna the medium between the radar and the target radar pulse width pulse rest time the time interval between pulses frequency of operation signal to noise ratio
NOISE GENERATED WITHIN THE RECEIVER • When the noise in the radar receiver is high, the echo signal will be masked. • The noise can be made minimum by reducing the beamwidth. • Typical low noise receiver and its output are shown in below fig. Amplified Echo Signal Amplified Internal Noise
Receiver
External Amplified Noise External Noise
Echo Signal
Fig. Receiver output with low noise
• At the same time, high noise receiver and its output are shown in below fig. Amplified Echo Signal Amplified Internal Noise
Receiver
External Amplified Noise External Noise
Masked Echo Signal
Fig. High noise receiver and its output
EXTERNAL EMI DUE TO NATURAL PHENOMENA • The electromagnetic interference caused by natural phenomena is seasonal dependent and effects the radar operation. • However, the effect is minimum in modern radars operating between 3 and 30 GHz.
Storm Centre
Plane Echo Obscured
PPI Screen
Fig. Effect of clutter
EMI FROM LAND MASSES • Land masses screen an echo in the receiver display. • The reflected signals from land masses are useful in navigation and mapping radars. • But in radars used for detection, the reflected signals from land masses mask the required echo signals. • A typical situation in which the land masses create an EMI in the radar display is shown in below fig.
Aircraft No. 1
Aircraft No. 2 Aircraft No. 2 Echo Aircraft No. 1 Echo
Fig. Effect of land masses
EFFECT OF EARTH CURVATURE ON RADAR OPERATION • The curvature of earth creates shadow zones. • It prevents the detection of targets at faraway distances. • The radar horizon reduces the maximum range of the radar. • A typical situation in which the curvature of the earth is affecting the radar operation in the detection of objectives is shown in below fig.
Fig. Effect of Earth’s Curvature
EFFECT OF SIZE, SHAPE OF THE OBJECT AND MATERIAL • The radar electromagnetic waves are reflected from all objects in their path. • But the strength of the reflected wave depends on size, shape of the object and the material with which it is made. • The reflected wave is strong from metal, large and close and flat objects.
• Echoes from different objects are shown in below fig. Metal Object
Strong Echo
Large Object
Strong Echo
Close Object
Strong Echo
Flat Object
Strong Echo
Irregular Object
Weak Echo
Small Object
Weak Echo
Distant Object
Weak Echo
Wood Object
Weak Echo
Display
Receiver
Fig. Echoes from different objects
EFFECT OF TRANSMITTER POWER ON RADAR OPERATION
• The radar with high transmitter power has long range of detection. • The low power radar transmitter prevent the detection of objects.
• A typical situation in which the effect of transmitter power effects echoes is shown in below figs.
High Power Transmitter
High Resolution
Fig. Effect of high power transmitter
Low Power Transmitter
Low Resolution
Fig. Effect of low power transmitter
EFFECT OF RECEIVER SENSITIVITY • The sensitivity of the receiver depends on the level of noise generated by it. • The quality of the receiver is usually described by noise figure. • Ideally noise figure is unity. • The noise generated in the receiver is amplified and affects the detection of the objects. • A typical situation in which the effect of sensitivity on the radar detection is shown in below fig.
Amplified Echo Signal Amplified Internal Noise
Echo
Receiver Total Noise External Amplified Masked Echo External Noise Noise Signal
Fig. Effect of receiver sensitivity
EFFECT OF BROAD BEAM • The broad beam makes discrimination to be poor.
target
• A typical situation in which two aircrafts in a broad beam of the radar antenna create a single echo pulse in the radar display is shown in below fig.
Broad Beam
Fig. Effect of broad beam : Poor discrimination of targets
• The improved discrimination of the targets with a narrow beam is shown in fig. 1
2
Narrow Beam
1
2
Fig. Effect of narrow beam : Good discrimination of targets
EFFECT OF THE FAN BEAMS • The fan beam form radar antennas are useful for search the targets with less number of scans of the beam.
EFFECT OF NARROW SEARCHLIGHT BEAMS • The narrow searchlight beam provides accurate determination of range, bearing and elevation angles of the targets.
EFFECT OF TIME INTERVAL BETWEEN PULSES • The time interval between pulses should be sufficiently long to receive the echo signals before the next pulse is transmitted. • The short intervals create confusion in the radar display.
EFFECT OF PULSE DURATION • Narrow pulse width provides good target discrimination. • The rage is obtained from CRT by measuring distance between the leading edge of the transmitter pulse and leading edge of receiving pulse. (below fig.). Transmitted pulse
Received pulse
Range
Fig. Range measurement
•
The effect of transmitted pulse width is shown in below figs.
Transmitted pulse Ambiguous echo pulse
Fig. Effect of broad pulse
Transmitted pulse Unambiguous echo pulse
Fig. Effect of narrow pulse
The time interval between pulses should be long to receive all echoes with clarity before the next pulse is transmitted.
SUMMARY OF EFFECT OF DIFFERENT FACTORS ON RADAR OPERATION S. No.
Parameter
Advantage
Disadvantage
1.
External EMI
nil
searching and position finding becomes different
2.
Internal EMI
nil
searching and position finding becomes different
3.
Land masses
the reflected signals from land masses are useful in navigation and mapping radars
detection become different as echo signals from land masses mask the required signals.
4.
curvature of earth
nil
reduce the radar range
5.
size of the object
beam can be narrow for detection
echo becomes weak
6.
irregular object
nil
echo becomes weak
7.
metal object
echo becomes strong
detected by enemy easily
8.
Insulator object
not detected by enemy
echo becomes negligible
S. No.
Parameter
Advantage
Disadvantage
9.
high transmitted power
radar range becomes high
not economical
10.
low transmitted power
radar range becomes small
economical
11.
low frequency
loss of power in atmosphere is small
angle discrimination is poor
12.
High frequency
angle discrimination is better
loss of power in atmosphere is high
13.
large pulse width
searching is good
range discrimination is poor
14.
small pulse width
range discrimination is good
searching is poor
15.
high receiver sensitivity
easy to detect weak echos
nil
16.
low receiver sensitivity
nil
not easy to detect weak echos
17.
low PRF
nil
flow of information is not smooth
18.
high PRF
flow of information is smooth
nil
19.
high radar cross-section of the target
easy detection of target
helps enemy to detect the targets
20.
low radar cross-section
enemy cannot detect the target
not easy to detect target
SIGNAL TO NOISE RATIO (SNR) • The noise is either internal or external. • It disturbs the ability of the receiver to detect the required signal. • The noise is internally generated within the receiver. • It also may come from external man-made and natural sources. • Ideally, SNR is infinite.
INTERNAL NOISE OR EMI •
One such noise is thermal noise. This is also called Johnson noise. This is generated by the thermal motion of the conducted electrons in receiver.
The thermal noise depends on • bandwidth, B n • absolute temperature, T • Boltzman constant, Joules/degree Kelvin. •
In fact, its magnitude of thermal noise power proportional to B n and T. That is,
p n TB n p n kTB n
Here,
23 o k = Boltzman constant, = 1.38 10 J / K
T = temperature,
B n= receiver bandwidth or noise bandwidth
RADAR CROSS –SECTION OF TARGETS (RCS), • The radar cross-section is the targets relative reflecting/scattering size. • It represents the magnitude of the echo signal returned to the radar by the target. • It is defined as the ratio of power reflected towards the radar receiver per unit solid angle to the incident power density per 4.
• That is,
Power reflected towards the radar receiver /unit solid angle incident power density 4
4R 2
Er Ei
2 2
Here, = radar cross-section, R = the range of the target from the radar, m E i = incident electronic field on the target, V/m E r = reflected electronic field strength, V/m
• From the above definition, the radar crosssection is obtained by measuring the received echo amplitude, incident signal amplitude and the target range. • It is a part of target radar signature. • The signature depends on radar crosssection and the Doppler spectrum of a target.
RCS has 3 components. • Area of the target • The reflectivity of the target • The antenna-like gain of the target
•
The radar cross-section of different targets are shown in the following table S. No.
Target
2
RC S m
1.
Bird
0.01
2.
Small open boot
0.02
3.
Conventional missile
0.5
4.
Man or Women
1.0
5.
Small single engine aircraft
1.0
6.
Small pleasure boat
2.0
7.
Bicycle
2.0
8.
Small fighter plane
2.0
9.
Large fighter aircraft
6.0
10.
Cabin Cruisers
10.0
11.
Insect
10.5
12.
Medium Bomber
20.0
13.
Large Bomber
40.0
14.
Jumbo Jet
100
15.
Automobile
100
16.
Pickup Truck
17.
Small Insect
200 10 4
18.
Large Insect
19.
Helicopter
10 5 3.0
