radar-principles

Radar PrinciplesRadar Principles
&&
SystemsSystems
AKHIL MOHAN SHARMAAKHIL MOHAN SHARMA
ECEECE
16114828071611482807

INTRODUCTIONINTRODUCTION
Radar is an object-detection system that uses electromagnetic waves -
specifically radio waves - to identify the range, altitude, direction or speed of
both moving and fixed objects such as aircraft, ships, spacecraft, mountain
ranges, radio and TV towers, guided missiles, motor vehicles, weather
formations, and terrain. The radar dish, or antenna, transmits pulses of radio
waves or microwaves which bounce off any object in their path. The object
returns a tiny part of the wave's energy to a dish or antenna which is usually
located at the same site as the transmitter.
The term RADAR was coined in 1940 by electronics engineers working for
the U.S. Navy as an acronym for RAdio Detection And Ranging.

BRIEF HISTORYBRIEF HISTORY
Before the Second World War developments by the British, the Germans, the
French, the Soviets and the Americans led to the modern version of radar. In
1934 the French Émile Girardeau stated he was building a radar system
"conceived according to the principles stated by Tesla" and obtained a patent
(French Patent n° 788795 in 1934) for a working dual radar system, a part of
which was installed on the Normandie liner in 1935.The same year, American
Dr. Robert M. Page tested the first monopulse radar and the Soviet military
engineer P.K.Oschepkov, in collaboration with Leningrad Electrophysical
Institute, produced an experimental apparatus RAPID capable of detecting an
aircraft within 3 km of a receiver. Hungarian Zoltán Bay produced a working
model by 1936 at the Tungsram laboratory in the same vein.
The British were the first to fully exploit radar as a defence against aircraft
attack. This was spurred on by fears that the Germans were developing death
rays. The Air Ministry asked British scientists in 1934 to investigate the
possibility of propagating electromagnetic energy and the likely effect

BASIC PRINCIPLESBASIC PRINCIPLES
A radar system has a transmitter that emits radio waves called radar signals in
predetermined directions. When these come into contact with an object they are
usually reflected and/or scattered in many directions. Radar signals are reflected
especially well by materials of considerable electrical conductivity - especially
by most metals, by seawater, by wet land, and by wetlands. Some of these make
the use of radar altimeters possible. The radar signals that are reflected back
towards the transmitter are the desirable ones that make radar work. If the
object is moving either closer or farther away, there is a slight change in the
frequency of the radio waves, due to the Doppler effect.
Radar receivers are usually, but not always, in the same location as the
transmitter. Although the reflected radar signals captured by the receiving
antenna are usually very weak, these signals can be strengthened by the
electronic amplifiers that all radar sets contain. More sophisticated methods of
signal processing are also nearly always used in order to recover useful radar
signals.

The weak absorption of radio waves by the medium through which it passes is
what enables radar sets to detect objects at relatively-long ranges - ranges at
which other electromagnetic wavelengths, such as visible light, infrared light,
and ultraviolet light, are too strongly attenuated. In particular, there are
weather conditions under which radar works well regardless of the weather.
Such things as fog, clouds, rain, falling snow, and sleet that block visible light
are usually transparent to radio waves. Certain, specific radio frequencies that
are absorbed or scattered by water vapor, raindrops, or atmospheric gases
(especially oxygen) are avoided in designing radars except when detection of
these is intended.
Finally, radar relies on its own transmissions, rather than light from the Sun or
the Moon, or from electromagnetic waves emitted by the objects themselves,
such as infrared wavelengths (heat). This process of directing artificial radio
waves towards objects is called illumination, regardless of the fact that radio
waves are completely invisible to the human eye or cameras.

Reflection
Brightness can indicate reflectivity as in this 1960 weather radar image (of Hurricane Abby). The
radar's frequency, pulse form, polarization, signal processing, and antenna determine what it can
observe.
Electromagnetic waves reflect (scatter) from any large change in the dielectric constant or
diamagnetic constants. This means that a solid object in air or a vacuum, or other significant change
in atomic density between the object and what is surrounding it, will usually scatter radar (radio)
waves. This is particularly true for electrically conductive materials, such as metal and carbon fiber,
making radar particularly well suited to the detection of aircraft and ships. Radar absorbing
material, containing resistive and sometimes magnetic substances, is used on military vehicles to
reduce radar reflection. This is the radio equivalent of painting something a dark color so that it
cannot be seen through normal means (see stealth technology).
Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and
the shape of the target. If the wavelength is much shorter than the target's size, the wave will
bounce off in a way similar to the way light is reflected by a mirror. If the wavelength is much
longer than the size of the target, the target may not be visible due to poor reflection. Low
Frequency radar technology is dependent on resonances for detection, but not identification of
targets. This is described by Rayleigh scattering, an effect that creates the Earth's blue sky and red
sunsets. When the two length scales are comparable, there may be resonances. Early radars used
very long wavelengths that were larger than the targets and received a vague signal, whereas some
modern systems use shorter wavelengths (a few centimeters or shorter) that can image objects as
small as a loaf of bread.

RADAR EQUATIONRADAR EQUATION
The power Pr returning to the receiving antenna is given by the radar equation:
P_r = {{P_t G_t A_r sigma F^4}over{{(4pi)}^2 R_t^2R_r^2}}
where
* Pt = transmitter power
* Gt = gain of the transmitting antenna
* Ar = effective aperture (area) of the receiving antenna
* σ = radar cross section, or scattering coefficient, of the target
* F = pattern propagation factor
* Rt = distance from the transmitter to the target
* Rr = distance from the target to the receiver.

In the common case where the transmitter and the receiver are at the same
location, Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the
range. This yields:
This shows that the received power declines as the fourth power of the range,
which means that the reflected power from distant targets is very, very small.
The equation above with F = 1 is a simplification for vacuum without
interference. The propagation factor accounts for the effects of multipath and
shadowing and depends on the details of the environment. In a real-world
situation, pathloss effects should also be considered.

DOPPLERS EFFECTDOPPLERS EFFECT
Ground-based radar systems used for detecting speeds rely on the
Doppler effect. The apparent frequency (f) of the wave changes with
the relative position of the target. The doppler equation is stated as
follows for vobs (the radial speed of the observer) and vs (the radial
speed of the target) and f0 frequency of wave :
f = {{v+v_{obs}}over{v-v_s}}f_0
However, the change in phase of the return signal is often used instead
of the change in frequency. It is to be noted that only the radial
component of the speed is available. Hence when a target moving at
right angle to the radar beam, it has no velocity while one parallel to it
has maximum recorded speed even if both might have the same real
absolute motion.

Two Basic Radar TypesTwo Basic Radar Types
Pulse TransmissionPulse Transmission
Continuous WaveContinuous Wave

Range vs. Power/PW/PRFRange vs. Power/PW/PRF
•Minimum Range: If still transmitting when return
received RETURN NOT SEEN.
•Max Range: PRFPWPRT
PW
PeakPower
erAveragePow
*==
As min Rh max Rh
PW
PRF

2. Pulse repetition frequency (PRF)
a. Pulses per second
b. Relation to pulse repetition time (PRT)
c. Effects of varying PRF
(1) Maximum range
(2) Accuracy
3. Peak power
a. Maximum signal power of any pulse
b. Affects maximum range of radar

4. Average power
a. Total power transmitted per unit of time
b. Relationship of average power to PW and PRT
5. Duty cycle
a. Ratio PW (time transmitting) to PRT (time of entire cycle,
time transmitting plus rest time)
b. Also equal to ratio of average power to peak power

Determining Range With Pulse RadarDetermining Range With Pulse Radar
2
*tc
Range =
c = 3 x 108
m/sec
t is time to receive return
divide by 2 because pulse traveled to object and back

Pulse Width (PW)Pulse Width (PW)
Length or duration of a given pulse
Pulse Repetition Time (PRT=1/PRF)Pulse Repetition Time (PRT=1/PRF)
PRT is time from beginning of one pulse to the
beginning of the next
PRF is frequency at which consecutive pulses are
transmitted.
PW can determine the radar’s minimum detectionPW can determine the radar’s minimum detection
range; PW can determine the radar’s maximumrange; PW can determine the radar’s maximum
detection range.detection range.
PRF can determine the radar’s maximum detectionPRF can determine the radar’s maximum detection
range.range.

D.D. Describe the components of a pulse radarDescribe the components of a pulse radar
system.system.
1. Synchronizer
2. Transmitter
3. Antenna
4. Duplexer
5. Receiver
6. Display unit
7. Power supply

Pulse Radar Block DiagramPulse Radar Block Diagram
Power
Supply
Synchronizer
Transmitter
Display
Duplexer
(Switching Unit)
Receiver
Antenna
Antenna Bearing or Elevation
Video
Echo
ATRRF
TR

Continuous Wave RadarContinuous Wave Radar
Employs continualEmploys continual
RADAR transmissionRADAR transmission
Separate transmit andSeparate transmit and
receive antennasreceive antennas
Relies on theRelies on the
“DOPPLER SHIFT”“DOPPLER SHIFT”

Doppler Frequency ShiftsDoppler Frequency Shifts
Motion Away:
Echo Frequency Decreases
Motion Towards:
Echo Frequency Increases

Continuous Wave RadarContinuous Wave Radar
ComponentsComponents
Discriminator AMP Mixer
CW RF
Oscillator
Indicator
OUTOUT
ININ
Transmitter Antenna
Antenna

Pulse Vs. Continuous WavePulse Vs. Continuous Wave
Pulse EchoPulse Echo
Single AntennaSingle Antenna
Gives Range, usuallyGives Range, usually
Alt. as wellAlt. as well
Susceptible ToSusceptible To
JammingJamming
Physical RangePhysical Range
Determined By PWDetermined By PW
and PRF.and PRF.
Continuous WaveContinuous Wave
Requires 2 AntennaeRequires 2 Antennae
Range or Alt. InfoRange or Alt. Info
High SNRHigh SNR
More Difficult to JamMore Difficult to Jam
But Easily DeceivedBut Easily Deceived
Amp can be tuned toAmp can be tuned to
look for expectedlook for expected
frequenciesfrequencies

RADAR Wave ModulationRADAR Wave Modulation
Amplitude Modulation
– Vary the amplitude of the carrier sine wave
Frequency Modulation
– Vary the frequency of the carrier sine wave
Pulse-Amplitude Modulation
– Vary the amplitude of the pulses
Pulse-Frequency Modulation
– Vary the Frequency at which the pulses occur

AntennaeAntennae
Two Basic Purposes:Two Basic Purposes:
Radiates RF Energy
Provides Beam Forming and Focus
Must Be 1/2 of the Wave Length for theMust Be 1/2 of the Wave Length for the
maximum wave length employedmaximum wave length employed
Wide Beam pattern for Search, NarrowWide Beam pattern for Search, Narrow
for Trackfor Track

Beamwidth Vs. AccuracyBeamwidth Vs. Accuracy
Beamwidth vs Accuracy
Ship A Ship B

Azimuth AngularAzimuth Angular
MeasurementMeasurement
Azimuth Angular Measurement
Relative Bearing = Angle from ship’s heading.
True Bearing = Ship’s Heading + Relative Bearing
N
Ship’s Heading
Angle
Target Angle

Determining AltitudeDetermining Altitude
Determining Altitude
Slant Range
Altitude
Angle of Elevation
Altitude = slant range x sin0 elevation

Concentrating Radar EnergyConcentrating Radar Energy
Through Beam FormationThrough Beam Formation
Linear ArraysLinear Arrays
Uses the Principle of wave summation (constructive
interference) in a special direction and wave
cancellation (destructive interference) in other
directions.
Made up of two or more simple half-wave antennas.
Quasi-opticalQuasi-optical
Uses reflectors and “lenses” to shape the beam.

Reflector ShapeReflector Shape
Paraboloid - Conical Scan used for fireParaboloid - Conical Scan used for fire
control - can be CW or Pulsecontrol - can be CW or Pulse
Orange Peel Paraboliod - Usually CWOrange Peel Paraboliod - Usually CW
and primarily for fire controland primarily for fire control
Parabolic Cylinder - Wide search beam -Parabolic Cylinder - Wide search beam -
generally larger and used for long-rangegenerally larger and used for long-range
search applications - Pulsesearch applications - Pulse

Wave Shaping -Quasi-Optical SystemsWave Shaping -Quasi-Optical Systems
Reflectors Lenses

Wave GuidesWave Guides
Used as a medium forUsed as a medium for
high energy shielding.high energy shielding.
Uses A Magnetic Field toUses A Magnetic Field to
keep the energy centeredkeep the energy centered
in the wave guide.in the wave guide.
Filled with an inert gasFilled with an inert gas
to prevent arcing due toto prevent arcing due to
high voltages within thehigh voltages within the
waveguide.waveguide.

Factors That Affect RadarFactors That Affect Radar
PerformancePerformance
Signal ReceptionSignal Reception
Receiver BandwidthReceiver Bandwidth
Pulse ShapePulse Shape
Power RelationPower Relation
Beam WidthBeam Width
Pulse RepetitionPulse Repetition
FrequencyFrequency
Antenna GainAntenna Gain
Radar Cross Section ofRadar Cross Section of
TargetTarget
Signal-to-noise ratioSignal-to-noise ratio
Receiver SensitivityReceiver Sensitivity
Pulse CompressionPulse Compression
Scan RateScan Rate
Mechanical
Electronic
Carrier FrequencyCarrier Frequency
Antenna apertureAntenna aperture

Radar Receiver PerformanceRadar Receiver Performance
FactorsFactors
Signal-to-Noise RatioSignal-to-Noise Ratio

• Only a minute portion of the
RF is reflected off the target.
• Only a fraction of that returns
to the antenna.
• The weaker the signal that
the receiver can process, the
greater the effective range .

Signal-to-Noise RatioSignal-to-Noise Ratio
Measured in dB!!!!!Measured in dB!!!!!
Ability to recognize target in random noise.Ability to recognize target in random noise.
Noise is always present.
At some range, noise is greater that target’s return.
Noise sets the absolute lower limit of the unit’sNoise sets the absolute lower limit of the unit’s
sensitivity.sensitivity.
Threshold levelThreshold level used to remove excess noise.used to remove excess noise.

Is the frequency range the receiver can process.Is the frequency range the receiver can process.
Receiver must process many frequenciesReceiver must process many frequencies
Pulse are generated by summation of sine waves of
various frequencies.
Frequency shifts occur from Doppler Effects.
Reducing the bandwidthReducing the bandwidth
Increases the signal-to-noise ratio(good)
Distorts the transmitted pulse(bad)

RADAR SIGNAL PROCESSING
Distance measurement
Transit time
Pulse radar
Sonar radar
One way to measure the distance to an object is to transmit a short pulse of radio signal
(electromagnetic radiation), and measure the time it takes for the reflection to return. The
distance is one-half the product of the round trip time (because the signal has to travel to
the target and then back to the receiver) and the speed of the signal. Since radio waves
travel at the speed of light (186,000 miles per second or 300,000,000 meters per second),
accurate distance measurement requires high-performance electronics.
In most cases, the receiver does not detect the return while the signal is being
transmitted. Through the use of a device called a duplexer, the radar switches between
transmitting and receiving at a predetermined rate. The minimum range is calculated by
measuring the length of the pulse multiplied by the speed of light, divided by two. In
order to detect closer targets one must use a shorter pulse length.

A similar effect imposes a maximum range as well. If the return from the target
comes in when the next pulse is being sent out, once again the receiver cannot tell
the difference. In order to maximize range, longer times between pulses should be
used, referred to as a pulse repetition time (PRT), or its reciprocal, pulse repetition
frequency (PRF).
These two effects tend to be at odds with each other, and it is not easy to combine
both good short range and good long range in a single radar. This is because the
short pulses needed for a good minimum range broadcast have less total energy,
making the returns much smaller and the target harder to detect. This could be
offset by using more pulses, but this would shorten the maximum range again. So
each radar uses a particular type of signal. Long-range radars tend to use long
pulses with long delays between them, and short range radars use smaller pulses
with less time between them. This pattern of pulses and pauses is known as the
pulse repetition frequency (or PRF), and is one of the main ways to characterize a
radar. As electronics have improved many radars now can change their PRF
thereby changing their range. The newest radars fire 2 pulses during one cell, one
for short range 10 km / 6 miles and a separate signal for longer ranges 100 km /60
miles.

SPEED MEASUREMENT
Speed is the change in distance to an object with respect to time. Thus the existing system for
measuring distance, combined with a memory capacity to see where the target last was, is enough to
measure speed. At one time the memory consisted of a user making grease-pencil marks on the radar
screen, and then calculating the speed using a slide rule. Modern radar systems perform the
equivalent operation faster and more accurately using computers.
However, if the transmitter's output is coherent (phase synchronized), there is another effect that can
be used to make almost instant speed measurements (no memory is required), known as the Doppler
effect. Most modern radar systems use this principle in the pulse-doppler radar system. Return
signals from targets are shifted away from this base frequency via the Doppler effect enabling the
calculation of the speed of the object relative to the radar. The Doppler effect is only able to
determine the relative speed of the target along the line of sight from the radar to the target. Any
component of target velocity perpendicular to the line of sight cannot be determined by using the
Doppler effect alone, but it can be determined by tracking the target's azimuth over time.

Smallest return signal that is discernibleSmallest return signal that is discernible
against the noise background.against the noise background.
Milliwatts range.
An important factor in determining theAn important factor in determining the
unit’s maximum range.unit’s maximum range.

Pulse Effects on RadarPulse Effects on Radar
PerformancePerformance
Pulse WidthPulse Width
Pulse PowerPulse Power

Determines range accuracy andDetermines range accuracy and
minimum and maximum range.minimum and maximum range.
Ideally we want a pulse with verticalIdeally we want a pulse with vertical
leading and trailing edges.leading and trailing edges.
Very clear signal – easily discerned when
listening for the echo.

Pulse WidthPulse Width
Determines the range resolution.Determines the range resolution.
Determines the minimum detectionDetermines the minimum detection
range.range.
Can also determine the maximum rangeCan also determine the maximum range
of radar.of radar.
The narrower the pulse, the better theThe narrower the pulse, the better the
range resolution.range resolution.

Increases frequency of the wave withinIncreases frequency of the wave within
the pulse.the pulse.
Allows for good range resolution whileAllows for good range resolution while
packing enough power to provide a largepacking enough power to provide a large
maximum range.maximum range.

Pulse PowerPulse Power
The “Ummph” to get the signal out aThe “Ummph” to get the signal out a
long way.long way.
High peak power is desirable to achieveHigh peak power is desirable to achieve
maximum ranges.maximum ranges.
Low power means smaller and moreLow power means smaller and more
compact radar units and less powercompact radar units and less power
required to operate.required to operate.

Other Factors Affecting PerformanceOther Factors Affecting Performance
Scan Rate and Beam WidthScan Rate and Beam Width
Narrow beam require slower antenna rotation rate.
Pulse Repetition FrequencyPulse Repetition Frequency
Determines radars maximum range(tactical factor).
Carrier FrequencyCarrier Frequency
Determines antenna size, beam directivity and target size.
Radar Cross SectionRadar Cross Section (What the radar can see(reflect))(What the radar can see(reflect))
Function of target size, shape, material, angle and carrier
frequency.

Summary of Factors and CompromisesSummary of Factors and Compromises
Summary of Factors and Compromises
Pulse Shape Sharp a rise as possible Better range accuracy Require infinite bandwidth, more complex
Tall as possible More power /longer range Requires larger equipment/more power
Pulse Width Short as possible Closer minimum range Reduces maximum range
More accurate range
Pulse Repetition Freq. Short Better range accuracy Reduces maximum range
Better angular resolution
Better detection probability
Pulse Compression Uses technique Greater range More complex circuitry
Shorter minimum range
Power More Greater maximum range Requires larger equipment & power
Beam Width Narrow Greater angular accuracy Slow antenna rate, Detection time
Carrier Frequency High Greater target resolution Reduces maximum range
Detects smaller targets
Smaller equipment
Receiver Sensitivity High Maximizes detection range More complex equipment
Receiver Bandwidth Narrow Better signal-to-noise ratio Distorts pulse shape
Factor Desired Why Trade-off Required

Specific Types of RadarSpecific Types of Radar
Frequency Modulated CW RadarFrequency Modulated CW Radar
Use for radar altimeters and missile guidance.
Pulse DopplerPulse Doppler
Carrier wave frequency within pulse is compared with a reference
signal to detect moving targets.
Moving Target Indicator (MTI) SystemMoving Target Indicator (MTI) System
Signals compared with previous return to enhance moving targets.
(search radars)
Frequency Agile SystemsFrequency Agile Systems
Difficult to jam.

Specific Types of RadarSpecific Types of Radar
SAR / ISARSAR / ISAR
Phased Array - AegisPhased Array - Aegis
Essentially 360° Coverage
Phase shift and frequency shift allow the planar
array to “steer” the beam.
Also allows for high / low power output
depending on requirements.

Transm
ission
Echo
W
asted
Echo

APPLICATIONAPPLICATIONThe information provided by radar includes the bearing and range (and therefore position) of the
object from the radar scanner. It is thus used in many different fields where the need for such
positioning is crucial. The first use of radar was for military purposes: to locate air, ground and sea
targets. This evolved in the civilian field into applications for aircraft, ships and roads.
In aviation, aircraft are equipped with radar devices that warn of obstacles in or approaching their
path and give accurate altitude readings. They can land in fog at airports equipped with radar-
assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on
radar screens while operators radio landing directions to the pilot.
Marine radars are used to measure the bearing and distance of ships to prevent collision with other
ships, to navigate and to fix their position at sea when within range of shore or other fixed
references such as islands, buoys, and lightships. In port or in harbour, vessel traffic service radar
systems are used to monitor and regulate ship movements in busy waters. Police forces use radar
guns to monitor vehicle speeds on the roads.
Radar has invaded many other fields. Meteorologists use radar to monitor precipitation. It has
become the primary tool for short-term weather forecasting and to watch for severe weather such
as thunderstorms, tornadoes, winter storms, precipitation types, etc. Geologists use specialised
ground-penetrating radars to map the composition of the Earth's crust.

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