Radar Principles Radar Principles & &

SystemsSystems

Two Basic Radar Two Basic Radar TypesTypes

Pulse Pulse

TransmissionTransmission

Continuous Continuous

WaveWave

Pulse TransmissionPulse Transmission 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 PW can determine the radar’s minimum detection range; PW can determine the radar’s detection range; PW can determine the radar’s masimum detection range , PW can determine masimum detection range , PW can determine the radar’s minimum range resolution.the radar’s minimum range resolution.

PRF can determine the radar’s maximum PRF can determine the radar’s maximum detection range. Faster PRF means greater detection range. Faster PRF means greater accuracy!accuracy!

Pulse DiagramPulse Diagram

PRTPRT

PWPW

““Listening”Listening”TimeTime

PRT=1/PRFPRT=1/PRF

Carrier Freq.

Pulse Radar Pulse Radar ComponentsComponents

SynchronizerSynchronizer TransmitterTransmitter

Display UnitDisplay Unit ReceiverReceiver

PowerPowerSupplySupply

ANT.ANT.DuplexerDuplexer

RF Out

Ech

o In

Antenna Control

Continuous Wave Continuous Wave RadarRadar Employs continual Employs continual

RADAR RADAR transmissiontransmission

Separate transmit Separate transmit and receive and receive antennasantennas

Relies on the Relies on the “DOPPLER SHIFT”“DOPPLER SHIFT”

Doppler Frequency Doppler Frequency ShiftsShifts

Motion Away: Echo Frequency Decreases

Motion Towards:Echo Frequency Increases

Continuous Wave Continuous Wave Radar ComponentsRadar Components

Discriminator AMP Mixer

CW RFOscillator

Indicator

OUTOUT

ININ

Transmitter Antenna

Antenna

Pulse Vs. Continuous Pulse Vs. Continuous WaveWave

Pulse EchoPulse Echo Single AntennaSingle Antenna Gives Range, Gives Range,

usually Alt. as usually Alt. as wellwell

Susceptible To Susceptible To JammingJamming

Physical Range Physical Range Determined By Determined By PW and PRF.PW and PRF.

Continuous WaveContinuous Wave Requires 2 Requires 2

AntennaeAntennae Range or Alt. InfoRange or Alt. Info High SNRHigh SNR More Difficult to Jam More Difficult to Jam

But Easily DeceivedBut Easily Deceived Amp can be tuned Amp can be tuned

to look for expected to look for expected frequenciesfrequencies

RADAR Wave RADAR Wave ModulationModulation

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 Must Be 1/2 of the Wave Length for the maximum wave length the maximum wave length employedemployed

Wide Beam pattern for Search, Wide Beam pattern for Search, Narrow for TrackNarrow for Track

Beamwidth Vs. Beamwidth Vs. AccuracyAccuracy

Beamwidth vs Accuracy

Ship A Ship B

Azimuth Angular Azimuth Angular MeasurementMeasurement

Azimuth Angular MeasurementRelative Bearing = Angle from ship’s heading.True Bearing = Ship’s Heading + Relative Bearing

NShip’s Heading Angle

Target Angle

Determining AltitudeDetermining Altitude

Determining Altitude

Altitude

Angle of Elevation

Altitude = slant range x sin0 elevation

Concentrating Radar Concentrating Radar Energy Through Energy Through Beam FormationBeam 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 Paraboloid - Conical Scan used for fire control - can be CW or Pulsefire control - can be CW or Pulse

Orange Peel Paraboliod - Usually Orange Peel Paraboliod - Usually CW and primarily for fire controlCW and primarily for fire control

Parabolic Cylinder - Wide search Parabolic Cylinder - Wide search beam - generally larger and used beam - generally larger and used for long-range search applications for long-range search applications - Pulse- Pulse

Wave GuidesWave Guides Used as a medium Used as a medium

for high energy for high energy shielding.shielding.

Uses A Magnetic Uses A Magnetic Field to keep the Field to keep the energy centered in energy centered in the wave guide.the wave guide.

Filled with an inert Filled with an inert gas to prevent gas to prevent arcing due to high arcing due to high voltages within the voltages within the waveguide.waveguide.

Factors That Affect Factors That Affect Radar PerformanceRadar Performance

Signal ReceptionSignal Reception Receiver Receiver

BandwidthBandwidth Pulse ShapePulse Shape Power RelationPower Relation Beam WidthBeam Width Pulse Repetition Pulse Repetition

FrequencyFrequency Antenna GainAntenna Gain Radar Cross Radar Cross

Section of TargetSection of Target

Signal-to-noise Signal-to-noise ratioratio

Receiver Receiver SensitivitySensitivity

Pulse CompressionPulse Compression Scan RateScan Rate

Mechanical Electronic

Carrier FrequencyCarrier Frequency Antenna apertureAntenna aperture

Radar Receiver Radar Receiver Performance FactorsPerformance Factors

Signal ReceptionSignal Reception Signal-to-Noise RatioSignal-to-Noise Ratio Receiver BandwidthReceiver Bandwidth Receiver Sensitivity Receiver Sensitivity

Signal ReceptionSignal Reception

• Only a minute portion of theRF is reflected off the target.

• Only a fraction of that returnsto the antenna.

• The weaker the signal thatthe receiver can process, thegreater the effective range .

Signal-to-Noise RatioSignal-to-Noise Ratio

Measured in dB!!!!!Measured in dB!!!!! Ability to recognize target in random Ability to recognize target in random

noise.noise. Noise is always present. At some range, noise is greater that target’s

return. Noise sets the absolute lower limit of Noise sets the absolute lower limit of

the unit’s sensitivity.the unit’s sensitivity. Threshold levelThreshold level used to remove excess used to remove excess

noise.noise.

Receiver BandwidthReceiver Bandwidth

Is the frequency range the receiver Is the frequency range the receiver can process.can process.

Receiver must process many Receiver must process many frequenciesfrequencies 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)

Receiver SensitivityReceiver Sensitivity

Smallest return signal that is Smallest return signal that is discernible against the noise discernible against the noise background.background. Milliwatts range.

An important factor in An important factor in determining the unit’s determining the unit’s maximum range.maximum range.

Pulse Effects on Radar Pulse Effects on Radar PerformancePerformance

Pulse ShapePulse Shape Pulse WidthPulse Width Pulse CompressionPulse Compression Pulse PowerPulse Power

Pulse ShapePulse Shape

Determines range accuracy Determines range accuracy and minimum and maximum and minimum and maximum range.range.

Ideally we want a pulse with Ideally we want a pulse with vertical leading and trailing vertical leading and trailing edges. edges. Very clear signal – easily discerned

when listening for the echo.

Pulse WidthPulse Width

Determines the range Determines the range resolution.resolution.

Determines the minimum Determines the minimum detection range.detection range.

Can also determine the Can also determine the maximum range of radar.maximum range of radar.

The narrower the pulse, the The narrower the pulse, the better the range resolution.better the range resolution.

Pulse CompressionPulse Compression

Increases frequency of the Increases frequency of the wave within the pulse.wave within the pulse.

Allows for good range Allows for good range resolution while packing resolution while packing enough power to provide a enough power to provide a large maximum range.large maximum range.

Pulse PowerPulse Power

The “Ummph” to get the signal The “Ummph” to get the signal out a long way.out a long way.

High peak power is desirable High peak power is desirable to achieve maximum ranges.to achieve maximum ranges.

Low power means smaller and Low power means smaller and more compact radar units and more compact radar units and less power required to less power required to operate.operate.

Other Factors Affecting Other Factors Affecting PerformancePerformance

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 (What the radar can

see(reflect))see(reflect)) Function of target size, shape, material, angle and

carrier frequency.

Summary of Factors and Summary of Factors and CompromisesCompromises

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 rangeMore 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 circuitryShorter 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 rangeDetects smaller targetsSmaller 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 Specific Types of RadarRadar

Frequency Modulated CW Radar Frequency 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 Specific Types of RadarRadar

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.

Questions?Questions?

Tran

smiss

ion

Echo

Wasted Echo