radar principles & systems
Post on 07-Feb-2016
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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
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