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LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Integrated Systems & Solutions – S & R Systems
Introduction to Synthetic Aperture Radar (SAR)
Floyd Millet
August 2005
Integrated Systems & Solutions – S & R Systems
Introduction to Synthetic Aperture Radar (SAR)
Floyd Millet
August 2005 Work was done under US Government contract.
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Air Traffic ControlAir traffic and weather
Ground Control approach landing
Aircraft NavigationAltimeter
Doppler navigation
Weather avoidance
Law EnforcementPolice speedometers
Intrusion alarms
MilitarySurveillance and reconnaissance
Weapon guidance and control
Proximity fuzes for weapons
Bomb damage assessment
Remote Sensing the EarthFlood monitoring
Crop and forest assessment
Location of archeological ruins
Remote Sensing the Solar SystemPlanetary rotation rates
Range to the moon and planets
Meteor tracking
Ship SafetyCollision avoidance
Piloting in restricted waters
SpaceRendezvousing of spacecraft
Spacecraft docking and landing
Satellite tracking
RaDAR is an acronym for Radio Detection And Ranging
Radar ApplicationsRadar Applications
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Individual image points (pixels) must be discriminated in two dimensions, range and azimuth
Form a terrain image using a radar in a moving airborne vehicle
Problem
Simplest approach: Real-Beam Imaging Radar
Example: Plan Position Indicator (PPI)
PPI Display
Range
Azimuth
Radar ImagingRadar Imaging
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Target Not Identified When Coarse Resolution Used
Cell Size: 1/5 Major Dimension Corresponding Map
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Target Identified When Fine Resolution Used
Cell Size: 1/20 Major Dimension Corresponding Map
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Resolution Required for Various Mapping Applications
Features to be resolved
Coast lines, large cities, outlines of mountains
Major highways, variations in fields
“roadmap” details: city streets, large buildings, small air fields
Vehicles, houses, small buildings
Cell Size
150m
10-20 m
1-3 m
20-35 mResolution Cell
da
dr
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High-Resolution MappingSynthetic Aperture Radar (SAR)
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What is High Resolution Radar Mapping?
• HRM involves breaking up real antenna beam into fine resolution cells
• The map is made by forming cells and measuring signal intensity in each cell
Footprint of mainlobe on ground
dAZ
dr Resolution Cell
dAZ = Azimuth Resolution
dr = Range Resolution
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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What Does a Radar Measure?
Amplitude versus Time
All other SAR parameters are derived:
Range (known time relationship)
Phase - coherent transmission plus demodulation
Doppler frequency
Range resolution (pulse compression)
Azimuth Resolution
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Range Discrimination
2²d
²d
²d
The transmitted pulse travels at the speed of light 300,000 km/second 3.3 nanoseconds/meterRound-trip “radar time” 6.7 nanoseconds/meter(²d = 2 meters ² = 13.3 nanoseconds)
But target returns overlap if targets are separated by less than c/2
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Short Pulse Range Resolution
2(R1 + ΔR/c)
R2 = R1 + ΔR
R1
Received Pulses
Pulses Just Resolvable
Transmitted Pulse
O Time
2R1/c
2ΔR/c
2ΔR/c
ΔR = c/2
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Shorter Pulses
So for better resolution, just make the transmitted pulse SHORTER
However, the shorter pulses must somehow transmit the SAME ENERGY to the target
Peak power gets MUCH to high before pulse length even approaches high resolution
ProblemProblem
As the pulse gets SHORTER, the peak power gets HIGHER
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Coded Pulses
Transmit a long coded pulse that can be decoded (compressed) after reception into a much shorter pulse
SolutionSolution
f1 f2
Linear Frequency Modulation (FM)Linear Swept Frequency“Chirp”
Note: A typical 200 microsecond pulse extends over 60 km resulting in a range resolution of 30 km
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Pulse Compression
Delay Time
f1
f2
Δf
Time
f1
f2
Δf
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Time
f1
f2Δf 1/Δf
Frequency
Transmitted/Received Pulse Resolution = c/2
Variable Delay Line “Compression” Filter
Decoded/”Compressed” Output Resolution = c/2Δf = (/2)(fo/ Δf)F number = fo/ 2Δf
Resolution varies as 1/ Δf , that is, it varies with transmitted bandwidth
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Linear FM (Chirp) Waveform
A
-A
t
Sx
n, t A cos f0tαt2
t
Ý f fc
f0 f
c BWt
2
fc BWt
2
ft
n , t
Swept frequency having bandwidth BWt across the pulse length
Transmit waveform Frequency modulation of pulse
Phase function given by: Transmit frequency given by: Transmit bandwidth given by: Linear FM has desirable properties over other waveform types: • Easy to generate • "Stretch mode" demodulation
ft n ,t 1
2d n, t
dt fo +
, t 2 f0 t , 0 t
, 0 t
BWt ατ
n αt2
αt
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Pulse Compression
Received Signal Given By: Complex Signal (after demodulation and/or sideband removal)* Given By: Goal of Pulse Compression: • Produce finest possible resolution, which is analogous to: • Maximizing the signal-to-noise ratio
A
-A
* does not apply to "Stretch Mode" signals after the IF mixer stage--we will cover this shortly
BUT: We have the additional requirement that we minimize artifacts in the image ( Good sidelobe control on the impulse response (IPR) )
Sx 0, t S
r0, t
2 Rtgt
c2 R
tgt
c
Pulse n=0
Sr n, t C n , t cos 2 f0
t TR
α t TR
2
Sv
n,t Sr
n, t e j 0 t C n, t e j0 TR exp jα t TR
2 D n, t exp jα t T
R 2
Rtgt = Range-to-Target
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Fine Range Resolution Requires Large Radar Bandwidth
1/
Time Frequency
Fourier Transform Pair
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Pulse Compression Advantages
Range resolution independent of transmit pulse length
• Transmit long pulses
• Keep peak power comfortably low
Set range resolution with transmitted bandwidth
• Resolution inversely proportional to bandwidth
– 150 MHz 1 meter resolution
– 300 MHz 0.5 meter resolution
• Resolution independent of slant range
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Ability to Resolve Closely Spaced Targets is Beamwidth Dependent
A
B
The half-power (3dB) beamwidth is a measure of angular resolution of radar
A B B BA A(1/2)3dB 3dB (3/2)3dB
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Azimuth Considerations
SAR
Synthetic-Aperture Radar
Antenna beamwidth is inversely proportional to the number of wavelengths in its length (aperture)
L
= C/f
= /L radians R
R
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Azimuth Discrimination
L
L
R/L
R/L
Δd
R
As the collection vehicle moves along the flight path, targets are detected as they move in and out of the antenna pattern
But target returns overlap if the targets are separated in azimuth by less than the antenna beamwidth
• So achievable azimuth resolution degrades with range
Real-beam imaging radar
Flight Path
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Azimuth Discrimination
So for better azimuth resolution, just make the antenna beam NARROWER!
• Generate more wavelengths in the antenna aperture by lengthening the antenna or by shorting the wavelength (increasing the frequency)
However, very LONG antennas are difficult to carry and position and very HIGH frequencies limit performance in weather and at long ranges
Problem
Antennas get MUCH too long and frequencies MUCH to high before the beamwidth even approaches high resolution
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Synthetic-Aperture
Solution
Synthesize a long antenna aperture using a physically short antenna
SAR
Synthetic-Aperture Radar
Store the data collected sequentially and coherently across a long aperture and then process the data to
synthesize a full aperture collection
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Design Options for Improving Resolution Before SAR
Range Resolution
•Decrease pulsewidth, at expense of power and range
•Operate at short range/decrease power
Azimuth Resolution
• Increase operating frequency to Ku and Ka-bands or higher, with increased atmospheric and weather attenuation, lower available power sources
• Increase antenna aperture, with attendant installation and stabilization problems
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The Resolution Breakthrough
Range -
Pulse Compression - Increased range resolution without loss of power
Azimuth -
Synthetic Aperture - Increased azimuth resolution without large antenna installation
Note: 1. Both use special waveforms
2. Both use signal processing techniques
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Phase History of a Scatterer
From Hovanessian, “Introduction to Synthetic Array and Imaging Radars”
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AzimuthBeam Coverage
RangeResolution
Maximum SyntheticAperture Length Lmax
Ro
Beamwidthbeam
Minimum Resolution Ro
2 Lmaxbeam Ro
dRoLmax
Minimum Resolution d2
where d is the antenna length
Resolution Limitation on Sidelooking SAR
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-Resolution limitation on sidelooking SAR:
Maximum θINT is limited by azimuth antenna beamwidth
Synthetic Aperture
RoINT
Tgt 1
Ls
"Integration Angle"
INT Ls
Ro0.886k A
2WAZ
, Lant is length of linear arrayantLaz
Waz 2 INT
Wazmin
2 az
2Lant
Lant
2
-This limitation does not apply to spotlight collection
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Point-Target Phase HistoryCompressed in Both Range and Azimuth
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ADTS Advanced Detection Technology Sensor
Ft. Devens, MA
35 GHZ
HH Polarization
± 20 Deg Depression Angle
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Where Do You Fit in?Where Do You Fit in?
Future Topics
Implementation of Theory
September 6 Algorithm Architecture for SAR Ground Processing
Creating an Image
September 20 Concepts in Image Processing
From Idealization to Realization
October 11 PACE: An Autofocus Algorithm for SAR
Future Topics
Implementation of Theory
September 6 Algorithm Architecture for SAR Ground Processing
Creating an Image
September 20 Concepts in Image Processing
From Idealization to Realization
October 11 PACE: An Autofocus Algorithm for SAR
LOCKHEED MARTININTEGRATED SYSTEMS & SOLUTIONS
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Texts and Software
Texts
Curlander, John and McDonough, Robert Synthetic-Aperture Radar - Systemsand Signal Processing
Skolnik, Merrill Introduction to Radar Systems
Nathanson, Fred Radar Design Principles
Carrara, Walter et al Spotlight Synthetic-Aperture Radar
Oppenheim, Alan and Schafer, Ron Discrete Time Signal Processing
Skolnik, Merrill Radar Handbook
Software
Mathcad V6
Matlab