lockheed martin integrated systems & solutions 1 integrated systems & solutions – s &...

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.


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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


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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


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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


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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


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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


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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


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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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Azimuth Resolution


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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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Synthetic Aperture Radar


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Phase History of a Scatterer

From Hovanessian, “Introduction to Synthetic Array and Imaging Radars”


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Anamorphic Hologram


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SAR Collection Geometry


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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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First SAR Imagery


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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


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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

lockheed martin integrated systems & solutions 1 integrated systems & solutions – s &...

Documents

lockheed martin

range resolution slide

high resolution radar

radar applications

coarse resolution

radar measure

azimuth resolution d

fine resolution cells