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Physics of SARPhysics of SAR

Summer 2003

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Synthetic-Aperture RadarSynthetic-Aperture Radar

SARSAR

Radar - Transmits its own illumination• a "Microwave flashlight"

RAdio Detection And RangingRAdio Detection And Ranging

SAR RadarSAR Radar

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Azimuth

Form a terrain image using a radar in a Form a terrain image using a radar in a moving airborne/orbital vehiclemoving airborne/orbital vehicle

ProblemProblem

Simplest Approach - Real-Beam Imaging RadarSimplest Approach - Real-Beam Imaging RadarExample: Example: PPlan lan PPosition osition IIndicator (PPI)ndicator (PPI)

Individual image points (pixels) must Individual image points (pixels) must be discriminated in two dimensions, be discriminated in two dimensions, range and azimuthrange and azimuth

Range

AzimuthAzimuth

0°

90°

180°

270°

PPI DisplayPPI Display

Radar ImagingRadar Imaging

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dd

dd

dd

The transmitted pulse travels at the speed of light109 feet/second 1 nanosecond/foot

Round trip "radar time" 2 nanoseconds/foot

(d = 2 feet = 4 nanoseconds)

But target returns overlap if targets are separated by less than /2

The transmitted pulse travels at the speed of light109 feet/second 1 nanosecond/foot

Round trip "radar time" 2 nanoseconds/foot

(d = 2 feet = 4 nanoseconds)

But target returns overlap if targets are separated by less than /2

Range DiscriminationRange Discrimination

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So for better range resolution, just make So for better range resolution, just make the transmitted pulse the transmitted pulse SHORTERSHORTER

However , the shorter pulses must somehow However , the shorter pulses must somehow transmit the transmit the SAME ENERGYSAME ENERGY to the target to the target

As the pulse gets As the pulse gets SHORTERSHORTER, the peak power gets , the peak power gets HIGHERHIGHER

Peak power gets MUCH too high beforepulse length even approaches high resolution

Problem

== == ==

Shorter PulsesShorter Pulses

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Linear F.M. (Frequency Modulation)Linear F.M. (Frequency Modulation)

Transmit a long coded pulse that can be decoded (compressed) afterreception into a much shorter pulse

Solution

Linear Swept Frequency Linear Swept Frequency "Chirp""Chirp"

Note: A typical 200 microsecond pulse extends over more than 16 nautical miles in radar space

ff11 ff22

Coded PulsesCoded Pulses

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Transmitted/Received PulseTransmitted/Received Pulse f1

f2

Pulse compression ratio = pulse "time-bandwidth product"Pulse compression ratio = pulse "time-bandwidth product"

Variable Delay Line"Compression" FilterVariable Delay Line"Compression" Filter

Decoded / "Compressed“Output Decoded / "Compressed“Output

Time

Frequency

f1

f2

f1

f2

DelayTime

Frequency

Time

Frequency

f

f

f

0

f1

Pulse CompressionPulse Compression

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Pulse CompressionAdvantagesPulse CompressionAdvantages

• 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

300 MHz ñ 2-foot resolution

600 MHz ñ 1-foot resolution

– Resolution independent of slant range

• 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

300 MHz ñ 2-foot resolution

600 MHz ñ 1-foot resolution

– Resolution independent of slant range

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Synthetic-Aperture RadarSynthetic-Aperture Radar

SARSAR

Antenna beamwidth is inversely proportional to the number of wavelengths in its length (aperture)

Antenna beamwidth is inversely proportional to the number of wavelengths in its length (aperture)

= L

radians

= = ccff

LL

Azimuth ConsiderationsAzimuth Considerations

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Real-beam imaging radar Real-beam imaging radar

• 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 decreases with range

• 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 decreases with range

LL

LL

RR

R L

R L

dd

Flight PathFlight Path

Azimuth DiscriminationAzimuth Discrimination

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ProblemProblemAntennas get MUCH too long and frequencies MUCH too high before the beamwidth even approaches high resolution

Antennas get MUCH too long and frequencies MUCH too high before the beamwidth even approaches high resolution

Narrower BeamwidthNarrower Beamwidth

• 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

• 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

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Store the data collected sequentially and coherently across a long aperture and then process the data to

synthesize a full aperture collection

Store the data collected sequentially and coherently across a long aperture and then process the data to

synthesize a full aperture collection

Synthesize a long antenna apertureusing a physically short antenna Synthesize a long antenna apertureusing a physically short antenna

SolutionSolution

Synthetic-ApertureSynthetic-Aperture

Synthetic-Aperture RadarSynthetic-Aperture Radar

SARSAR

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LPLP

/LS/LS

Real BeamReal Beam

Synthetically Synthetically Processed Processed BeamBeam

SyntheticallyProcessedAperture(LS)

SyntheticallyProcessedAperture(LS)

PhysicalAntenna

(LP)

PhysicalAntenna

(LP)

Flight PathFlight Path

Azimuth ConsiderationsAzimuth Considerations

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Synthetic-ApertureAdvantagesSynthetic-ApertureAdvantages

• Increased angle of collection on a target allows increased resolution

– High resolution capability with short physical antenna

• Processed aperture size is easily increased as imaging distance increases

– Azimuth resolution independent of slant range

• Increased angle of collection on a target allows increased resolution

– High resolution capability with short physical antenna

• Processed aperture size is easily increased as imaging distance increases

– Azimuth resolution independent of slant range