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Physics of SAR. Summer 2003. SAR Radar. SAR. Synthetic-Aperture Radar. RAdio Detection And Ranging. Radar - Transmits its own illumination • a "Microwave flashlight" . Radar Imaging. Form a terrain image using a radar in a . Problem. moving airborne/orbital vehicle.

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physics of sar

Physics of SAR

Summer 2003

sar radar
SAR Radar

SAR

Synthetic-Aperture Radar

RAdio Detection And Ranging

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

radar imaging
Radar Imaging

Form a terrain image using a radar in a

Problem

moving airborne/orbital vehicle

Simplest Approach - Real-Beam Imaging Radar

Example:

P

lan

P

osition

I

ndicator (PPI)

Range

Azimuth

Azimuth

270°

90°

Individual image points (pixels) must

be discriminated in two dimensions,

range and azimuth

180°

PPI Display

range discrimination
Range Discrimination

2D

d

t

t

t

D

d

D

d

The transmitted pulse travels at the speed of light

109 feet/second Þ1 nanosecond/foot

Round trip "radar time" Þ 2 nanoseconds/foot

(Dd = 2 feet Þ Dt = 4 nanoseconds)

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

shorter pulses
Shorter Pulses

So for better range resolution, just make

SHORTER

the transmitted pulse

However , the shorter pulses must somehow

SAME ENERGY

transmit the

to the target

=

=

=

SHORTER

HIGHER

As the pulse gets

, the peak power gets

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

Problem

coded pulses
Coded Pulses

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

Solution

f

f

1

2

t

Linear F.M. (Frequency Modulation)

Linear Swept Frequency

"Chirp"

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

pulse compression
Pulse Compression

Frequency

f

2

D

f

Transmitted/Received Pulse

f

1

t

Time

t

t

1

2

Frequency

f

2

Variable Delay Line"Compression" Filter

D

f

f

1

Delay

Time

t

0

Frequency

f

2

1

Decoded / "Compressed“Output

D

f

D

f

f

1

Time

Pulse compression ratio = pulse "time-bandwidth product"

pulse compression advantages
Pulse 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
azimuth considerations
Azimuth Considerations

SAR

Synthetic-Aperture Radar

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

l

L

q

=

radians

L

c

l

=

f

azimuth discrimination

l

R

L

l

R

L

Azimuth Discrimination

Flight Path

L

D

d

L

R

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
narrower beamwidth
Narrower 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

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

Problem

synthetic aperture
Synthetic-Aperture

Synthesize a long antenna apertureusing a physically short antenna

Solution

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

azimuth considerations13
Azimuth Considerations

Flight Path

SyntheticallyProcessedAperture(LS)

Synthetically

Processed

Beam

l

LP

PhysicalAntenna(LP)

l/LS

Real Beam

synthetic aperture advantages
Synthetic-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