Design Optimization Aspects for Reflector Base Synthetic Aperture Radar

M. Younis

Design Optimization Aspects for Reflector Base Synthetic Aperture Radar

Marwan Younis, Anton Patyuchenko, Sigurd Huber, and Gerhard Krieger,Microwaves and Radar Institute, German Aerospace Center (DLR)

International Geoscience and Remote Sensing Symposium July 24-29, 2011 – Vancouver, Canada

Microwaves and Radar InstituteM. Younis – IGARSS’11 – [email protected]

Viewgraph 2

SAR Instrument Requirements

Parameter Value frequency 9.65 GHz (X-Band) coverage > 300 km resolution ≤ 1 x 1 m ambiguity-to-signal ratio ≤ -20 dB noise-equivalent sigma zero ≤ -20 dB

System and Requirement Parameters

• Reflector based SAR Systemarchitecture and operation

• System Performancerange- & azimuth-ambiguity-to-signal ratio, noise-

equivalent sigma zero, pulse extension loss

• Performance Optimizationbeamforming in elevation and azimuth


Viewgraph 3

Operation of Transmit in Elevation

swath

Tx illumination

ground range

reflectorflight

direction

slant range

• transmit with all feed elements• narrow beam of feed array• illuminate small portion of reflector

wide and low gain beam illuminating complete

swath

Transmit in Elevation


Viewgraph 4

reflector

swathground range

Rx window

Operation of Receive in Elevation

flight direction

slant range

SCan-On-REceive (SCORE)• follow the pulse echo on the ground

by activating corresponding elements• cycle through all elements within on

PRI

Rx element activation matrix

• energy from a small portion of the ground illuminates complete reflector

• focused on individual elements of feed narrow and high gain beam

Receive in Elevation


Viewgraph 5

Azimuth Operation

flight

dir

ectio

n

Transmit in Azimuth

• transmit with all feed elements• narrow beam of feed array• illuminate small portion of reflector

wide and low gain beam

swath width


Viewgraph 6

Azimuth Operation

flight

dir

ectio

n

Doppler span 4

beam 3

beam 1 Doppler span 1

Doppler span 2

Doppler span 3

beam 2

beam 4

Receive in Azimuth

• each azimuth channel is sampled• each azimuth channel covers a

narrow Doppler spectrum low PRF• combining the azimuth channels

yields a wide Doppler bandwidth high resolution


Viewgraph 7

single azimuth channel T/R-Module

Nel

1

2

ADC

ADC

ADC

feed elements

AMP

AMP

AMP

Dig

ital B

eam

form

ing

Hardware Functional Block Diagram

flight direction

slant range

mem

ory

signal gen.

reflector

• digital feed array in elevation directionSCan-On-REceive (SCORE)


Viewgraph 8

Hardware Functional Block Diagram

reflector

flight direction

slant range

mem

ory

signal gen.

Nel

1

2

ADC

ADC

ADC

feed elements

AMP

AMP

AMP

Dig

ital B

eam

form

ing

T/R-Modulesingle azimuth channel

single azimuth channel

single azimuth channel

• digital feed array in elevation directionSCan-On-REceive (SCORE)

• digital feed array in azimuth direction good azimuth resolution

2D Digital Feed Array


Viewgraph 9

• deployable reflector are mature technology• flight heritage in space telecommunications

satellites• Lightweight mesh reflectors spanning

diameters > 20 m exist

Deployable Reflector Antennas

X-Band Reflector System

Parameter Value

reflector

diameter (elevation x

azimuth)12 x 12 m

focal length 12 m

elevation offset 0.5 m

feed

patches & TRMs 114 x 10

digital feeds 38 x 5

approx. size 3.5 x 0.3 m


Viewgraph 10

• image any single swath within access range

• conventional stripmap processing

swath 1

swath 2

swath 3

swath 4

Operation Mode and Timing

95 k

m

82 k

m

70 km

75 km access range 315 km

orbit height 745 km

receive window

Tx Tx

time

PRI = 1/PRF

PRI·dc PRI pulse repetition intervalPRF pulse repetition frequencydc duty cyclessw sub-swath


Viewgraph 11

Range-Ambiguity-to-Signal Ratio

range-ambiguity-to-signal ratio

signal

ambig

Tx

Rx

elevation patterns

2-way

good range ambiguity suppression due to narrow Rx pattern increase of PRF is possibleBut: timing issues limit the swath width


Viewgraph 12

Azimuth-Ambiguity-to-Signal Ratioazimuth-ambiguity-to-signal ratio

• AASR shows degradation at swath edges due to degraded azimuth patterns

• improvement through: higher PRF, antenna optimization, azimuth beamforming, or waveform encoding

proc. Doppler 595x10 Hzoversampling 3.8

azimuth resolution 10.3/10 m

signalambig

Tx Rx

azimuth patterns

2-way

near range

mid rangeNESZ does not meet requirement


Viewgraph 13

Noise-Equivalent Sigma Zero (NESZ)

Noise-Equivalent Sigma-Zero

• lower average power per swath than planar antenna systems• a sub-set of the TRMs are activated for each swath• the number of TRMs determine the total power• reducing the swath width does not improve the NESZ

Pav = 900 W

720 W

600 W

540 W

2-way loss 2 dB

sys. noise temp. 450 K

duty cycle 10%

Av. power per TRM 2 W

NESZ performance does not meet requirement


Viewgraph 14

3 Rx elements active

pulse extension on ground

s

nadir

receive beam

reflecto

r

pattern steering

pulse

Pulse Extension Loss (PEL)

The pulse extension loss (PEL) is the integral effect over multiple points simultaneously illuminated by the pulse.

pulse extension loss


Viewgraph 15

Pulse Extension Loss (PEL)near range

3 R

x ac

tive

elem

ents

4 R

x ac

tive

elem

ents

far range

wide beam:low PEL but low gain

PEL not critical at far range


Viewgraph 16

SCORE beam 1

S

feed 1 ADC

ADC

ADC

ADC

OnOff

OnOff

OnOff

OnOff

feed 2

feed 3

feed 4

reflector

swath 1

On/Off Beamforming in Elevation

On/Off : switch element On or Off


Viewgraph 17

SCORE beam 1

SCORE beam 2

S

S

feed 1 ADC

ADC

ADC

ADC

ADC

ADC

ADC

OnOff

OnOff

OnOff

OnOff

OnOff

OnOff

OnOff

feed 2

feed 3

feed 4

feed 5

feed 6

feed 7

reflector

swath 1

swath 2

Two-Swath On/Off Beamforming

On/Off : switch element On or Off


Viewgraph 18

SCORE beam 1

w

w

w

w

w

w

w

SCORE beam 2

S

S

i

i

i

i

i

i

i

ADC

ADC

ADC

ADC

ADC

ADC

ADC

reflector

feed 1

feed 2

feed 3

feed 4

feed 5

feed 6

feed 7

Time Varying Beamforming

i : range sample (discrete time) : complex time-varying weightw i


Viewgraph 19

SCORE beam 1

w4w3w2w1

w4w3w2w1

w4w3w2w1

w4w3w2w1

w4w3w2w1

w4w3w2w1

w4w3w2w1

SCORE beam 2 S

S

S

S

S

S

S

i+3i i+2i+1

i+3i i+2i+1

i+3i i+2i+1

i+3i i+2i+1

i+3i i+2i+1

i+3i i+2i+1

i+3i i+2i+1

reflector

swath 1

ADC

ADC

ADC

ADC

ADC

ADC

ADC

feed 1

feed 2

feed 3

feed 4

feed 5

feed 6

feed 7

FIR Filter Beamforming

i : range sample (discrete time) : complex time-varying weightw i


Viewgraph 20

elevation angle in degree

patte

rn g

ain

[dB

]

Noise-Equivalent Sigma-Zeroelevation beamforming gain

ground range in km

NE

SZ

[dB

]• Use elevation beamforming to increase antenna gain• Most effective at large scan angel, where beams overlap (defocus) • In best case increase the gain (NESZ) by 3dB to 5dB

3 dB3 dB

5 dB

3 dB

5 dB3 dB

3 dB

Elevation Beamforming to Increase Antenna Gain

MVDR: Minimum Variance Distortionless ResponseLCMV: Linear Constraint Minimum Variance


Viewgraph 21

• The reflector is only partially illuminated in elevation

• The illumination is a function of pulse duty cycle

reflector height reduction

• Although all azimuth elements are active on receive no sub-illumination occurs.

X-Band Reflector System

diameter 6 x 12 mfocal length 12 melevation offset 0.5 m

center elements

edge elements

Reflector Illumination 6 x 2 Active Patches

Reflector Illumination Efficiency


Viewgraph 22

Noise-Equivalent Sigma-ZeroAzimuth Beamforming Gain

ground range in km

NE

SZ

[dB

]

SN

R g

ain

[dB

]

PRF [kHz]

far range

0.8 dB.8 dB

2.2 dB

.8 dB

near range

• Due to wide azimuth beams, several elements share common Doppler spectra.

• Combine azimuth channels to increase signal engery• Increase the gain (NESZ) by .8dB to 2.2dB

PRF range

Azimuth Beamforming for SNR Improvement

LCMV: Linear Constraint Minimum Variance


Viewgraph 23

AASR without Beamforming

ground range in km

far range

near range

PRF range

PRF [kHz]

AA

SR

[dB

]

AA

SR

[dB

]

AASR with LCMV Beamforming

-28 dB-40 dB

-28 dB-28 dB

• The LCMV algorithm uses overlapping beams to place nulls at the ambiguity positions

• However the azimuth channels are sampled adequatly, i.e. no reconstruction needed.

• Azimuth-ambiguity suppression better than -38dB

Azimuth Beamforming for AASR Improvement


Viewgraph 24

Reflector based systems allow for high-resolution wide-swath operation using digital beamforming

• High performance reflector SAR is feasible at X-band.

• The power consumption per swath is less than for planar

systems.

• Time varying digital beamforming is required in elevation

to reach full antenna gain.

• On-Ground digital beamforming is required in azimuth to

suppress ambiguities .

Conclusion

Design Optimization Aspects for Reflector Base Synthetic Aperture Radar

Documents

radar institutem

younis igarss11

sampledeach azimuth

azimutheach azimuth

reflector digital feed

elevation microwaves

high gain beam

scoredigital feed array