4_igarss11_younis_nogifv4.pptx

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 ParametersSystem 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 ElevationOperation 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 ElevationOperation 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 OperationAzimuth Operation

flight

dire

ctio

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 OperationAzimuth Operation

flight

dire

ctio

n

Doppler span 4

beam 3

beam 1 Doppl

er 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

Bea

mfo

rmin

g

Hardware Functional Block DiagramHardware 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 DiagramHardware 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

Bea

mfo

rmin

g

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 AntennasDeployable Reflector Antennas

X-Band Reflector System 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 TimingOperation Mode and Timing

95 k

m

82 k

m

70 k

m

75 k

m

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 RatioRange-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 possible

But: timing issues limit the swath width


Viewgraph 12

Azimuth-Ambiguity-to-Signal RatioAzimuth-Ambiguity-to-Signal Ratio

azimuth-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 Hz

oversampling 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 (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

na

dir

receive beam

reflecto

r

pattern steering

pulse

Pulse Extension Loss (PEL)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)Pulse Extension Loss (PEL)

near range3

Rx

activ

e el

emen

ts4

Rx

activ

e el

emen

tsfar 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 ElevationOn/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 BeamformingTwo-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 BeamformingTime 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 BeamformingFIR 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 GainElevation Beamforming to Increase Antenna Gain

MVDR: Minimum Variance Distortionless Response

LCMV: 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 X-Band Reflector System

diameter 6 x 12 m

focal length 12 m

elevation offset 0.5 m

center elements

edge elements

Reflector Illumination 6 x 2 Active Patches

Reflector Illumination EfficiencyReflector 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 ImprovementAzimuth 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 ImprovementAzimuth 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 .

ConclusionConclusion

4_igarss11_younis_nogifv4.pptx

Technology

azimuth beamforming

azimuth operationreceive

azimuth operationtransmit

degraded azimuth patterns

narrow beam of feed

access range

doppler span

score digital feed array