Folie 1

Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems

DLR - Institut für Hochfrequenztechnik und Radarsysteme

F. Bordoni, M. Younis, G. Krieger

IGARSS 2011, 24-29 July, Vancouver, Canada

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Outline

o Introduction

o APC (Azimuth Phase Coding) technique

o APC in multichannel SAR (Synthetic Aperture Radar) systems

o Figure of merit

o Numerical analysis

o APC performance versus system parameters

o Example: two multichannel systems for high resolution wide swath imaging

o Conclusions

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Introduction

Current spaceborne SAR systems limitation: trade-off spatial resolution v.s. swath width

Research in two main directions:

Processing methods for removing the ambiguities

APC- low implementation complexity

- effectiveness for point and distributed ambiguities

New, more flexible SAR systems- Multichannel systems-Digital Beamforming (DBF) on receive- Multichannel processing

APC is conceived for conventional SAR systems:

APC in multichannel systems based on DBF on receive?

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Review of the APC Technique

APC is a technique for range ambiguity suppression, conceived for conventional (1 Tx and 1 Rx) SAR systems [Dall, Kusk 2004]

mod ( )l

mod( ) ( )dem n n m

azimuth sample number, order of range ambiguity, APC shift-factor

APC residual phase

[Dal04] J. Dall, A. Kusk, “Azimuth Phase Coding for Range Ambiguity Suppression in SAR”, IGARSS 2004.

@ round-trip delay

( , , )res n k M

APC modulation phase

Tx pulse number

3) Azimuth filtering over the processing bandwidth

APC demodulation phase

APC is based on three main steps:

1) Azimuth, i.e. pulse to pulse, phase modulation on Tx

2) Azimuth phase demodulation on Rx

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APC residual phase Doppler shift

order of range ambiguity (0 useful signal)

M=2 maximum Doppler shift of the 1st order range ambiguity Larger oversampling Larger ambiguity suppression / pPRF B

, ( ) ( ) exp ( , , )k apc k resx n x n j n k M

Time domain: linear phase Frequency domain: Doppler shift

, ( ) ( )k apc kX f X f f

2( , , )res n k M k n

M

res

1 M…

2

x0

0( / )

n

t n PRFx

x

x

+

+

2

M

…

k = 1

k =

2

k = 12

kM

… …

res

1 M…

2

x0

0( / )

n

t n PRFx

x

x

+

+

2

M

…

k = 1

k =

2

k = 12

kM

… …

f0

PRF

Bp

k =

0

k =

1

k =

2

f (1, M)

PRF

M2

PRF

M

f (2, M)

f0

PRF

Bp

k =

0

k =

1

k =

2

f (1, M)

PRF

M2

PRF

M

f (2, M)

Az.

FIL

TE

R

Az.

FIL

TE

R

/ 2

( , ) modPRF

PRFf f k M k

M

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Application to Multichannel Systems

mod ( , )l M

MULTICHANNEL PROCESSING

N Rx az. signals sampled at PRF

APC residual phase:

APC residual phase: ( , , )mc mcres n k M

, ( ) ( )k apc kX f X f f k

1 2 N

( , )dem n M

( , , )res n k M

( , )dem n M

Multichannel SAR system: 1 transmitter, N receivers

The behavior of the APC changes when applied to a multichannel system

, ( )rk apcX freconstructed multichannel signal sampled at PRFeff =N PRF:

, ( )rk apcX f

PRF << Bp

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APC & Reconstructed Multichannel Signal

The APC residual phase has no more a linear trend versus the azimuth sample (pulse) number no shift of the Spectrum

The residual phase a “stair” shape (<≠> Doppler shift):

The ambiguity spectrum: , ( ) ( ) exp ( , , )r r mck apc k resX f X f FT j n k M

N M 2 mcres

2

2

00

/( )

mc

mc

n

t n N PRF

k= 1, 3, …x

xx4

xx

xx

mcres

2

2

00

/( )

mc

mc

n

t n N PRF

k= 1, 3, …x

xx xx4

xx xx

xx xx

f0

PRFeff = 2 PRF

Bp

PRF

f0

PRFeff = 2 PRF

Bp

PRF

2( , , ) int

mcmc mcres

nn k M k

M N

,

,

PRF

(uniform PRF*)

*PRF matched to the antenna length and No. of apertures > regular sampling in azimuth results

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Figure of Merit

Measurement of the ambiguity suppression induced by APC

p

p

p

p

B / 22r

1B / 2

apc B / 22r

1,apcB / 2

X ( f ) df

G

X ( f ) df

PSD (Power Spectral Density) range ambiguity of 1st order if APC is not applied

processed bandwidth

PSD range ambiguity of 1st order if APC is applied

APC Gain:

r r1 0,apcX ( f ) X ( f )

useful signal after multichannel reconstruction (neglect. elev.)

Computed on the SAR signal after multichannel processing

Note: the Gapc depends on the azimuth pattern shape

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Parameter System #

1 2 3(Ref.) 4

Orbit height [km] 520

Carrier frequency [GHz] 9.600

Rx antenna total length [m] 3 6 12 24

Tx antenna length [m](and Rx subapert. length)

3

No. of az. Rx channels 1 2 4 8

PRF [Hz] (uniform) 5068 2534 1267 633.5

PRFeff [Hz] 5068

APC Performance Analysis

Reference Multichannel Planar Systems

PRFeff

Bp

PRFeff

Bp

The systems have the same azimuth patterns

Effect of the Doppler oversampling The effect of the pattern shape is not evident

pN PRF B Behavior of APC versus the number of Rx channels, N

Processing bandwidth 2316 Hz ≤ Bp ≤ 4168 Hz

Investigation:

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Numerical Results: Gapc

0.1dB ≤ Gapc ≤ 3.13dB

for a given N, the Gapc increases with the oversampling factor,

the Gapc decreases for increasing number of channels, N

the sensitivity of Gapc to decreases with increasing N

N=1

N=2

N=4

N=8

N=1

N=2

N=4

N=8

APC Gain v.s. oversampling factor

For the considered systems, for M=2:

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Numerical Results: PSD v.s. N

larger N, the upper profile PSD with or without APC are similar and Gapc reduces

Normalized PSD 1st range ambiguity after multichannel reconstruction

BpBp BpBpBpBpBpBp

N = 8

with APC

N = 2N = 1

without APC

BpBpBp

N = 1, 2, 8

The thickness of the curves is a fast variation of the spectrum,

due to aliasing

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HRWS (High-Resolution Wide-Swath) SAR Systempromoted by the German Aerospace Centre (DLR)conceived to obtain high resolution and wide swaths

(1 m resolution, 70 km swath width in stripmap mode)

Different Rx azimuth patterns & multichannel reconstruction

HRWS SAR Multichannel Systems

Parameter Planar Reflector

Orbit height [km] 520 745

Carrier frequency [GHz] 9.600 9.650

Tx/Rx antenna total length [m] 8.75

Paraboloid diameter (elev., az.) [m] 10, 12

Total number of feeds (elev., az.) 60, 10

No. of az. Rx channels 7 10

PRF [Hz] 1750 2792

Processed bandwidth [Hz] 6252 5946

Oversampling factor 1.960 4.696

Planar system: currently adopted design

Reflector system:alternative design option,

studied in DLR

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Peculiarities HRWS Systems

Reflector systemPlanar system

The pattern of each Rx channel covers Bp

Multichannel processing: Multi-Aperture Reconstr.

The patters do not change along the swath

PRF

Be

PRF

Be

e pB B / N

pB

N PRF

pB

The pattern of each Rx channel covers 1/N of Bp

Multichannel processing: Spectral decomposition

The patters change along the swath

Evidence of the dependence of the APC performance on the pattern shape

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Numerical Results: Planar HRWS System

For M=2, Gapc = 0.69 dB

The high number of channels (7) and the small oversampling (1.96) associated low Gapc

BpBpBp BpBpBp

with APCwithout APC

Normalized PSD 1st range ambiguity used to compute the Gapc

(after multichannel reconstruction)

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Numerical Results: Reflector HRWS System

For M=2, 3.2 dB ≤ Gapc ≤ 8.6 dB over the swath, depending on the azimuth pattern

The azimuth pattern strongly affects the APC performance

The reflector based system, characterized by a higher oversampling factor (4), takes better advantage from the application of APC

Normalized PSD 1st range ambiguity used to compute the Gapc

(before multichannel reconstruction, single Rx channel)

BeBeBe

BeBe

without APC with APC

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Conclusions

o In multichannel systems, the APC effect is no more a frequency shift of the range ambiguity.

o Also in multichannel systems, the APC allows for improved ambiguity suppression.

o The azimuth pattern strongly affects the APC performance.

o For a given azimuth pattern, the suppression is directly proportional to the oversampling factor and inversely proportional to the number of receive channels.

o In a conventional SAR system with = 2, the achievable suppression of each ambiguity of odd order is about 3 dB. In multichannel systems based on planar antenna architectures, the suppression is generally poorer.

Reflector based systems reach better performance, because of the higher oversampling.

o In the planar and reflector based HRWS systems the APC suppression is about 0.7 dB and between 3 and 8 dB, respectively.