Ambiguity suppression by azimuth phase coding in multichannel sar systems
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Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems. F. Bordoni , M. Younis, G. Krieger. DLR - Institut für Hochfrequenztechnik und Radarsysteme. IGARSS 2011, 24-29 July, Vancouver, Canada. Outline. Introduction APC (Azimuth Phase Coding) technique

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Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems

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Ambiguity suppression by azimuth phase coding in multichannel sar systems

Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems

F. Bordoni, M. Younis, G. Krieger

DLR - Institut für Hochfrequenztechnik und Radarsysteme

IGARSS 2011, 24-29 July, Vancouver, Canada


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Outline

  • Introduction

  • APC (Azimuth Phase Coding) technique

  • APC in multichannel SAR (Synthetic Aperture Radar) systems

  • Figure of merit

  • Numerical analysis

    • APC performance versus system parameters

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

  • Conclusions


Ambiguity suppression by azimuth phase coding in multichannel sar systems

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?


Ambiguity suppression by azimuth phase coding in multichannel sar systems

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]

[Dal04] J. Dall, A. Kusk, “Azimuth Phase Coding for Range Ambiguity Suppression in SAR”,IGARSS2004.

APC is based on three main steps:

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

APC modulation phase

Tx pulse number

2) Azimuth phase demodulation on Rx

APC demodulation phase

@ round-trip delay

APC residual phase

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

3) Azimuth filtering over the processing bandwidth


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Az. FILTER

Az. FILTER

APC residual phase  Doppler shift

Time domain: linear phase

Frequency domain: 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


Ambiguity suppression by azimuth phase coding in multichannel sar systems

PRF << Bp

APC residual phase:

reconstructed multichannel signal sampled at PRFeff =N PRF:

Application to Multichannel Systems

Multichannel SAR system: 1 transmitter, N receivers

N

2

1

N Rx az. signals sampled at PRF

APC residual phase:

MULTICHANNEL PROCESSING

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


Ambiguity suppression by azimuth phase coding in multichannel sar systems

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

(uniform PRF*)

PRF

,

,

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

 The ambiguity spectrum:

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


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Figure of Merit

Measurement of the ambiguity suppression induced by APC

APC Gain:

Computed on the SAR signal after multichannel processing

PSD (Power Spectral Density) range ambiguity of 1st order

if APC is not applied

useful signal after multichannel reconstruction (neglect. elev.)

processed bandwidth

PSD range ambiguity of 1st order if APC is applied

  • Note: the Gapcdepends on the azimuth pattern shape


Ambiguity suppression by azimuth phase coding in multichannel sar systems

APC Performance Analysis

Reference Multichannel Planar Systems

The systems have the same azimuth patterns

Processing bandwidth 2316 Hz ≤ Bp≤ 4168 Hz

  • Behavior of APC versus the number of Rx channels, N

Investigation:

  • Effect of the Doppler oversampling

  • The effect of the pattern shape is not evident


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Numerical Results: Gapc

APC Gain v.s. oversampling factor

For the considered systems, for M=2:

  • 0.1dB ≤ Gapc ≤ 3.13dB

  • for a given N, the Gapcincreases with the oversampling factor, 

  • the Gapcdecreases for increasing number of channels, N

  • the sensitivity of Gapc to  decreases with increasing N


Ambiguity suppression by azimuth phase coding in multichannel sar systems

The thickness of the curves is a fast variation of the spectrum, due to aliasing

N = 1, 2, 8

Numerical Results: PSD v.s. N

Normalized PSD 1st range ambiguity after multichannel reconstruction

without APC

with APC

N = 8

N = 1

N = 2

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


Ambiguity suppression by azimuth phase coding in multichannel sar systems

HRWS SAR Multichannel Systems

HRWS (High-Resolution Wide-Swath) SAR System

promoted 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

Planar system:

currently adopted design

Reflector system:

alternative design option, studied in DLR


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Peculiarities HRWS Systems

Planar system

Reflector system

The pattern of each Rx channel covers Bp

Multichannel processing: Multi-Aperture Reconstr.

The patters do not change along the swath

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


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Numerical Results: Planar HRWS System

Normalized PSD 1st range ambiguity used to compute the Gapc

(after multichannel reconstruction)

without APC

with APC

For M=2, Gapc = 0.69 dB

  • The high number of channels (7) and the small oversampling (1.96) associated lowGapc


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Numerical Results: Reflector HRWS System

Normalized PSD 1st range ambiguity used to compute the Gapc

(before multichannel reconstruction, single Rx channel)

without APC

with APC

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


Ambiguity suppression by azimuth phase coding in multichannel sar systems

Conclusions

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

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

  • The azimuth pattern strongly affects the APC performance.

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

  • In a conventional SAR system with g = 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.

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


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