COMPARISON%20OF%20GEOMETRY-BASED%20DOPPLER%20AMBIGUITY%20RESOLVER%20IN%20SQUINT%20SAR%20

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COMPARISON OF GEOMETRY-BASED DOPPLER AMBIGUITY
RESOLVER IN SQUINT SAR
AN INDIRECT DOPPLER RATE ESTIMATION SCHEME OF SAR
IN LOW-CONTRAST SCENE
Wenchao Li University of Electronic Science and
Technology of China
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Introduction
Synthetic Aperture Radar (SAR)
An important remote sensing tool.

• Monostatic SAR
• Bistatic SAR
• Stripmap SAR
• Scan SAR
• Spotlight SAR
• Side-looking SAR
• Squint-looking SAR

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Introduction
Squint SAR
Squint angle
Geometry model
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Introduction
Squint SAR
Range History

• The linear term is range walk
• The quadratic term is range curvature

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Introduction
Signal model
After range walk correction, the azimuth data can
be modeled as a linear frequency modulated (LFM)
signal,

• Doppler centroid ( )
• Doppler rate ( )

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Introduction
In principle, it is possible to compute Doppler
parameter from prior information provided by
GPS/INS, but measurement uncertainties on the
information will limit the accuracy.
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Introduction

• An incorrect Doppler centroid
• a loss of SNR
• an increase in the azimuth ambiguity level
• a shift in the location of the target
• ...

An incorrect Doppler rate would blur the image.
Correct Doppler rate
Incorrect Doppler rate
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Introduction
Therefore, Doppler parameter Estimation from echo
data is an essential procedure for high quality
SAR processing
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Doppler centroid estimation (Resolve Doppler
centroid ambiguity)
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Doppler centroid estimation
As azimuth data are sampled by pulse repetition
frequency (PRF), there is always Doppler centroid
ambiguity, and the bigger the squint angle is,
the more serious the doppler ambiguity will be.
Doppler centroid is usually expressed in two
parts the baseband Doppler centroid and Doppler
ambiguity number .
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Doppler centroid estimation

• Multiple PRF
• Wavelength diversity algorithm (WDA)
• Multilook algorithms (MLCC,MLBF)
• Geometry-based algorithms

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Geometry-based algorithms
The relationship between Doppler centroid and
range walk slope,
Then Doppler centroid can be computed via
estimating the range walk slope.
Since this method is based on the geometric
characteristics of the range compressed data, it
is named as a geometry-based Doppler ambiguity
resolver.
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Geometry-based algorithms
Conventional Radon transform scheme

• Radon transform
• A well-known tool for detecting slope.
• It integrates intensity along every possible
  direction in the image and maps this information
  into a feature space.
• It involves two-dimensional search.

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Geometry-based algorithms
Iterative scheme
The slope is seen as a searching variable, and
entropy is used to measure the robustness of
signal. It utilizes the prior information of
platform, and replaces the Radon transform with a
simple integration of image intensity over
azimuth.
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Geometry-based algorithms
Improved Radon transform scheme
This scheme utilizes prior information of
platform, convert the data into binary data, and
conduct two-step Radon transform, which would
make it is sparse and reduce the need to search
the whole feature space .
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Comparisons
It is obviously that they can give the accurate
Doppler ambiguity number. While the speed of the
iterative scheme and the improved Radon transform
scheme is much faster than conventional Radon
transform.
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Doppler rate estimation (Autofocus)
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Doppler rate estimation

• Parametric Method
• Metric-based Autofocus (Entropy/Contrast)
• Map Drift (MD)
• Shift and correlate (SAC)

High contrast scene.

• Nonparametric Method
• Prominent Point Processing (PPP)
• Phase Gradient Autofocus (PGA)
• Time-Frequency Analysis

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Doppler rate estimation

• Low contrast scene
• Low SNR
• Without prominent point
• What can we do ?

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Estimation Scheme
According to the fact that Doppler rate is
inversely proportional to range,
It indicates that the Doppler rate in low-
contrast scene can be estimated indirectly.
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Estimation Flowchart
Doppler rate in high-contrast scene is estimated
first using Radon-Wigner transform, and then
using the fact that Doppler rate is inversely
proportional to range, Doppler rate in
low-contrast scene is estimated indirectly.
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Results
High Contrast Scene With Prominent Point
Time frequency analysis
Radon transform


Low Contrast Scene Without Prominent Point
Time frequency analysis
Radon transform
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Conclusions

• The geometry-based scheme can be used to
  resolve
• Doppler ambiguity of SAR with large
  migration.
• The speed of improved Radon transform scheme
  and
• the iterative scheme is about ten times
  faster than
• conventional Radon transform scheme.
• An indirect estimation scheme of Doppler rate
  for low
• contrast scene is developed.
• The indirect scheme is effective, and it can
  reduce
• computation time greatly.

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Thank you for your listening!