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COMPARISON OF GEOMETRY-BASED DOPPLER COMPARISON OF GEOMETRY-BASED DOPPLER
AMBIGUITY RESOLVER IN SQUINT SARAMBIGUITY RESOLVER IN SQUINT SAR
&&
Wenchao LiWenchao Li
University of Electronic Science and Technology of ChinaUniversity of Electronic Science and Technology of China
AN INDIRECT DOPPLER RATE ESTIMATION AN INDIRECT DOPPLER RATE ESTIMATION
SCHEME OF SAR IN LOW-CONTRAST SCENESCHEME OF SAR IN LOW-CONTRAST SCENE
IntroductionIntroduction
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
……
IntroductionIntroduction
Squint SAR
z
y
O
BR
v
x
P
Geometry model
Squint angle
IntroductionIntroduction
Squint SAR
2 22
0 0 00
cos( ) ( ) sin ( ) ( )
2 ( )
vR t R t v t t t t
R t
Range History:
The linear term is range walk
The quadratic term is range
curvature
IntroductionIntroduction
21( ) exp[ 2 ( )]
2dc drs t j f t f t
• Doppler centroid ( )
• Doppler rate ( )drfdcf
After range walk correction, the azimuth data can be modeled as a linear frequency modulated (LFM) signal,
Signal model
IntroductionIntroduction
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.
2 22 cosdr
vf
R
2sindc
vf
IntroductionIntroduction
An incorrect Doppler centroid • a loss of SNR;
• an increase in the azimuth ambiguity level;
• a shift in the location of the target; …...
Incorrect Doppler rate Correct Doppler rate
An incorrect Doppler rate would blur the image.
IntroductionIntroduction
Therefore, Doppler parameter Estimation from echo data is an essential procedure for high quality SAR processing
Doppler centroid estimation(Resolve Doppler centroid ambiguity)
Doppler centroid estimationDoppler 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 .
,dc amb dc basef M PRF f
Doppler centroid estimationDoppler centroid estimation
• Multiple PRF
• Wavelength diversity algorithm (WDA)
• Multilook algorithms (MLCC,MLBF)
• Geometry-based algorithms
Geometry-based algorithmsGeometry-based algorithms
0
2 ( ) 2dc
t t
dR tf K
dt
The relationship between Doppler centroid and
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.
Then Doppler centroid can be computed via
estimating the range walk slope.
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.
Geometry-based algorithmsGeometry-based algorithms
Range compressed data
Calculate the range walk slope
Output the Doppler centroid
Radon transform
Initialize the range walk slope
Integrate energy over azimuth Update the range walk slope
Range compressed data
Output the corresponding range walk slope
Output Doppler centroid
Compute waveform entropy
Has minimum entropy been obtained ?
No
Yes
Range walk correction
Iterative schemeIterative schemeThe 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.
Geometry-based algorithmsGeometry-based algorithms
Range compressed data
Edge detection
Calculate the range walk slope
Output the Doppler centroid
Set transform region and step size, and do the coarse Radon transform
Update the transform region and step size, and do the fine Radon transform
Improved Radon transform schemeImproved 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 .
Geometry-based algorithmsGeometry-based algorithms
ComparisonsComparisonsCOMPARISONS FOR DIFFERENT DOPPLER AMBIGUITY NUMBER
Geometry-based
Doppler resolver
True
value
Estimated
value
Computation
time
Conventional Radon
transform 6 6 14.57 s
Iterative scheme 6 6 1.48 s
Improved Radon
transform scheme 6 6 0.62 s
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.
Doppler rate estimation(Autofocus)
Parametric Method • Metric-based Autofocus
(Entropy/Contrast)• Map Drift (MD)• Shift and correlate (SAC)
Doppler rate estimationDoppler rate estimation
Nonparametric Method • Prominent Point Processing (PPP)• Phase Gradient Autofocus (PGA)• Time-Frequency Analysis
High contrast scene.
Low contrast scene
• Low SNR
• Without prominent point
What can we do ?
Doppler rate estimationDoppler rate estimation
Estimation SchemeEstimation Scheme
2 2
11
2 cosdr
vf
R
2 2
22
2 cosdr
vf
R
12 1
2dr dr
Rf f
R
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.
Range compressed data
Choose the range bin that contains bright line
Radon transform
Output the Doppler rate of the range bin
Wigner-Ville distribution
Fit the Doppler rate of the range bin in low-contrast scene
Range walk correction
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.
ResultsResults
High Contrast Scene With Prominent Point
Time frequency analysis Radon transform
Low Contrast Scene
Without Prominent Point
Time frequency analysis Radon transform
ConclusionsConclusions
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.
Thank you for your listening!Thank you for your listening!
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