COMPARISON OF GEOMETRY-BASED DOPPLER AMBIGUITY RESOLVER IN SQUINT SAR & Wenchao Li University of Electronic Science and Technology of China AN INDIRECT DOPPLER RATE ESTIMATION SCHEME OF SAR IN LOW-CONTRAST SCENE
Introduction Synthetic Aperture Radar (SAR) An important remote sensing tool.
Introduction Squint SAR Geometry model Squint angle
Introduction Squint SAR Range History ：
The linear term is range walk
The quadratic term is range curvature
Doppler centroid ( )
Doppler rate ( )
After range walk correction, the azimuth data can be modeled as a linear frequency modulated (LFM) signal, Signal model
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.
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.
Introduction Therefore, Doppler parameter Estimation from echo data is an essential procedure for high quality SAR processing
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 .
Doppler centroid estimation
Wavelength diversity algorithm (WDA)
Multilook algorithms (MLCC,MLBF)
Geometry-based algorithms 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
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.
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. 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 . Geometry-based algorithms
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.
Doppler rate estimation (Autofocus)
Metric-based Autofocus (Entropy/Contrast)
Map Drift (MD)
Shift and correlate (SAC)
Doppler rate estimation
Prominent Point Processing (PPP)
Phase Gradient Autofocus (PGA)
High contrast scene.
Low contrast scene
Without prominent point
What can we do ?
Doppler rate estimation
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.
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.
Results High Contrast Scene With Prominent Point Time frequency analysis Radon transform Low Contrast Scene Without Prominent Point Time frequency analysis Radon transform
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