合成孔径雷达图像特征

Courtesy of CCRS
Courtesy of CCRS
为什么要削弱斑点噪声
The presence of speckle noise must be considered before further analysis Reduction of speckle noise allow:
第三章 Characteristics of SAR Images（合成孔径雷达图像特征）
SAR Imaging - Slant Range Projection （斜距投影）
Antenna 像平面
e
fg
R H
Nadir
E
F G
Y
Across track
The location of pixels in the image plane is dependent on the slant ranges between the antenna and the ground objects 地物目标在像平面上的位置取决于目标与雷达天线之间的距离
(after ESA)
SAR Image Expression （SAR图像表达）
Each pixel is represented by a complex value, i.e.,
ω = a +eflectivity (intensity) of the imaged patch
PDF(φ) = ∫ PDF( A, φ)dA =
0 ∞
1 , −π ≤ φ < π 2π
(Franceschetti & Lanari, 1999)
Example: Histogram of SAR Data （直方图）
6000
6000
Pixel count
4000
Pixel count
-2 0 2 Imaginary component (Im) 4
Degrade spatial resolution
Averaging adjacent pixels （相邻像素平均）
Degrade spatial resolution
Speckle filtering（斑点滤波）
Not sacrificing the resolution （不牺牲空间 分辨率） The adoption of classical enhancement tools developed for imagery from optical sensors （可以借鉴经典的方法）
光学与SAR成像差异，地形起伏引起的位移方向相反 (Copyright, CCRS)
Incident Angle（入射角）
(Nominal) Incident angle (名义入射角)
--angle between the radar illumination and the normal to the ground surface. （雷达入射方向和地面法线的夹角） -- the incident angle increases from the near range to the far range （雷达图像的近距到远距，入射角增大）
Example of radar shadow effects under large incidence angle (>45°) illumination. (Copyright, CCRS)
Diagram of Layover （叠影）
目标上部的回波比底部 的回波先到达接收天线 目标的顶部发生位移， 相对与底部倒置 叠影是透视收缩的极端 情况 名义入射角小时，叠影 更显著
4000
2000
2000
0 -4
0 -4
-2 0 2 Real component (Re)
4
From test data set over Yuan Long, Hong Kong
Example: Histogram of SAR Data （直方图）
4000 3000
Pixel count 5000 4000 3000 2000 1000 0 -4 -2 0 Phase ( φ ) 2 4
Local incident angle（局部入射角）
-- angle between the radar LOS and the line normal to the local slope. （雷达入射方向与局部坡度法线的夹角）
θloc
αi
θi
Incident angle（入射角） Local incident angle（局部 入射角）
Average multiple amplitude image （多图平均）
Degrade temporal resolution
Separate the maximum synthetic aperture into smaller subapertures, generate independent looks (with reduced resolution) at target areas, register and add the looks （子孔 径叠加）
System parameters State vectors of SAR platform
Amplitude component
Phase component
Statistical Model of SAR Data （SAR数据统计模型）
Real part（实部）
Re2 PDF(Re) = exp − 2 2σ 2 2πσ 1 Im2 PDF(Im = ) exp − 2 2σ 2 2πσ 1
Sketch map of joint contribution（综合贡献示意图）
Fading and speckle（衰退和斑点）
Fading is the variation of amplitude of total radar echo wave （衰退之总雷达回波幅度的变化） Speckle refers the constructive (相长) and destructive(相消) interference of radar echo waves, appearing in the image as bright or dark （斑点指雷 达回波的相长或相消叠加，在影像上表现为明或 暗）
A = a2 + b2
Phase（相位）: proportional to sensor-to-target range
b ψ = tan ∈(-π, π ] a
−1
Example: SAR Image（SAR图像）
Range
Complex image
Azimuth Azimuth
Pixel count
2000 1000 0
0
2 4 Amplitude (A)
6
Content of a SAR Image File（SAR图像内容）
Key parameters: (example) Header
Scene identification: ORBIT 18795, DATE 24-NOV-1998, 2:54:44 Scene location: FRAME 3159, LAT 22.24, LON 114.34 Pulse_repetition_frequency (actual, Hz): 1679.9020000 Total_azimuth_band_width (Hz): 1378.0000000 Weighting_azimuth: HAMMING Xtrack_f_DC_constant (Hz, early edge): -117.6290000 Xtrack_f_DC_linear (Hz/s, early edge): 444430.0000000 Xtrack_f_DC_quadratic (Hz/s/s, early edge): -2016000000.0000 Range_time_to_first_pixel (2way) (ms): 5.5272710 Range_sampling_rate (leader file, MHz): 18.9624680 Total_range_band_width (MHz): 15.5500000 Weighting_range: HAMMING Radar_wavelength (m): 0.0566660 First_pixel_azimuth_time (UTC): 24-NOV-1998 02:54:44.888 First_pixel_slant_range (m): 828424 Pixel_spacing (m): 7.904000 Line_spacing (m): 3.988000 Projection mode: Zero-Doppler
θi
layover
Exercise: 区分几何形变?
Understanding geometric distortions from local incident angle 理解几何形变
强 叠影
弱
强 透视收缩
弱
弱 阴影 900
强
00 局部入射角
(Courtesy: CCRS)
ERS-2 image, incidence angles between 24°to 26°
Fading（衰退） Speckle（斑点）
Fading and speckle（衰退和斑点）
Fading and speckle are the inherent “noise-like” processes in a coherent imaging system （衰退和斑 点是相干成像系统固有的噪声过程） Radar total echo wave in a resolution cell is the joint contribution of echo wave from different small scatters within the resolution cell.（分辨单元的雷 达回波是分辨单元内小散射体回波的综合贡献）