4.5 Image Reconstruction of SAR

4.5 Image Reconstruction of SAR

Raw data from SAR are records of backscattering in time sequence which are returned from the ground targets. Return signal from a point P is recorded in the expanded range in the range direction which is identical to the pulse width. In addition, the return signal from a point P is also expanded in the azimuth direction because the point P continues to be radiated by microwave pulses during the flight motion(see Figure 4.5.1).

Data processing to generate an image of gray tone corresponding to the backscattering intensity of each point on the ground is called image reconstruction of synthetic aperture radar (SAR). Figure 4.5.3 shows the flow of image construction of SAR.

The image reconstruction is divided into range compression and azimuth compression which make compression of expanded signals in both range and azimuth directions into a point signal. The compression is usually carried out by adopting Fourier transformation to achieve convolution of received signals and a reference function.

The reference function of range compression is the complex conjugate of the transmitted signal, while the reference of azimuth compression is a complex conjugate of the modulated signal by chirp modulation.

The slant range to a point on the ground is expressed as the quadratic function of time with respect to the movement of the platform. The change of the slant range is called range migration. The first order term is called range walk resulting from the earth rotation, while the second order term is called range curvature.

The range migration correction is to relocate the quadratic distribution (see Figure 4.5.4 (c)), in which range walk and range curvature may be separately processed.

In image reconstruction, there is a major problem, called speckle which is due to high frequency noise, as seen in the example of Figure 4.5.2. In order to reduce the speckle, mulch-look processing is applied in which range compression and azimuth compression with respect to subdivided frequency domains are independently overlaid three or four times termed the number of looks. Sometimes a median filter or local averaging may be applied to reduce the speckle. The speckle will be reduced by the square root of the number of looks, although the spatial resolution declines in proportion to the number of looks.