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Mapping without the sun
Zhang, Jixian

Fused image
2.2 SAR texture extraction
The Lee filter for SAR speckle removal is used to reduce the
effect of the multiplicative noise on homogeneous areas at first.
Thus, the ratio image M of despeckled SARcfe image to its
lowpass approximation SAR L , obtained by the a trous wavelet
algorithm as approximation at level L, is computed. The ratio
image M has unity mean value. Due to imperfect despeckling,
intrinsic signal variability of rough surfaces, and instability of
image ratioing, the spurious fluctuations in homogeneous areas
around the mean value m = 1 appear obviously.
By denoting with of the modulating SAR texture M, a soft thresholding with
threshold 0 = k interval [1 -k cr t ; 1 +k <7 t ] are diminished in modulus by
k (7 t t and retained, while values lying within this interval are
set to the mean value. The resulting texture map M s contains
spatial features which are easily detected by the SAR image. It
can be properly modulated into wavelet planes of the
panchromatic image produced by the a trous wavelet algorithm
to obtain new wavelet planes.
2.3 HPFM (high pass filtering based on modulation) fusion
High pass filtering based on modulation fusion technique aims
at preserving the spectral characteristics of the MS image by
keeping the result independent of the spectral property of the
higher resolution image used for modulation (Pohl and
Genderen, 1998). The HPFM processing formula for the case of
the multi-spectral images sharpening by means of a co
registered high resolution Panchromatic image P is the
j)=MS k (i, j)+ K(i, j) . P„(i, j) (3)
where MS k is the resampled k-th band of the MS image, P H (i,
j) is high frequence details of high resolution Panchromatic
image. In the proposed method, it is replaced by the wavelet
planes of the panchromatic image, produced by a trous wavelet
algorithm, modulated with the SAR texture. K(i, j) is decided
by the following forma
K(i J)= MS k (i, j)/P L (i, j) (4)
where P L (i, j) is low frequence information of high resolution
Panchromatic image, and P(ij)= P k 0 > j)+ Ph (*> j)-
The proposed fusion scheme applied to obtain the fused image
is shown in Figure. 1.
Figure 1 The proposed fusion schedule
The input dataset is composed of an ENVISAT SAR image of
urban area of Wuhan, Landsat-5 TM images (band 3, 4, 5) and a
SPOT Pan image. All the data have been accurately co
registered to the SPOT Pan image, as shown in Figures 2(a), 2(b)
and 2(c). Texture will be blurred in the fusion image if the
accuracy of co-registration among the SAR image, the TM
images and the SPOT Pan image is not high enough. Figure 3 is
the fused images by proposed fusion method. The fused images
are evaluated in visual examination and quantitative analysis.
2(a) ENVISAT SAR image