smooth curve is fitted through these points. A multiplicative correction factor (C i ) is
then obtained for each range sample by dividing the maximum value on this curve
by the value at a given range sample:
C i ¼ S max =S i for I ¼ 1 . . . ::N
ð14:1Þ
Where Si, is the value on the smooth curve at a specific range sample i and S max is
the maximum value encountered. Thus, a corrected data value V, for this range
sample may be obtained by
V
0
i ¼ C i V i with V i being the uncorrected data value
ð14:2Þ
Sophisticated SAR systems use a Sensitivity Time Control (STC) function to
accommodate large variations of range focused radar returns over a uniform
surface. If the antenna pattern and the terrain type are well known, a STC function
can be applied to incoming signals. The STC function has the effect that systematic
variations in the processed image intensity (Pirasteh et al. 2010) in range is at a
minimum. The new CCRS C- and X-band SARs, for example, offer five choices of
STC functions: ‘test’, ‘land’, ‘smooth water’, ‘rough water’ and ‘ice’. The ‘test’-mode corresponds to an STC setting of 1, i.e. no modification of the range focused
return signal. For the other modes nominal reflectance laws are modeled for each
respective surface type. These models are then applied, together with the appropriate antenna pattern model, platform altitude and swath mode, to correct for systematic radiometric variations.
14.4 Geometric Correction of Digital SAR Imagery
In order to obtain a high degree of accuracy in the position of surface features in a
SAR image, geometric correction algorithms are required to compensate for geometric distortions through image processing. Geometric distortions may be introduced internally by the SAR system itself. They are related to the inherent slantrange viewing geometry. External factors responsible for geometric distortions
include changes in platform velocity, earth rotation, in the case of spaceborne
SAR, and the map projection of the output imagery.
14.4.1 Internal Geometric Distortions
Recall that the natural coordinate system of the side-looking imaging radar is the
slant-range plane along which the distance of an object relative to the SAR is
defined. However, for image interpretation and reasons of geometric fidelity it is
more desirable to measure the distance of objects from the ground- or nadir track of
286
S. Pirasteh et al.
then obtained for each range sample by dividing the maximum value on this curve
by the value at a given range sample:
C i ¼ S max =S i for I ¼ 1 . . . ::N
ð14:1Þ
Where Si, is the value on the smooth curve at a specific range sample i and S max is
the maximum value encountered. Thus, a corrected data value V, for this range
sample may be obtained by
V
0
i ¼ C i V i with V i being the uncorrected data value
ð14:2Þ
Sophisticated SAR systems use a Sensitivity Time Control (STC) function to
accommodate large variations of range focused radar returns over a uniform
surface. If the antenna pattern and the terrain type are well known, a STC function
can be applied to incoming signals. The STC function has the effect that systematic
variations in the processed image intensity (Pirasteh et al. 2010) in range is at a
minimum. The new CCRS C- and X-band SARs, for example, offer five choices of
STC functions: ‘test’, ‘land’, ‘smooth water’, ‘rough water’ and ‘ice’. The ‘test’-mode corresponds to an STC setting of 1, i.e. no modification of the range focused
return signal. For the other modes nominal reflectance laws are modeled for each
respective surface type. These models are then applied, together with the appropriate antenna pattern model, platform altitude and swath mode, to correct for systematic radiometric variations.
14.4 Geometric Correction of Digital SAR Imagery
In order to obtain a high degree of accuracy in the position of surface features in a
SAR image, geometric correction algorithms are required to compensate for geometric distortions through image processing. Geometric distortions may be introduced internally by the SAR system itself. They are related to the inherent slantrange viewing geometry. External factors responsible for geometric distortions
include changes in platform velocity, earth rotation, in the case of spaceborne
SAR, and the map projection of the output imagery.
14.4.1 Internal Geometric Distortions
Recall that the natural coordinate system of the side-looking imaging radar is the
slant-range plane along which the distance of an object relative to the SAR is
defined. However, for image interpretation and reasons of geometric fidelity it is
more desirable to measure the distance of objects from the ground- or nadir track of
286
S. Pirasteh et al.