2.2.2 Methods of Computer Simulation in Remote
Sensing
Computer simulation was applied long
before in calculating atmosphere radiative
transfer function and determining the point
distributed function of ground reflecting
through atmosphere. When RT function was
introduced into the research of vegetation
reflect, Monte Carlo was introduced naturally
into the scope at the same time (Li, 1995).
We found the irrationality of RT function
using computer simulation in vegetation
reflection soon, so the computer simulation
developed to structure real model simulation
making use of strong graphics function of
computer. Structure real model includes two
parts: one is to simulate realistic structure of
ground objects, the other is to simulate the
interaction between microwave and ground
objects
Presently, there are four methods to simulate
realistic structure of ground objects: (1)fractal
method(Asrar, et al, 1991); (2) methods based
on special graphics software warp. For example,
DIANA model based on computer graphics
(Goel et al, 1991,1999; Qin, et al 1995, 1999).
In additional, “Rayshade” graphics software
package (Borel, C et al, 1991,1999); (3) Method
based on the vegetation growth model (Dauzat,
et al, 1991); (4) Method based on
photogrammetry and the vegetation growth
mechanism model (Barnsley, et al,1991).
2.2.3 Application Status
Structure real model can almost mix the false
with the genuine. When Prof. Goel showed crop
land and orchard made by his DIANA model
using videotape, audience thought it be a
videotape of real scene. When using Monte Carlo
method acting on fractal tree to gain
s0, Prof.
Sarabandi and Lin in Michigan University also
have obtained good effects (Lin Y., 1999).
Both Institute of Remote Sensing Application
of Chinese Academy of Sciences and Institute of
Remote Sensing in Peking University have
acquired some results in these aspects. So we can
say that computer simulation at the scattering
characteristic of ground objects has established
good basement for utilizing computer 3D
simulation SAR system.
2.3 SAR Image Simulation
Firstly, we explain the process of radar image
simulation: the simulation of radar image is a
technique which is based upon the ground real
situation.map or remote sensing materials or the
existed radar image according to the imaging
mechanism of radar image to produce different
frequency.wavelength.polarimetric ways radar
images which are applied to various occasion.
The selected physical model used in radar
image simulation is point scattering model. The
selected mathematics models are respectively
the radar imaging equation, the radar equation
and the gray equation. These functions will be
presented in section 3.
The data sources for SAR simulation image
include radar system parameter.database of
geography information and database of ground
scattering characteristic. The database of
geography information includes DEM and
database of land use. The database of ground
scattering characteristic includes diversified
ground objects scattering characteristics at
different time and different radar parameters,
that is,
s or
s0.
When radar imaging, the ground is divided
into grid cells according to the size of resolution
cell. Each cell of image pixel is corresponding to
each ground cell. Based on the imaging
mechanism of radar image and scattering
characteristic of ground objects, we make use of
the radar equation and the gray equation to
calculate the gray value of ground object of each
cell according to point scattering model, and
then form radar image. For gaining a better radar
image, we should consider the geometry
characteristics of radar image, for examples,
foreshortening and layover and etc(Shu,1997).
Though radar image simulation is of
embryonic form of computer simulation SAR
system which this paper proposed, it is only
static process based on two databases existed,
and can’t realize transmitting and receiving
analog signals in real-time.dynamic way. A
great gap has existed between its function and
the final goal of computer 3Dsimulation
integrated system we have proposed in the
introduction.
3 Simulation of SAR system imaging
mechanism
If we simulate the work status of real SAR
sensor using computer, we should firstly
analyze on the characteristics and imaging
mechanism of real SAR system, and on the
basis of this we can set up the signal
transferring model of SAR imaging simulation
system.
3.1 Principle of real SAR system
The principle of Synthetic Aperture Radar can
be explained from the three viewpoints of
synthetic aperture, frequency analysis and
microwave holography. In this paper, we mainly
adopt the view of synthetic aperture to explain it
(Lou, 1997). It can be generalized as follows:
through this method of the phase compensation
of the signals received by the same antenna at
different times and at different positions, we
could form the equivalent large antenna array for
the ground objects with radar going forward. The
signals for the same object received by the
antenna of synthetic aperture do not obtain at the
same time, which need to record the amplitude
and phase of a signal echo at each position.
Because the distance between the aerocraft and
ground objects is different, the phase and
intensity of each signal is also different. So, a
coherent image can be obtained in this way,
which could be reverted to the practical image of
ground objects only if after processed. In the real
SAR system, the emphasis is to research on the
radar imaging system. It is also the main part of
our research work.
3.2 Imaging mechanism model for SAR
simulation system
The physical model that is used for simulating
radar image is the point scatter
model:
Sr(R,x)=s(R,x)h1 h2(R,x)
where
s(R,x)=1,
The functions h
1(R,x) and h
2(R,x) are respectively the range impulse
response function and the azimuth impulse
response function.
The selected mathematics model equations are
respectively the radar imaging equation, the radar
equation and the gray equation. The radar imaging
equation is:
where (X
p, Y
p, Z
p) and
(X
s, Y
s, Z
s) are the ground
coordinates of the ground point P and the side-looking
radar's antenna S.
(V
x, V
y, V
z) is the
speed vector.
The radar equation for point objects is:
The radar equation for area objects is:
where G(
q) is the antenna plus;
l is the
wavelength;
s is the efficacious scattering
section;
s0
is the average scattering coefficient;
W
t is the transmitting power of SAR transmitter;
R is the distance between ground objects and
antenna; A is the SAR irradiation area.
The radar gray-level equation is:
where
g is the transferring function of film; R
n
is the Guass normalized stochastic variable; N is
the independent sample number; k is constant; M
is the transferring function of the receiver;
4 Image processing and information
retrieval
As described earlier, we have explained that
the direct goal of computer simulation SAR
system is to obtain the SAR image. Next what
we need to do is to process the SAR image and
then extract information. Image processing is
convenient for information extraction, and
information extraction is used for extracting the
interested information from the SAR image, for
instance geological structure, crop yield
estimation and etc., and directly severing
application. Therefore, the image processing and
information retrieval are also the important part
in the computer simulation SAR system.
Because the simulating model is an ideal model,
the system error and stochastic error derived
from radar process would not be considered
here.
This part of content has been described in
many books relating to remote sensing image
processing, so it would not be introduced at
large here. Presently, the most of functions and
methods of image processing and information
extraction can be performed well by PCI or
ERDAS, which are popular business software
package. Of course, it can also develop
independent software module serving for special
purpose.
