Programing languages for a ray tracing and for generation of
an echo signal were Maxscript and Matlab language,
respectively.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
Hardware OS: Win7 Professional. CPU: core 17,
RAM:24GB, GPU:GeForce275 x 2
Software 3ds Max 2011 x64 (3DCG space)
(Autodesk)
natFX (Bionatics) (Forest (tree)
modeling)
Maxscript (Autodesk) (Ray tracing)
Matlab 2010b (Echo signal
(Mathworks) simulation)
Table 1. The development environment
4. MODEL DEVELOPMENT
The developed simulator considers the following features: the
sensor configurations, the laser beam definitions, the forest and
terrain objects, the echo signal generation and the visualization
of intersections.
4.1 Sensor configuration
The illumination angle, the footprint shape and wavelength of
laser beam as the sensor configuration were considered. The
illumination angle was variable. The footprint shape above the
target was defined as a round with specific diameter. The
illumination angle was rotated to Y axis. Therefore, the
footprint shape on the ground depended on both the illumination
angle of the laser beam and the shape of terrain surface. In this
study, the beam divergence wasn't considered. The effect of
wavelength of laser beam was considered as reflectance of the
target at a specific wavelength.
4.2 Laser beam definition
Laser beam was considered in two features. One was spatial
feature, the other was time feature. In point of view of spatial
feature, a laser beam was defined as multiple sub laser beams.
Spatial density of sub laser beams was 1/0.09m? as default. In
order to decide the value of spatial density, gap distributions
were examined using a tree model of full polygons before. As a
result, this value was reasonable in point of view of uniformly
hitting sub laser beams to objects. Moreover, the intensity on
cross section of laser beam also was considered. The intensity
of the edge of footprint was defined as 1/e? (TEM), since an
actual distribution of intensity on cross section of laser beam
was Gaussian distribution. On the other hand, in point of view
of time feature, sub laser beams were defined as multiple
particles based on the sampling rate and the pulse width. Each
particle had different value of intensity based on an actual pulse
intensity.
4.3 Forest and terrain objects
The forest consisted of several trees, and these trees created
by the plant growth model (natFX). Each tree was full polygons
in order to avoid the effect of a gap function. Each tree was
created by initial parameters which are species and planting
year. If tree shape is needed to fit an actual shape, its shape is
able to be modified to any shape in 3DCG software. On the
other hand, distribution of trees was able to be located based on
an ideal condition or an actual condition by manual.
There are two ways to deal with terrain data in the developed
simulator. One way is automatically generating a plane object
under forest object as the ideal condition. The other way is
importing an external DEM data of DXF format.
Furthermore, each object had a specific reflectance derived
from literatures or actual hyperspectral data.
4.4 Echo signal generation
Echo signal was generated by synthesizing wave propagation
of each sub laser beam. At first, multiple cells were generated
by using number of the sampling and a range of each cell had
the distance calculated by light speed and the sampling rate.
Secondly, the start position of cell number corresponding to
particles of sub laser beam was calculated by using initial
positions of sub laser beams and its intersections. And the
intensity at each cell was calculated by using both the intensity
of particles and the reflectance of target. The echo signal was
generated by the summation of each value at cells.
4.5 Visualization of intersections
In order to understand the interaction between sub laser
beams and targets, the developed simulator implemented
visualization function. The position the particle with highest
intensity hit against was visualized to understand the
interactions between sub laser beam and targets.
5. SIMULATION METHODOLOGY
The echo signal was simulated by the following two steps:
(1) the calculation of intersections and (2) the generation of
echo signal.
5.1 Input and output parameters
In order to calculate intersections, initial input variables were
the diameter of footprint (m), the illumination angle (degree
against Y axis) and the Z value at the center of the forest object
(m). The illumination angle defined the direction vector of laser
beam. The diameter of footprint and the illumination angle
defined the initial the position of each sub laser beams. The Z
value defined an adjustment value to fix a viewpoint, in order to
compare results from any simulation at several illumination
angles. Output parameters in this procedure were the
followings: the initial position of each sub laser beam,
intersections between each sub laser beam and the target, the
intensity of each sub laser beam and a flag of target type. Next,
in order to generate an echo signal, input variables were the
sampling rate and the reflectance of target based on the flag.
Output parameters in this procedure were a csv file of echo
signal and display of a graph of png format.
5.2 Simulation flow
The echo signal was generated by the following steps. The
flow chart was a case of using a plane object as the ground.
-Calculation of intersections-
(1) Creation of the forest object using natFX in 3DCG software.
(2) Input of the diameter of footprint size, the illumination angle
and the Z value at the center of the forest object, as initial
parameters.
(3) Generation of initial points of sub laser beams with in the
footprint area.
(4) Calculation of the intensity of each initial point based on
TEMoo- si
(6) Rotation of initial points to Y axis using the illumination
angle and moving it to above the forest object.
(7) Creation of the ground plane object under the forest object.
(8)
(9)1
( 10
- Ge
(1)
|
(3)
(4)
(5) |
6):
7)
perf
simi
pe
size
0.5%
