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3D ANALYSIS OF SCATTERING EFFECTS BASED ON RAY TRACING TECHNIQUES
Stefan Auer1, Xiaoxiang Zhu
1, Stefan Hinz
2, Richard Bamler
1,3
1Remote Sensing Technology, Technische Universitt Mnchen2Photogrammetry and Remote Sensing, Universitt Karlsruhe
3Remote Sensing Technology Institute, German Aerospace Center (DLR), Oberpfaffenhofen
ABSTRACT
The side-looking geometry of SAR sensors hampers the
interpretation of SAR images of urban areas. Simulation
tools for illuminating 3D models of man-made objects by
means of a virtual sensor support the interpretation of
scattering effects by providing artificial images in the
azimuth-range plane. In this paper, a simulation approach is
presented which extends SAR simulation to three
dimensions in order to focus detected intensity contributionsin azimuth, range and elevation. Based on the simulation
output, a concept for creating scatterer histograms
displaying the number of scatterers within one resolution
cell is introduced. Methods for analyzing simulated
elevation data by means of selected slices are presented for
an urban test site. Eventually, the number of scatterers
extracted for a selected pixel by tomographic analysis, using
a stack of spotlight TerraSAR-X images, is confirmed by
results provided by the simulator.
Index TermsSAR Simulation, Ray Tracing, SAR
Tomography, 3D SAR, TerraSAR-X
1. INTRODUCTION
High resolution SAR sensors like TerraSAR-X or Cosmo-
SkyMed provide SAR images of a spatial resolution below
1 meter in spotlight mode. Adjacent scatterers, which were
condensed in the same image pixel of SAR images offering
moderate geometrical resolution, are now well separated
into several resolution cells in the azimuth-range plane.
Hence, image features like corner lines or point scatterers
become more dominant with respect to the background [1]
and more reliable for extracting object parameters, e.g. the
shape or height of buildings. However, interpreting SAR
images of urban scenes is still challenging due to the side-
looking imaging geometry. In particular, lay-over effectshamper the identification, characterization and monitoring
of single objects in urban areas.
While across-track InSAR offers the possibility to
reconstruct 3D surfaces by means of two SAR images
captured from slightly different positions in space, SAR
Tomography [2] [3] [4] makes use of a stack of SAR images
in order to focus radar echoes in azimuth, range and
elevation.
The work presented in this paper deals with 3D localization
of scatterers by means of a SAR tomogram simulator, based
on a given 3D site model, and comparing them with real
space-borne tomographic data. The simulation concept is
based on ray tracing algorithms [5] and enables 3D analysis
of scattering effects in azimuth, range and elevation.
Simulation of artificial SAR images may support the visual
interpretation and offers the possibility to concentrate on theanalysis of deterministic effects. Depending on the quality
of 3D model scenes used for the simulation process and the
reflection models, the position and 3D intensity of focused
scatterers may be reconstructed and used as a-priori
knowledge, for instance, in the case of image analysis based
on parametric models.
2. RAY TRACING FOR 3D ANALYSIS OF
SCATTERERS
Artificial images in SAR geometry as well as results of 3D
analysis presented in this paper are derived by a simulation
tool based on ray tracing algorithms provided by the open-
source software POV Ray. Own developments have been
added to POV Ray's source code in order to provide output
data for creating reflectivity maps in the azimuth-range
plane [6] as well as elevation information for separating
several scatterers located in one resolution cell [7]. The
SAR imaging system is approximated by a cylindrical light
source and an orthographic camera for providing the
possibility to focus intensity contributions backscattered
from 3D models of urban areas in azimuth, range and
elevation. The focus of the simulation approach is on
geometrical correctness while speckle effects are neglected
for the purpose of getting undisturbed reflectivity maps to
be analyzed.
For comparing simulation results with findings fromtomographic analysis, the Wynn hotel in Las Vegas, USA,
was chosen. The urban scene also includes some adjacent
man-made structures surrounding the building complex
(Figure 1). The 3D model for simulation purposes was taken
from the Google Earth database, and a stack of 16
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TerraSAR-X high resolution spotlight images was available
for tomographic analysis with a synthetic aperture length in
elevation of 270 m. Figure 2 shows a SAR image of the
scene where straight and curved linear features as well as
dominant point scatterers can be clearly distinguished.
Signal parts backscattered from the building are overlayed
with contributions from adjacent buildings and vegetation
located in front of the hotel. Surprisingly, the hotel faade is
almost invisible in the SAR image apart from three rows of
point scatterers aligned in range direction, likely caused by
dihedrals or trihedrals at each floor of the building,.
Figure 1: 3D Model of Wynn Hotel, Las Vegas (USA);
Google Earth
For providing artificial images in SAR geometry, both
geometric and radiometric parameters had to be adapted in
the 3D model. Although all buildings seem to be included in
the 3D model, their level of detail is moderate as, e.g.,
building faades are approximated by flat surfaces.
However, interpretation of the layover effect as well as
analysis of dominant scattering effects is still possible. After
adapting the SAR imaging geometry by selection of aspect
angle and incidence angle, the 3D model of the Wynn Hotel
has been illuminated by the virtual SAR sensor and a
reflectivity map is created in the azimuth-range plane
(Figure 2, right) based on the distribution of scatterers
detected while tracing rays through the 3D scene. Analyzing
the image from near range to far range, the layover effect
mentioned above is confirmed and deterministic scattering
effects in front of the hotel are displayed with high contrast.
A strong curved double bounce line is detected at the
bottom of the hotel faade (feature A) followed by fourfold
scattering effects appearing within the shadow area (feature
B).
Figure 2: SAR image vs. simulation; left: high resolution
spotlight TerraSAR-X image; right: simulated reflectivity
map; range: top-down; cross marking pixel selected for
elevation analysis in Section 4; arrows indicating slice in
range; elevation is perpendicular to azimuth-range plane
3. ANALYSIS IN ELEVATION
3.1 Scatterer Histogram
Since elevation coordinates of backscattered signals are
available after ray tracing, scatterers within the same
resolution cell can be separated. Figure 3 illustrates the
histogram in the azimuth-range plane showing the number
of scatterers for the Wynn hotel model. The step width
along both axes of the histogram as well as the image
margins are adapted to the reflectivity map shown in the
right part of Figure 2. According to the elevation resolution
of the current TerraSAR-X stack, the minimum distance in
elevation for distinguishing two scatterers is chosen to be
40.5m. Layover areas are visible in green, yellow and
orange color for two, three and four scatterers, respectively.
Compared to the SAR image displayed in Figure 2, it isinteresting to see that areas of several scatterers do not
necessarily show bright backscattering characteristics. This
is due to the fact that most of the overlayed signals are
single bounce contributions which show lower intensity
than, for instance, focused double bounce lines. In contrast,
double bounce reflection appearing at two surfaces
perpendicular to each other are focused at the same height
in elevation.
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Scatterer histograms (as shown in Figure 3) for multi-body
3D site models may support the interpretation of results
derived from SAR Tomography. On the other hand, such
simulation can be used to include a-priori knowledge into
tomographic analysis based on parametric models.
Figure 3: Scatterer histogram indicating the number of
scatterers for each resolution cell; range: top-down
3.2 Analysis of slices for height determination
Besides scatterer histograms, the developed simulator offers
further tools to analyze elevation information of scattering
effects. Single pixels can be selected for defining slices
oriented in azimuth, range and elevation. Afterwards,
intensity contributions are gathered for each slice according
to the dimensions of the pixel in azimuth and range and the
selected interval in elevation to be displayed.
In the right part of Figure 2 the marked pixel located at the
roof of the Wynn hotel has been selected. The
corresponding slice in range direction is shown in Figure 4.
Height values are calculated with respect to the height level
of the surrounding ground. While single bouncecontributions - indicated by blue color - are distributed all
over the building surfaces and the ground, double bounce
contributions appear at building walls and are focused into
points (green). The zoom into the slice (Figure 5) shows that
double bounce effects are focused to corner lines at the
faade where there are no physical building corners in
reality. This is due to the fact, that signals interacting with
the faade and two flat roofs of the building in front of the
hotel are concentrated at the same position in range and
elevation. Focused triple and fourfold reflections marked in
red and magenta, respectively, are even located within the
hotel since the spatial distance travelled by corresponding
signals was longer than for the double bounce phenomena
mentioned before.
Figure 4: Slice in range; blue: single bounce, green: double
bounce, red: triple bounce, magenta: fourfold bounce
Figure 5: Zoom into slice in range; artificial corners
appearing at the front wall of the hotel; multiple reflection
effects located within the hotel
While slices along the range or azimuth axis only represent
the 3D position of scatterers, slices in elevation offer the
possibility to analyze the intensity distribution within
resolution cells. For creating this kind of slice, simulated
intensities distributed in elevation are imposed with a
regular grid, e.g., of 2m and summed up for each grid cell.
Figure 6 shows the simulated slice displaying thenormalized intensity of three scatterers which have been
assigned to the selected pixel in the reflectivity map of
Figure 2. All intensities are linked to discrete height values
with respect to the height level of the ground surrounding
the hotel and are marked with blue color indicating single
bounce scattering. From left to right, the contributions are
detected at the ground, the faade and the roof of the hotel.
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Figure 6: Simulated elevation of normalized intensity
contributions. Height information is displayed with respectto the ground around the building. Signals are backscattered
at the ground, the faade and the roof of the hotel.
Figure 7: Reflectivity function derived from tomographic
analysis or real data. Three dominant peaks distinguishable
what corresponds to the simulation result shown in Figure 6.
Figure 7 shows a reflectivity function in elevation derivedby tomographic analysis using the above mentioned SAR
images stack for a pixel selected in the same region on the
roof as for the simulation before. Despite the appearance of
strong sidelobes, three main peaks can be distinguished,
which likely occur due to the same backscattering effects
detected by means of the simulation. In the case of the
central peak, a different position in elevation has been
detected. In order to increase the reliability of the
comparison in the future, reflectivity maps will have to be
co-registered more precisely with the stack of SAR images
to ensure the selection of identical pixels in the simulated
reflectivity map and in the real SAR image. Comparison of
the height of intensity peaks indicates that diffuse
backscattering from the ground has been overestimated in
the 3D model scene used for the simulation process.
5. CONCLUSION AND OUTLOOK
In this paper, simulation results for 3D analysis of scattering
phenomena based on ray tracing methods have been
presented. Artificial scattering histograms have been
introduced for supporting the interpretation and analysis of
SAR images displaying urban areas affected by lay-over
effects. Slices in range direction displaying elevation
information of scatterers have shown to be reasonable to
verify the 3D position of physical corner lines. Slices in
elevation enable the separation of signal intensities assigned
to one pixel in the azimuth-range plane. Intensities detected
for a selected pixel in the artificial reflectivity map showedgood correspondance to the reflectivity function derived
from SAR Tomography.
Potentials for the integration of the presented 3D simulation
tool and SAR Tomography will have to be investigated in
the future. Moreover, improved reflection models may
improve the radiometric correctness of the simulated slices
in elevation. Further test sites for tomographic analysis will
have to be chosen in combination with the application of 3D
models of increased complexity to be included into the
simulation process.
7. REFERENCES
[1] N. Adam, M. Eineder, N. Yague-Martinez, R. Bamler, HighResolution Interferometric Stacking with TerraSAR-X, In:
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[3] G. Fornaro, F. Serafino, F. Soldovieri, Three-dimensional
focusing with multipass SAR data, IEEE TGARS, 2003, 41,
507-517
[4] X. Zhu, N. Adam, R. Bamler, First Demonstration of Space-
borne High Resolution SAR Tomography in Urban
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[5] A.S. Glassner, An Introduction to Ray Tracing, Morgan
Kaufmann, 2002, 329
[6] S. Auer; S. Hinz, R. Bamler, Ray Tracing for SimulatingReflection Phenomena in SAR Images, In: Proceedings of
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[7] S. Auer, X. Zhu, S. Hinz, R. Bamler, Ray Tracing and SAR-
Tomography for 3D Analysis of Microwave Scattering at Man-
Made Objects, CMRT 2009. International Archives of
Photogrammetry, Remote Sensing and Spatial Information
Sciences, Vol. 38-3/W4, accepted
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