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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:

Proceedings of IGARSS 08, Boston, USA

[2] A. Reigber, A. Moreira, First demonstration of airborne SAR

tomography using multibaseline L-band data, IEEE TGARS,

2000, 38, 2142-2152

[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

Environment Using TerraSAR-X Data, Proceedings of CEOS

SAR Workshop on Calibration and Validation, 2008

[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

IGARSS 2008, 5, V -518-V -521

[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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