In Fig.4 (a) and Fig.5 (a), the cross-polarized intensity of Max(R_hh) is very low compared with the co-polarization intensity. As expected, Max(R_hh) exhibited co- and cross-polarized signature shapes similar to those of the simple backscattering model from an object with a horizontal component. Hence, it is assumed that the area near Max(R_hh) must be the scattering object to the radar illumination direction. Therefore, it is suggested that the area is a residential one where buildings are horizontal with respect to the radar illumination direction. In Fig.4 (b) and Fig.5 (b), the entire polarized intensity of Max(R_vh) is very low compared with those of Max(R_hh) and Max(R_vv). Max(R_vh) exhibited co- and cross-polarized signature shapes similar to those of the simple backscattering model from an object with a 45 degree inclined corner reflector component. In Fig.4 (c) and Fig.5 (c), Max(R_vv) exhibited co- and cross-polarized signature shapes similar to those of the simple backscattering model from an object with a vertical component. Since the R_vv value is high for tall buildings in commercial areas, it is suggested that VV polarization intensity has a relation with the building height.

Fig.4 Relationship between HH, VH, and VV polarization intensities for selected areas and maximum received intensities which are computed from co- and cross-polarizations

Co-polarization	Cross-polarization
(a) Max(Rhh) area
(b) Max(Rvh) area
(c) Max(Rvv) area
Fig.5 Co- and cross-polarization signatures for three areas

Comparison with Field Survey and Aerial Photographs
The characteristics of the building in each area extracted from Fig.3 were confirmed by the results of the field survey, and those of surrounding structures in urban areas were confirmed based on aerial photographs. The field survey was carried out in August 1999. The aerial photographs were mostly taken with SAR images at the same acquisitions (October-November, 1997). The actual structure at the Max(R_hh) area was found to be a wooden house with is a large wall almost horizontal to the radar illumination direction due to the field survey results. The surroundings were confirmed to be a highly dense residential district by the aerial photographs (Fig.6 (a)). The actual structure at the area of Max(R_vh) was confirmed to be a low-rise building built at a 45 degree inclination to the radar illumination direction based on the field survey results. It is confirmed from the aerial photograph that this area is a residential district of similar environment to that of the area with Max(R_vh) (Fig.6 (b)). The actual structure in the area Max(R_vv) is a tall and slender high-rise building compared with the surrounding buildings. On the basis of the aerial photograph (Fig.6 (c)), it is confirmed that this area is commercial district unique to metropolises.

(a) Area A around Max(Rhh)
(b) Area B around Max(Rvh)	(c) Area C around Max(Rvv)

Fig.6 Aerial photographs of the surroundings of selected areas

(© Nakanihon Air Service Co., Ltd.)

Conclusion
In this paper the relationship between the structure of urban areas and the backscattering characteristics was discussed using airborne SAR full-polarized images taken in the Tokyo Metropolitan area. As a result, it was suggested that the predominant polarization intensity differed with the height, form, and alignment direction of the buildings. Future study will be carried out to investigate the potential of polarimetric characteristics from airborne SAR for the evaluation of the seismic vulnerability of structures in urban areas.

References

• Communications Research Laboratory, and National Space Development Agency of Japan (1998). CRL/NASDA Airborne Synthetic Aperture Radar.
• Yamaguchi, Y. (1998), Fundamentals of Polarimetric Radar and its Applications, Realize Inc (in Japanese).
• Van Zyl, J.J., H.A. Zebker, and C. Elachi (1987), Imaging Radar Polarization Signatures: Theory and Observation, Radio Science, 22(4), pp.529-543. Radar illumination direction
• Zebker, H.A., J.J. Van Zyl, and D.N. Held (1987), Imaging Radar Polarimeter from Wave Synthesis, Journal of Geophysical Research, 92(B1), pp.638-701.