Abstract
In this study, we evaluated the relationship between the structure of an urban area and the backscattering characteristics using the CRL/NASDA airborne synthetic aperture radar (PI-SAR) which observed the Tokyo Metropolitan area, for the purpose of developing building inventory and grasping the elements of seismic risk. The specific buildings were selected from areas which show predominant intensities of HH, VH and VV polarizations. The polarization characteristics of these areas were examined from co- and cross polarization signatures computed from the PI-SAR data and were confirmed by field surveys and aerial photographs.

Introduction
The risk of seismic damage in urban areas is closely related to the structures and materials of buildings and distance between them. Hence it is important to study the plan, elevation and structure of buildings in urban areas in order to evaluate seismic vulnerability. This evaluation is very important for the prediction of human casualties, for the mitigation of disaster. Moreover, it is effective for the recovery and reconstruction of society. Inventory of structures in urban areas can be obtained by a field survey. However, a large amount of time and effort is required to do so. Remote sensing by aircraft is one of the most promising technologies for monitoring large areas covering the earth's surface. Airborne survey methods can be an effective solution since they can provide spatial information about a large area quickly and easily. Airborne SAR (Synthetic Aperture Radar) provides high-resolution images, which can be used for identifying individual buildings. The airborne SAR is able to provide full-polarization information. The complete polarization characteristics are obtained by the analysis of polarimetoric SAR images. The polarization characteristics are highly suitable for the identification of detailed surface conditions of objects because they differ according to factors such as building materials and the density of city blocks. If we can identify the urban structures on the basis of their areas from the polarization characteristics, the results can be used in the seismic vulnerability assessment. This study investigated backscattering signatures from the polarized characteristics of man-made objects such as low-rise buildings in a high-density area and tall buildings in a commercial area.

Airborne SAR images
The Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications of Japan and the National Space Development Agency of Japan (NASDA) have developed in collaboration an airborne high-resolution multiparameter synthetic aperture radar (Polarimetric and Interferometric SAR: PI-SAR). Mounted on an airplane, this synthetic aperture radar is a dual-frequency radar operating at L-band and X-band frequencies with polarimetric functions. The main characteristics of PI-SAR are listed in Table 1. The PI-SAR was used to observe the Tokyo Metropolitan area on September 30, 1997, and X-band, full polarization data were acquired. In this study, we used the data for a 3km X 3km area in Shinjyuku, Tokyo. The backscattering intensity images of HH and VH polarization are shown in Fig.1. Comparing the backscattering intensity images in HH, HV, VH, and VV polarizations, it was revealed that the intensities of co-polarization (HH, VV) were higher than those of cross-polarization (HV, VH), and HH polarization intensity was the maximum. High-rise buildings are indicated by strong backscattering reflection. Whereas low-rise buildings, forests on parks and ponds are indicated by weak backscattering reflection. Moreover, even in the case of areas which contain similar types of buildings, the predominant polarization differs. Then, a color composite image (red, green and blue are allocated to HH, VH and VV polarized intensity images, respectively) is created. The predominant polarization differs between each super-high-rise building and differs in segments, even within one super-high-rise building.
Table 1 Characteristics of PI-SAR X-band

Frequency	9.55 GHz
Bandwidth	100 MHz
Antenna, Length x Width	1.05 m L x 0.19 m W
Off-nadir Look Angle	10 – 75 deg. (Variable)
Observation mode	2-ch. Polarimetry/ Interferometry	4-ch. Polarimetry	6-ch. Polarimetry& Interferometry
Swath Width (Observed from12,000 m)	19.6/42.5 km	8.2/19.6 km	4.3/11.9 km
Slant Range Resolution	1.5/3 m
Azimuth Resolution (4/8-look)	1.5/3 m

(a) HH polarization



(b) VH polarization

Fig.1 Backscattering intensity images of PI-SAR