If you think that the image on the right looks like a city ravaged by fire, you have quite an imagination. In fact, it is a section of Albuquerque, N.M., where life is quite normal. This image is simply the way an interferometric SAR (IFSAR) "sees" an urban region.
      When compared to the image on the left, which is derived from aerial photography, an obvious reaction is that the photo derived product is superior. But what makes this IFSAR result a significant breakthrough is that it can be produced automatically from the source data in a matter of minutes. It requires several days to manually produce the height map from aerial photography.
      An IFSAR is a radar system that utilizes two standard antennas oriented to form a baseline orthogonal to the platform flight direction. Using this configuration the radar data can be processed so that each element in the resultant image contains information on the local topography. The IFSAR data shown above was produced by Sandia National Laboratories using their airborne Ku-band IFSAR system. It is approximately 1 meter horizontal resolution with better than 0.5 meter vertical accuracy.
      Sandia is interested in a variety of applications for high resolution interferometric SAR, one of which is the extraction of building heights and their footprints from IFSAR data. Ultimately this capability could provide a low cost solution to populate GIS databases for urban planning. Other applications include construction of electromagnetic interference models for siting of cellular telephone relay stations, military applications such as reconnaissance and targeting, and the creation of thematic maps for commercial applications.
      Since Vexcel is experienced in both traditional optical mapping techniques and in radar mapping, Sandia National Laboratory contracted the Boulder, Colo. company to support their study. Specifically, Vexcel is building high-resolution optically derived models, as shown above, for truth data sets to assist Sandia in the validation of their high-resolution IFSAR systems. These models are accurate to better than 6 inches. Additionally, Vexcel is researching new techniques to improve the usability of IFSAR derived elevation models in urban regions.
      While considerable knowledge and experience has been accumulated using IFSAR over open terrain, urban settings present a special challenge. There is multiple scattering of the radar wave between buildings; a layover effect where the steep slope of the building walls is compressed into a few pixels; and shadows where no information is available. These are just some of the challenges inherent in the use of radar in an urban environment.
      Vexcel is exploring the use of an IFSAR simulator to create an image that replicates the IFSAR measurement. If a building geometry can be found that exactly reconstructs the IFSAR image then this model should closely represent the structural characteristics of the city buildings.

About the Author:
Richard Carande is director of radar programs for Vexcel Corp. in Boulder, Colo. He is currently working in the area of interferometric synthetic aperture radar for use in applications such as building identification, elevation determination and automated mapmaking. He may be reached at 303-444-0094(phone) or 303-444-0470 (fax).

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