Scientists in China are using space radar images to locate and study two generations of the Great Wall of China that have been eroded and buried in places by centuries of blowing sand.
"In the images we can recognize two different dynasties that built the Great Wall. One was built in the Ming Dynasty and is about 600 years old. The other was built during the Sui Dynasty and is more than 1,000 years old," said Dr. Guo Huadong, a Spaceborne Imaging Radar C/X-band Synthetic Aperture Radar (SIR- C/X-SAR) science team member from the Institute of Remote Sensing Applications at the Chinese Academy of Sciences in Beijing.
The radar images were taken by the SIR-C/X-SAR instrument that flew on Space Shuttle Endeavour in April and October of 1994.
The Great Wall is one of several archeological sites being studied through the use of the space radar images. Other sites include Angkor, Cambodia, the Lost City of Ubar in Oman and the Silk Road along the desert of northwestern China.
"Archeology wasn't one of our original science objectives, but imaging radar data have been found to be very useful for this type of research. It's an exciting spin-off," said Dr. Diane Evans, the SIR-C project scientist at NASA's Jet Propulsion Laboratory.
Evans is discussing the radar's role in archeological research this week at a symposium at the University of Florida in Gainesville, organized by the World Monuments Fund, the Royal Angkor Foundation and the university, with support from the J.M. Kaplan Fund and the Samuel H. Kress Foundation.
The Great Wall of China dates back to the third century B.C., when it was built to protect the country from northern invaders. The wall, which spans more than 3,000 kilometers (1,860 miles), has been periodically rebuilt and modified throughout history by each reigning dynasty.
The scientists are studying a segment of the wall about 700 kilometers (430 miles) west of Beijing in a remote region of the north-central China desert. The most recent version of the wall was built by the Ming Dynasty during the 14th century and it is clearly visible both on the ground and in the radar data. An older version, built during the Sui Dynasty, runs parallel to the present wall.
"Part of the wall is visible on the surface, but part of it is buried by the strong winds that blow sand dunes across this part of the desert," Guo said. "In this region the wall was made out of loose soil and mud, not bricks or rocks. Usually you cannot find these segments even if you go there, so the radar data are helping to show us the whole wall."
"Using radar to look at archeological structures has been very powerful because the radar is sensitive to vertical structures, such as walls. Even if they are highly eroded, like these segments of the Great Wall, the radar is able to capture a reflection off it and the wall shows up quite clearly in the radar image," said Dr. Jeffrey J. Plaut, the SIR-C experiment scientist at JPL.
"This is a part of the world where we can also take advantage of the radar's ability to penetrate through layers of dry sand to image buried structures," Plaut added. "The multiple channels of the SIR-C/X-SAR system increase our ability to detect different kinds of structures that a single-channel radar system would not see."
The Spaceborne Imaging Radar project is managed by JPL for NASA's Office of Mission to Planet Earth, Washington, DC. SIR- C/X-SAR is a joint mission of the United States, German and Italian space agencies.
Radar images of the Great Wall at various resolutions are available via the World Wide Web at the following address:
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