The Great Wall of China
Photo ID: P-45924
August 3, 1995
These radar images show two segments of the Great Wall of China in a
desert region of north-central
China, about 700 kilometers (434 miles) west of Beijing. The wall
appears as a thin orange band,
running from the top to the bottom of the left image, and from the
middle upper-left to the
lower-right of the right image. These segments of the Great Wall were
constructed in the 15th
century, during the Ming Dynasty. The wall is between 5 and 8 meters
high (16 to 26 feet) in these
areas. The entire wall is about 3,000 kilometers (1,864 miles) long and
about 150 kilometers (93
miles) of the wall appear in these two images. The wall is easily
detected from space by radar because
its steep, smooth sides provide a prominent surface for reflection of
the radar beam. Near the center
of the left image, two dry lake beds have been developed for salt
extraction. Rectangular patterns in
both images indicate agricultural development, primarily wheat fields.
The images were acquired by
the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar
(SIR-C/X-SAR) onboard the
space shuttle Endeavour on April 10, 1994. SIR-C/X-SAR, a joint mission
of the German, Italian
and the United States space agencies, is part of NASA's Mission to
Planet Earth. The left image is
centered at 37.7 degrees North latitude and 107.5 degrees East
longitude. The right image is centered
at 37.5 degrees North latitude and 108.1 degrees East longitude. North
is toward the upper right. Each
area shown measures 25 kilometers by 75 kilometers (15.5 miles by 45.5
miles). The colors in the
image are assigned to different frequencies and polarizations of the
radar as follows: red is L- band
horizontally transmitted, horizontally received; green is L- band
horizontally transmitted, vertically
received; blue is C-band horizontally transmitted, vertically received.
![[THE GREAT WALL OF CHINA]](GREATWAL.GIF)