Image Processing Reference
In-Depth Information
flown on the Space ShuttleEndeavourin 1994 penetrated through cloud cover
and heavy vegetation to reveal the existence of previously unknown temple ruins
in the ancient city of Angkor in Cambodia. This system, known asSIR-C/X-SAR,
illuminated the earth's surface from an orbital altitude of about 160 km. Figure 3.15
is a model of the radar antenna array. This assembly, which measures 4 m by 12
m, fits into the Shuttle's cargo bay. When the bay doors are opened, the antenna
can scan the Earth's surface, sending thousands of pulses per second of microwave
light that bounce off the Earth's surface 160 km below and back to the antenna.
The complex irrigation system of canals and reservoirs around Angkor is
revealed in Fig. 3.16, along with the main temple structures. Archaeologists had
only been able to study remnants of the system from the ground, and were unable
to “get the big picture” as to how water was distributed. Studying the city from the
air with visible or infrared imaging is nearly impossible, as much of the previously
cleared area is now hidden by dense rainforest canopy, and the whole region is often
covered with clouds. Years of civil war made flying over the Cambodian jungle
dangerous, as aircraft sometimes drew ground fire. Radar imaging effectively “sees
through” the clouds and much of the rainforest covering, giving a sense of what the
city must have looked like in antiquity. The temple structures show up in Fig. 3.16
as rectangular shapes long known to archaeologists. But just above those shapes in
the image are subtle, rectilinear patterns that are the ruins of a previously unknown
city that predates Angkor.
The SIR-C/X-SAR imaging system flown onEndeavouralso imaged objects
through layers of dry sand. The Sahara desert regions of Northeast Africa were
not always as devoid of water as they are today. Approximately 7000 years ago
rainfall in the region was sufficient to support the existence of rivers that have long
since been buried under several meters of windblown sand. Figure 3.17 shows one
of these ancient drainage systems near the Kufra Oasis in southeast Libya. The
Figure3.15 Model of SIR-C/X-SAR imaging radar antenna array.
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