SIR-A data indicate that the Sahara region was not always the dry desert it is today
November 12, 2001
The Birth of a Geological Tool
Researchers in 1981 gained the first indication of ancient river channels beneath the sands of the Sahara Desert without the use of picks or shovels. Amazingly, nobody even had to walk on the sand of the hot African desert. Instead, the channels were found by researchers analyzing data from a radar instrument on the then-new technological wonder, the Space Shuttle Columbia.
Launched Nov. 12, 1981, the Shuttle Imaging Radar-A flew as an idea and an assemblage of spare parts from the 1978 Seasat Synthetic Aperture Radar.
"It was just a fluke," recalls JPL Director Dr. Charles Elachi, who was the principal investigator of the Shuttle Imaging Radar-A.
The radar onboard the shuttle was comprised of a single-frequency, single-polarization antenna capable of acquiring imagery at only one angle. Yet, the results were proof that certain radar frequencies could actually take images from as deep as 3 meters (9 feet) below the sand.
Building a Better Radar
The success of the mission paved the way for a follow on, the Shuttle Imaging Radar-B. In designing it, scientists tried to improve the quality of the images by building an instrument capable of collecting data at more than one angle. Scientists were aware of a relationship between the intensity of the image and the incidence angle of the radar at the surface of Earth.
Comparing data collected by the Shuttle Radar Imaging-A and the first ever Earth-orbiting satellite designed for remote sensing of Earth's ocean, the SeaSat Synthetic Aperture Radar, it was clear that not all terrains were easily mapped with just one angle. To address the problem, the Shuttle Imaging Radar-B antenna was designed to be mechanically tilted.
The Shuttle Imaging Radar-B mission launched Oct. 5, 1984, aboard the Space Shuttle Challenger for an eight-day mission. An international team of investigators led by Elachi conducted experiments in geology, oceanography, calibration techniques and other scientific areas.
The antenna design allowed images to be acquired at several different angles. Multiangle images could be used to generate perspective views such as these of Mt. Shasta in California.
Because of technical issues involving communication malfunctions, NASA granted another go to the team. However, after the 1986 Challenger disaster, a planned re-flight of the Shuttle Imaging Radar-B was aborted and the entire shuttle program placed on hold.
Back on the Space Shuttle
By the time the shuttle program resumed operations in 1988, JPL had developed a third set of instruments in the Shuttle Imaging Radar series. A cooperative project of NASA, the German Space Agency and the Italian Space Agency, Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar was the third radar to be flown on a space shuttle. This radar had an impressive list of characteristics.
For the first time, the synthetic aperture radar on board was fully polarimetric, that is capable of collecting information at any polarization, vertical or horizontal. In addition, the antenna was electronically steerable and operated at three frequencies. NASA rewarded the team with two flights in 1994, one in April and one in October. This allowed investigation into the radar's response to seasonal changes.
The multi-parameter images were combined and enhanced to produce some of the most spectacular radar images ever seen.
Ultimate Mapping Machine
But JPL scientists and engineers were not quite done in their search for the ultimate mapping machine.
In February 2000, with the aid of a 60-meter (200-foot) boom, the Shuttle Radar Topography Mission circled Earth for 10 days mapping 80 percent of the world's land area. The resulting high-resolution topographic map will be the most accurate available.
Radar's ability to image day or night and through cloud cover offers a unique tool for Earth science applications. Over the past 20 years JPL has been on the leading edge of developing radars that have led to important scientific and engineering discoveries. The next generation of spaceborne radars will continue to see JPL at the forefront of these exciting missions.
Shuttle Radar Topography Mission
http://southport.jpl.nasa.gov
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