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2. Elachi's pioneering achievement in remote environmental monitoring by space borne microwave radar
Elachi has been a pioneer in the field of precise observation technologies to monitor the global environment using spaceborne radars onboard space craft, including satellites, space shuttles, and other spacecraft. His important achievement is in optimizing the design of spaceborne radar systems for global environmental observation and using the data obtained to contribute to advances in scientific fields, such as geology, environmental science, oceanography, and archeology.
Since he was familiar with both elemental radar techniques and the earth sciences, he could lead the detailed design of the first spaceborne imaging radars for environmental monitoring, as well as a series of increasingly advanced successors onboard subsequent space missions.
It is necessary to apply short pulse waves and use a long antenna to obtain high-resolution images using radars. In the case of using a 9.5 GHz electric wave from an altitude of 12,000 meters, for example, it is necessary to use an antenna of over 250m in length to recognize subjects that are 1.5 m in size on the ground. Because of the difficulty in making such a long spaceborne antenna, a sophisticated new idea was considered. The Synthetic Aperture Radar (SAR) achieves high spatial resolution by recording backscatter signals from a target as its platform (aircraft or satellite) moves along the track in conjunction with the use of advanced signal processing technologies.
Using these sophisticated technologies, even a small antenna boarded on planes or satellites can get the same grade of high-resolution images as using the large size one in principle1).
Elachi understood well the role of microwave lengths and injection angles, the principles of polarimetory that use different reflection phenomena obtained from vertical polarization or horizontal polarization, and the methods to get vertical images using interferences of mutual waves. Appling this understanding, he developed spaceborne radar instruments to collect data on the Earth environment with excellent precision and accuracy.
In June 1978, the Synthetic Aperture Radar onboard the satellite SEASAT successfully presented radar imaging data on the Earth for the first time2).This satellite only obtained data on oceanographic phenomena for 100 days, but they were equal in quantity to those obtained in the past 100-years of observations on boats3).
SEASAT's SAR discovered the remains of an ancient irrigation system, which would have been impossible to discover using conventional optical sensors. Three kinds of radar systems (SAR, SCAT and ALT) were loaded on SEASAT and they provided information on wind rates and directions, currents or swirls and seabed structures in oceans globally. These successes based on spaceborne radar systems were a great advance in global environmental observation.
The Shuttle Imaging Radar-A(SIR-A), which was a synthetic aperture radar with HH polarization and 50°incidence angle at the surface, was launched on the Space Shuttle Columbia in November 19814). This experiment demonstrated the radar's ability to penetrate extremely dry surfaces, which resulted in the discovery of ancient river channels buried beneath the Sahara desert.
Shuttle Imaging Radar-B (SIR-B) was placed onboard the Space Shuttle Challenger in October 1984. This time, the incidence angle of the microwave was designed to be mechanically tilted. This modification resulted in clear 3-D images5).
The Spaceborne Imaging Radar C/X band Synthetic Aperture Radar (SIR C/X SAR) was launched via space shuttle twice, once in April 1984 and once in October 1994, and was one of. Elachi's most important contributions. This system consisted of three individual frequency antennas. Polarimetory and phased array systems were applied for two of them. The multi-frequency, multi-polarization capability created a new and more powerful tool for observing the global environment. This mission provided a wide range of information on the Earth's surface structures including those on the ground, ocean surface and polar regions6, 7, 8). (Fig 1) 9)
The effectiveness of the technologies developed by Elachi encouraged others to use spaceborne radar in other missions. The satellite "RADARSAT" was launched in November 1995 and uses SAR to collect data on global environmental changes and natural resources. It has obtained data on various phenomena, such as snow fall, crop growth, oil spill dispersion, forestry, flooding, ice melt, glacier movement, and so on.
Another radar, NSCAT, loaded on the satellite "ADEOS," was launched in August 1996 by H-2 rocket. This radar was specially designed to measure wind speed and direction. The system could take data covering over 90% of ocean surfaces with 50 km in range resolution every two days. This capacity is equal to nearly 100 times the information obtained by conventional observation from boats. The data collected has made substantial contributions to improving climate models and investigating the causes of the El Niño phenomenon.
The Shuttle Radar Topography Mission was launched by NASA in February 2000 via space shuttle and has resulted in a high-resolution, 3-D map of approximately 80% of the Earth's land mass - the most complete map ever assembled.
Recent state-of-the-art spaceborne imaging radar systems, which were led by . Elachi, presented new tools for observing the global environment for researchers in various fields. Researchers have used these tools to collect ecological data on land including plant distribution, crop cultivation, the estimation of biomass, and the monitoring of seaside areas or swamp regions. Other kinds of environmental data are also collected regardless of cloud condition and with day-night imaging over wide ranges of the Earth. Examples of such data include the water content in soil and snow, characteristics of ocean waves and currents, the distribution and classification of sea ice, changes in and movement of glaciers, crystal changes caused by earthquakes, volcanic activity, degradation of the Earth's crust, geographical information, hazard monitoring and the discovery of ancient geographical patterns.
Elachi has technical expertise in the design of radar instruments, as well in the earth sciences and their applications. These talents allow him to present the importance of spaceborne imaging radar technologies to scientists as well as to decision makers. His skills have also led him to be a leader of project teams, in charge of projects that include many scientists and or engineers.
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