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3. Development of spaceborne precipitation radar by Fugono and Okamoto
Rain circulates sunshine energy and greatly affects the global climate. Each year, two-thirds of the world's rainfalls occur in tropical areas. Since oceans or jungles largely cover tropical areas, it is difficult to get accurate data on precipitation. However, correct and detailed data is essential to predict weather or investigate the causes of climate changes.
Meteorological satellites cannot observe rainfall data directly, because visible and infrared sensors provide only the surface features of clouds. A passive microwave sensor, SSM/I (Special Sensor Microwave Imager), can provide rough rain data only on the sea surface and not on land, and cannot observe the three-dimensional structure of precipitation10).
In the mid-1970s, Fugono and Okamoto conducted research on satellite communication that increased interest in research on the attenuation of radio waves by rain and other phenomena. Since then, they have shifted their emphasis to research on more accurately measuring rainfall, which drives atmospheric circulation and water cycles. In 1979, they succeeded in measuring rainfall using airborne radar for the first time. Immediately after they publicized the experimental results, they received a proposal from NASA for a joint flight experiment at the end of 1980. After two joint flight experiments from 1983, the U.S.-Japan collaborative rainfall measurement project, the Tropical Rainfall Measuring Mission (TRMM), began 1986. The precipitation radar was the key sensor of the TRMM satellite. Fugono promoted the project, and Okamoto led the technology development. Although there were some difficulties, such as project discontinuation, after the Paris Summit in 1989 at which global environment subjects were discussed, the development of the TRMM went ahead. In 1997, TRMM was launched by H2 rocket at Tanegashima in Japan11).
The outstanding breakthroughs in this mission are the followings: 12)
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The echoes of rainfall are much weaker than those from land or sea surfaces. To observe weak echoes from an altitude of 350km, very low antenna side lobe level was established and the high 14.5 GHz operating frequency microwave was adopted. |
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The TRMM satellite is traveling at a very high velocity of 7km/sec. To observe rainfall data in a 215km swath width with contiguous coverage, a 128-element active phased array antenna was adopted. The precipitation radar is only one example that succeeded to develop active phased array of the frequency as high as 14GHz. Two-channel frequency agility technique of 13.796 and 13.802 GHz was used to increase S/N ratio. |
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Active array implies that each antenna array element has its own active devices, which includes transmit and receive amplifiers. The active array has advantages over its conventional passive array counterpart, such as a low system loss that is achieved by an almost direct connection between each antenna element and a transmitter/receiver unit. Moreover, for the distributed configuration, there is no need to use a high-voltage power transmit tube. |
The PR on this mission has provided a detailed 3-D precipitation structure of many instances of concentrated rainfall and typhoons, and the data gained by the PR is superior in quality than what was expected by many researchers in the world. This data is freely accessible to researchers all over the world and have been promoting scientific research.
The most important accomplishment of this mission is to measure precipitation with equal accuracy both on land and sea. It makes reliable records of data on tropical and subtropical regions on daily, monthly, and yearly time scales. The monthly summary of rainfall in tropical and subtropical zones between January 1998 and December 2001 clearly showed differences in the zone distribution of precipitation in May 1998 when El Niño was occurring compared to those of non El Niño periods13). (Fig.3) The PR data has also provided some information on diurnal cycles of precipitation from long term statistics14). Recently, TRMM observations are being combined with ground based observations by the Japan Meteorological Agency to help refine the precision of weather predictions15).
The PR radar collects precipitation data only about 10% of its total working hours, with the remaining hours spent collecting data on the Earth's surface. Oki, a professor at Tokyo University in Japan, has developed an algorithm to analyze data on water content in soil and land surface conditions globally derived from such extra data16, 17). He had reported in his research paper presenting the analytical results on the flooding damages in Mozambique and on the preceding increase of water contents along the river basin when Yangtze River in China had flooded.
Rosenfeld has also reported new ideas on changes of precipitation effected by the amounts of Suspended Particulate Materials (SPM) in the air. His idea, which is that if the amount of SPM increases precipitation decreases, is contrary to the accepted theory. His new theory is derived and supported by the data of PR18, 19). These findings will provide new and useful treatment against ought.
Recently, the system resolution limit of the Earth Simulator that create a "virtual earth" on a supercomputer to show what the world will look like in the future by means of advanced numerical simulation technology is 5 km square and it is nearly equal to that of PR in resolution level. Now, the precise and homogeneous precipitation data are being assimilated on the Earth Simulator, and the PR data have made great contributions to the improvements of simulation models on global warming or climate changes20).
Since the TRMM program has achieved great progress and the key technology is the PR, a innovative Global Precipitation Measurement (GPM) program is now planned by Japan, the United States and Europe, with the GPM mission satellites scheduled to launch in 2007. The aim of the program is to collect precipitation data with improved accuracy every 3 hours over the entire globe. The core satellite will use two frequencies of PR, which are being developed by Fugono and Okamoto, in collaboration with 8 other satellites carrying microwave radiometers21). |
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