The FORMOSAT-3/COSMIC mission is the product of a U.S.-Taiwan partnership sponsored jointly by Taiwan National Science Council and United States National Science Foundation to deploy a globe-spanning network of six low-earth-orbiting satellites. The mission is called COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) in the United States. The FORMOSAT-3/COSMIC mission is based on a system design provided by the University Corporation for Atmospheric Research (UCAR). Orbital Sciences Corporation designed the spacecraft. The rest of the satellite constellation was built and tested at National Space Organization (NSPO) in Taiwan.
The science goal of the FORMOSAT-3/COSMIC mission is to obtain the near-real time global distribution of air pressure, temperature, and water vapor of the atmosphere as well as the electron density of the ionosphere. The data collected are used for weather prediction simulations, global climate-change analysis, and ionosphere and gravity research. The World Meteorological Organization will distribute the FORMOSAT-3/COSMIC data to the weather centers of its member countries.
The FORMOSAT-3 satellites will be the first to provide global atmospheric data for both research and operations in near real time using the radio occultation technology by measuring the bending of radio signals from the U.S. Global Positioning System (GPS) as the signals pass through Earth's atmosphere and ionosphere. The radio occultation is a technique for determining the property of an inhomogeneous medium by measuring the refractive bending of radio signals passing through the medium. Just as the water molecules in a glass change the path of visible lights from the air into water (or vice versa) so that a pencil appears bent, molecules in the air bend GPS radio signals as they pass through the atmosphere. By measuring the amount of this bending, scientists can determine underlying atmospheric conditions, such as air density, temperature and moisture, and electron density in the ionosphere.
The GPS radio occultation technique has been deployed in satellites such as GPS-MET, CHAMP and SAC-C and the atmospheric data obtained have been shown to be highly accurate. However, these are single satellite operation without mutual coordination. The FORMOSAT-3 mission's six satellites use the GPS radio occultation technique on a large scale in near real time to provide continuous monitoring of worldwide atmospheric conditions. The satellites will take at least 2,500 measurements of vertical profiles of atmospheric air density, temperature and water vapor every 24 hours in a nearly uniform distribution around the globe, filling in current data gaps over the oceans and the polar region. This is about three times the daily ground-based observation of about 900 measurements, which are concentrated mostly in continental regions. The data's high vertical resolution will complement the high horizontal resolution of conventional weather satellite measurements. Temperature, air density and water vapor profiles derived from the FORMOSAT-3 GPS data will help meteorologists observe and forecast typhoons, hurricanes, and other storm patterns over the oceans and improve many areas of weather prediction. The stability, consistency, and accuracy of the measurements should also help scientists quantifying long-term climate change trend.
The FORMOSAT-3/COSMIC data will be processed at the Taiwan Analysis Center for COSMIC (TACC), which is located at the Central Weather Bureau (CWB). The scientific research of the FORMOSAT-3 mission will be performed by three science teams (the atmospheric science team, the ionospheric science team, and the geodesy science team), which consist of university faculty, researchers and students in Taiwan.
The FORMOSAT-3/COSMIC data will be open to scientists from all nations for collaboration on atmospheric and ionospheric research. Centers around the world will have access to this new information for both research and operational forecasting. The FORMOSAT-3 data can be accessed from TACC or CDAAC (COSMIC Data Analysis and Archive Center) by signing a user agreement with both NSPO and UCAR. The access information is given at the TACC website ( http://tacc.cwb.gov.tw ). The approved users can access the global distribution of atmospheric parameters such as refractivity, air pressure, temperature and water vapor, as well as global electron density distribution of the ionosphere. TACC, led by Mark Cheng, has established the necessary hardware and operating systems and is presently testing the analysis codes. TACC will be ready for operation when the FORMOSAT-3 satellites are launched.
The atmospheric science team, led by Prof. Ching-Yuang Huang, National Central University, has been using single satellite GPS radio occultation data to study their effectiveness on the improvement of typhoon prediction. His previous studies using only the German CHAMP GPS radio occultation data have shown remarkable improvement on 24-hour accumulated rainfall and 48-hour track predictions for typhoons. With at least six times more data from continuous monitoring by the six FORMOSAT-3 satellites, it is anticipated that the typhoon prediction can be greatly improved. Prof. Joseph Chang, Chinese Culture University, has been collaborating with CWB to assimilate GPS radio occultation data into the global weather model. It is expected that with the FORMOSAT-3/COSMIC data CWB would be able to improve the prediction of typhoon track, heavy rainfall area, and rainfall accumulation.
The FORMOSAT-3 ionospheric science team, led by Prof. Yen-Hsyang Chu, National Central University, studies the electron density distribution of the ionosphere. The measurements of global electron density distribution by the FORMOSAT-3 satellites are expected to improve analysis and forecasting of space weather--the geomagnetic storms that can interrupt sensitive satellite and communication systems and affect power grids on the ground. The FORMOSAT-3/COSMIC data allows establish a global ionospheric electron density model. The ionospheric science team has also established an ionospheric data center at the National Central University to process the FORMOSAT-3/COSMIC electron density data.
The geodesy science team, led by Prof. Ching-Liang Tseng of National Cheng-Kung University, computes the FORMOSAT-3 satellite orbits. To obtain accurate and continuous monitoring of atmospheric data using the radio occultation technique, it is necessary to know the satellite orbit in near real time. Moreover, with the precise orbit information of all satellites, one can obtain the Earth's mass distribution that determines the gravity field and the satellite orbits. With a continuous monitoring of the change in the Earth mass distribution, global change in ocean circulation, ground water table, and El Nino can be studied.
In March 2006, all six satellites of the FORMOSAT-3 mission will be stacked on a Minotaur rocket and launced from Vandenberg Air Force Base in US into initial parking orbits of about 500 km in altitude. It will take 13 months for all satellites to settle in their designed orbital planes with a longitudinal separation of 24 degree and altitude of about 700-800 km. During the first eight months after launch the FORMOSAT-3 satellites will stay relatively close to each other in longitude and the GPS radio occultation data points would be very dense. It is an excellent and rare opportunity for performing cross validation of the radio occultation data with other observations (ground based radiosonde, weather satellite, and balloons, etc.). To make use of this dense observation period, Prof. C. Z. Frank Cheng, Chief Scientist of National Space Organization, is planning an Intense Observation Period (IOP) campaign to conduct the cross data validation as well as to perform prediction studies of typhoon track, heavy rainfall area, and rainfall accumulation in the East Asia region. Researchers in the East Asia nations and from US and Europe will be invited to participate in the IOP campaign. |
