GPS & the Atmosphere - Atmospheric Profiling with Global Navigation Satellite Systems (GNSS)
The science behind COSMIC originates with a method called radio occultation that was developed in the 1960s to study the atmospheres of the outer planets of the solar system. More recently, scientists have begun to apply radio occultation to study the Earth's atmosphere. The method works by taking advantage of radio signals broadcast from Global Positioning System (GPS) satellites already in orbit. The U.S. Department of Defense maintains about 30 satellites that orbit approximately 20,000 kilometers (12,400 miles) above Earth to facilitate civilian and military navigation technology. When people use GPS receivers, they pick up signals from some of these satellites to accurately determine their longitude, latitude, elevation, and time.
As radio signals from the GPS satellites pass through the atmosphere, molecules and electrons bend their paths and slow their progress. COSMIC takes advantage of this effect by intercepting the radio signals and measuring their bending and signal delay. The satellites then downlink the data at ground stations in Fairbanks, Alaska, Tromso, Norway, and McMurdo Antarctica. Once the data are relayed back to the COSMIC Data Analysis and Archive Center (CDAAC) in Boulder, scientists retrieve profiles of bending angles, refractivity, temperature, pressure, and water vapor in the atmosphere, and profiles of electron density in the ionosphere. The COSMIC RO profiles start near Earth's surface and extend to the height of the COSMIC satellites at 800 kilometers (500 miles) above Earth. Since the six COSMIC satellites are distributed equally in local time around the Earth, COSMIC provides homogeneous sampling coverage that operational weather balloons and monitoring stations can not match, especially over the oceans and other areas traditionally sparse in data.
"Progress in many fields of research is hindered by lack of good data", says Bill Kuo, director of COSMIC at the UCAR Community Programs. "It is really exciting that we have shown that the GPS radio occultation technique works and can potentially have major impacts on weather forecasting, climate monitoring, and space weather prediction"
UCAR demonstrated the potential of using radio occultation to sound Earth's atmosphere with the GPS/MET experiment from 1995 to 1997. COSMIC has built on GPS/MET and subsequent radio occultation missions to produce real-time soundings that are currently used in operational weather prediction models.
"The independent nature of radio occultation soundings make them highly complementary to other atmospheric sounding systems, such as infrared and microwave sounders", says Rick Anthes, UCAR President Emeritus. "Two or three independent sounding systems, when combined properly, can yield atmospheric soundings of temperature and water vapor with higher accuracy and vertical and horizontal resolution than any one system alone".
(See the article written by Nicole Gordon in its entirety: A COSMIC Project, UCAR Staff Notes Monthly - September 2004 )