A revolutionary globe-spanning satellite network will furnish round-the-clock weather data, and monitor climate change by intercepting signals from the Global Positioning System (GPS). Using atmosphere-induced changes in the GPS radio signals, scientists will infer the state of the atmosphere above some 3,000 locations every 24 hours and over vast stretches of ocean inadequately profiled by current satellites and other tools. The $100 million mission will begin operation in 2005.

A U.S.-Taiwan partnership is developing the satellite network, called COSMIC, based on a system design provided by the University Corporation for Atmospheric Research (UCAR) in Boulder, Colo. Taiwan’s National Science Council and National Space Program Office (NSPO) and the U.S. National Science Foundation (NSF) are providing management and support for COSMIC. Additional support is provided by NASA, NOAA, and the Department of Defense.

“The increased coverage will improve weather forecasts by
providing data where there previously was none or not enough,” says Ying-Hwa Kuo, project director for the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), also called ROCSAT-3 in Taiwan. With six satellite receivers, COSMIC will collect a global, 3-D data set expected to improve analyses of both weather and climate change. By tracking temperature in the upper atmosphere up to 30 miles high, COSMIC could help clarify if these regions are cooling due to heat-trapping greenhouse gases closer to the earth’s surface. COSMIC will also measure high-altitude electron density, potentially enhancing forecasts of ionospheric activity and “space weather.” By taking measurements below 12 miles high, COSMIC will provide critical information on atmospheric water vapor, crucial for accurately predicting weather systems with precipitation.

“The potential value of cosmic observations for climate research and weather is enormous,” says Jay Fein, director of NSF’s climate dynamics program. “The system is relatively inexpensive, works in all weather conditions and does not require calibration. That means a measurement taken in 2005 can be compared with one taken by a different satellite in 2025, with no concerns about biases caused by instrument drift.”

COSMIC’s satellites will probe the atmosphere using a technique known as radio occultation, which was developed in the 1960s to study other planets but more recently applied to Earth’s atmosphere. Each satellite will intercept a GPS signal after it passes through (is occulted by) the atmosphere close to the horizon. Such a path brings the signal through a deep crosssection of the atmosphere. Variations in electron density, air density, temperature and moisture bend the signal and change its speed. By measuring these shifts in the signal, scientists can determine the atmospheric conditions that produced them. The result: profiles along thousands of angled, pencil-like segments of atmosphere, each about 200 miles long and a few hundred feet wide.

Rather than replacing other observing systems, COSMIC will blend with them, filling in major gaps and enhancing computer forecast models. Many satellite-based products are like topographic maps that trace the contours of atmospheric elements. COSMIC is more akin to a set of probes that drill vertically through the depth of atmosphere. Thus, says Kuo, “COSMIC will complement the existing and planned U.S. meteorological satellites.”

Radiosondes (weather sensors launched by balloon) have obtained vertical profiles since the 1930s. However, they are launched only twice a day in most spots, and few are deployed over the ocean. In contrast, the COSMIC data will be collected continuously across the globe. The GPS radio signals can be picked up by the low-orbiting COSMIC receivers even through clouds, which are an obstacle for satellite-borne instruments that sense infrared rays of the spectrum.