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Roundabout Way of Profiling Earth's Atmosphere

New method of measuring temperature, pressure, and humidity using GPS signals should improve weather forecasting

4 min read

If all went according to plan, on 31 March six microsatellites were carried into space stacked aboard a single U.S. Air Force Minotaur rocket. Each satellite is equipped with a radio receiver designed at the California Institute of Technology's Jet Propulsion Laboratory, in Pasadena, which will pick up signals from the U.S. Global Positioning System (GPS). Together, the six satellites make up a system called COSMIC, for Constellation Observing Systems for Meteorology, Ionosphere & Climate, whose mission is to take measurements of temperature and humidity in the atmosphere by means of a technique called radio occultation.

Conceived by Richard Anthes, president of the University Corporation for Atmospheric Research (UCAR) in Boulder, Colo., and several colleagues in Taiwan and the United States, the project is unusual and unprecedented in that the six satellites were assembled in Taiwan and largely paid for by the government of Taiwan. The system's method of sounding the atmosphere's temperature, pressure, and moisture and using the data for weather prediction is also without precedent. "It is the first mission to demonstrate the use of GPS radio occultation soundings for weather prediction in near real time," says Anthes.

The radio occultation technique works like this: each of the six COSMIC satellites in low-earth orbits will retrieve radio signals from the 28 civilian GPS satellites in higher orbits [see diagram, " "]. As the COSMIC and GPS satellites rise above the horizon or set relative to each other, the receivers will measure the changes in frequency of the radio000 signals as the signals pass through different layers and densities of Earth's atmosphere. From these frequency measurements, it is possible to compute the refraction angles of the radio waves, and from those bending angles, vertical profiles of the atmosphere's temperature, pressure, and humidity can be extracted.

Each of the COSMIC microsatellites is only 1 meter in diameter and weighs less than 70 kilograms. Once they are in space, the satellites will slowly drift apart over the course of 13 months; through a series of adjustments using four small thrusters, they will be placed at equal distances from each other, at an 800-kilometer altitude, with an inclination of 72 degrees relative to Earth's surface.

Traditional weather satellites rely on radiometers to take readings of both infrared and microwave radiation in the atmosphere. But as these satellites look down, they cannot distinguish clearly between thin atmospheric layers. What is more, traditional radiometers cannot see through clouds and are also subjected to various kinds of radiation from the sun and other parts of the universe, which can cause "instrument drift" to develop, affecting accuracy and sensitivity.

According to Bill Kuo, the director of the COSMIC Project at UCAR, the GPS radio occultation technique used by the COSMIC system will not suffer from any of these problems and therefore will deliver a precise measurement. That is because the COSMIC receivers will measure changes in the phase and amplitude of radio signals transmitted by GPS satellites, not changes in radiant energy.

The vertical resolution of the receivers will vary from about 100 meters in the lower part of the troposphere to about 1 kilometer in the upper troposphere and stratosphere. By contrast, traditional weather satellites in geostationary or polar orbits use infrared or microwave measurements and have a vertical resolution of several kilometers. Poor vertical resolution has been an important factor in the uncertainties regarding initial conditions that have plagued global weather prediction models, says Kuo.

Weather balloons carrying instrument packages (radiosondes) are another traditional method of gathering atmospheric data. Currently, there are more than 850 stations taking radiosonde data around the globe. But the instrument packages provide data of varying quality, and although the balloons have good vertical resolution, their horizontal resolution is poor. What's more, radiosonde observations are generally available only over land, leaving about 70 percent of Earth's atmosphere, the part above the oceans, uncovered.

In contrast, COSMIC will be able to provide 2500 soundings, distributed uniformly around the globe, per day. Once the atmospheric data have been recorded by the satellites, the information is downloaded to the COSMIC ground stations in Fairbanks, Alaska, and Kiruna, Sweden, then sent to a data analysis and archiving center in Boulder, where it is processed and transmitted to global weather prediction centers.

The COSMIC system is the first collaboration between the United States and the Taiwanese National Space Organization. It has a price tag of US $100 million for the first two years of operations, with 80 percent of the mission being funded by Taiwan's National Science Council, the country's equivalent of the U.S. National Science Foundation. NSF is the lead U.S. sponsor of COSMIC.

COSMIC is the Taiwanese National Space Organization's third satellite project; it is called FORMOSAT-3 in Taiwan. The country became interested in working on the project after the success of GPS/MET, a prototype GPS satellite launched by UCAR in 1995. GPS/MET gathered data for two years, proving radio occultation would provide a basis for more accurate weather and climate predictions.

One important reason for Taiwan's involvement is its concern about typhoons, the most serious weather threat facing the island. "COSMIC's soundings will tell us more about the environment of these tropical storms and contribute greatly to improved storm forecasts," says Bor-Han Wu, a senior scientist with Taiwan's space program.

According to Anthes, who initiated the collaboration with Taiwan in 1997, "Taiwan wanted to become an international partner in Earth system sciences." Evidently it also sees COSMIC as a way to get into the global business of producing Earth satellites.

Orbital Sciences Corp., of Dulles, Va., built the prototype COSMIC satellite and the Minotaur launch vehicle. The remaining five COSMIC microsatellites were then produced as kits and sent to Taiwan for assembly, with Taiwanese manufacturers contributing some of the parts. Both Orbital Sciences and UCAR have technology-transfer agreements approved by the U.S. Department of State to let them carry out the collaboration with Taiwan on the COSMIC project.

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