Sending 'weather balloons' up - way up

A new constellation of satellites works with GPS orbiters to monitor the atmosphere.

James Yoe didn't want to spoil his Easter weekend, so he tuned out the news. After all, rocket launches can fail. And to save money, the six tiny, revolutionary weather satellites he helped champion all sat atop one rocket as they awaited their Good Friday liftoff.

"I was nervous having all our eggs in one basket," he recalls. But angst has become elation. The satellites are up and apparently healthy, heralding what several researchers say will be a new era in weather forecasting and climate monitoring.

The satellites, collectively called the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), use signals from global navigation satellites to measure key traits in the atmosphere with unprecedented coverage and accuracy.

Such measurements now come largely from sensor-carrying balloons lofted worldwide. Balloons gather some 1,500 atmospheric profiles a day, measuring how wind, temperature, pressure, water vapor, and other traits change with altitude. These measurements become the snapshot of the atmosphere that sophisticated computer programs use to churn out weather forecasts several times a day.

Balloons have their limits, however. Most are launched from land, and most land-launched balloons rise over the northern hemisphere. They are costly; the sensor packages often are lost when the balloons come down.

COSMIC, by contrast, is designed to take 2,500 soundings a day and with truly global coverage.

While some of the satellites' measurements overlap those taken with weather balloons and larger weather satellites, these new kids on the orbital block also cover undersampled regions of the atmosphere. And their measurements are based on basic properties of the exquisitely precise timing signals that navigation satellites produce. This means COSMIC's measurements hold the promise of being more accurate and more stable and consistent over time. This stability is of special interest to climate scientists, who have had to struggle with a current generation of satellite sensors that can drift out of adjustment as they age or yield slightly different readings when engineers update sensor designs.

"We're not just adding redundant information," says Dr. Yoe, deputy director of the federal government's Joint Center for Satellite Data Assimilation in Camp Springs, Md. "This marks one of the first times we have a completely new kind of instrument."

The principle behind the idea has a long, honorable pedigree. For centuries, scientists have been able to calculate how light waves bend as they pass from air into water or glass - materials with different densities. Radio signals represent lower-energy manifestations of light. As radio waves pass through the atmosphere, they, too, bend and their frequency changes slightly. As a satellite passes behind a planet while locked onto a distant radio source, scientists can use the shifting frequencies to estimate how sharply the signal "bends" as it passes through different layers of the atmosphere. From those estimates, researchers can calculate how temperature, pressure, moisture, and other traits change with height.

Scientists first used the approach, known as radio occultation, to study Mars's atmosphere in 1965 with the Mariner 4 spacecraft, notes Thomas Yunck, who helped pioneer the technique for terrestrial work at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Radio occultation experiments have been a staple of unmanned missions to other planets ever since.

But Earth has been slow on the uptake.

Part of the reason is technological. Only with the advent of the Global Positioning Satellite system have enough transmitters been in the right place to make the approach feasible for home-planet use.

Also, the technique has been a tough sell to the weather and climate communities, Mr. Yunck adds. "For one thing, it's a totally different approach," he says. "And it looks so cheap that no one can take it seriously." Atmospheric "sounders" on today's weather satellites can cost $300 million to $400 million each, he continues. The GPS receivers are roughly $100,000 apiece.

Attitudes are changing, however. In the early 1990s, US researchers lofted a small GPS receiver on a low-cost satellite as a proof-of- concept experiment. The results yielded temperature measurements that matched balloon-borne measurements with stunning accuracy. Subsequent satellite-borne experiments have been equally encouraging.

COSMIC is the first set of satellites dedicated to testing the technique's weather-forecast potential. The $100 million project is a joint effort between the University Corporation for Atmospheric Research in Boulder, Colo., and the government of Taiwan. Indeed, the Taiwanese, who call the mission Formosat 3, put up $80 million.

During the five-year mission, the COSMIC team will feed the satellites' measurements into forecast models, but the results won't be applied to operational forecasts until the team can gauge the effect they have. The first rule, Dr. Yunck says, is "do no harm."

If the technique proves as useful as previous experiments suggest, the team would like to see a new suite of satellites that could use signals from Europe's Galileo navigation satellites as well as the US GPS system. Such capabilities would double almost overnight the number of profiles the new weather satellites could take.

Some project advocates say the satellites will complement, not replace, the venerable weather balloons. Others say the days of the weather balloons are numbered.

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