Sending 'Weather Balloons' Up - Way Up ; A New Constellation of Satellites Works with GPS Orbiters to Monitor the Atmosphere

Article excerpt

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. …