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
"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