| |
NSF PR 02-72 - September 6, 2002
Satellites to Profile Weather, Improve Forecasts
Through GPS
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
cross section 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.
|
|
