NSF PR 96-53 - October 3, 1996
Media contact: |
Cheryl Dybas |
(703) 306-1070 |
Program contact: |
Jay Fein |
(703) 306-1527 |
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Experiment Proves New Weather-Data Collecting Technique
Government, university, and private-sector partners
who 18 months ago launched a small radio-signal receiver
into space believe it proves that the existing network
of 24 Global Positioning System (GPS) satellites offers
tremendous opportunity for global weather prediction
and climate change research.
Greatly improved weather forecasts could be realized
within months after launch of a small fleet of microsatellites
carrying radio receivers similar to the prototype
instrument, say researchers collaborating in the Global
Positioning System/Meteorological (GPS/MET) Satellite
Program.
The GPS network is an advanced satellite system developed
by the military for precise navigation. It is being
used increasingly for scientific and commercial purposes.
Researchers who envisioned using the system to measure
properties of Earth's atmosphere formed an alliance
that got their program off the ground 22 months after
it was funded.
"Results of the experiment show that meteorologists,
for the first time, could have weather data of unprecedented
accuracy, every hour of the day, for anywhere on Earth
-- land or sea," says Jay Fein, program director in
NSF's division of atmospheric sciences, which funded
GPS/MET, "from the troposphere where conditions drive
weather and climate, up through the ionosphere, a
region where space weather affects radio wave communications
and occasionally produces major electrical power blackouts."
GPS/MET researchers piggybacked their eight-pound,
laptop-size sensor on the MicroLab-1 Satellite. MicroLab-1
was launched on a Pegasus rocket flown from California's
Vandenberg Air Force Base on April 3, 1995.
The receiver flies in a 450-mile Low Earth Orbit,
picking up radio signals emitted by the GPS satellites
orbiting at 12,500 miles. Radio signals are slowed
and bent by refraction as they pass downward through
increasingly dense layers of Earth's atmosphere. By
measuring the angle of refraction, program scientists
theorized, it should be possible to measure changes
in air density and, given that, calculate temperature,
pressure, and humidity with high precision for all
layers of the atmosphere, top to bottom. It would
be theoretically possible for a single GPS/MET instrument
to get 500 "soundings" of the Earth's atmosphere every
24 hours. Voyager scientists in the 1970s and 1980s
used the same radio "occultation" techniques to measure
the properties of atmospheres of the outer solar system
planets.
GPS/MET scientists reported initial results from their
proof-of-concept project in January in the Bulletin
of the American Meteorological Society. They have
steadily been accumulating data since: As of late
August, the team had cataloged approximately 45,000
radio occultations, says GPS/MET program manager Michael
Exner of UCAR, the University Corporation for Atmospheric
Research. The program goal originally had been to
accumulate one thousand such measurements.
Conventional space-based techniques measure average
temperature for layers of atmosphere 5-to-10 kilometers
thick. GPS/MET produces an accurate temperature every
500-to-1,000 meters. Also, GPS/MET takes data above
25-to-30 kilometers, where radiosonde weather balloons
burst. A single radiosonde station takes data only
once or twice a day, and only from land. Land covers
only a quarter of the Earth's surface, so there is
a void of data for the remaining three-quarters of
Earth's atmosphere over oceans, a void that GPS/MET
fills.
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