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November 30, 2006
PURVEYORS
OF THE COSMIC 'OCCULT'
To a non-scientist, the words 'radio
occultation' might
sound a little spooky. But this relatively simple NASA-developed
technology at
the heart of a new satellite network named Cosmic is proving to be a
powerful
new tool for weather and climate forecasting.
Launched April 14, 2006, the six spacecraft of the Constellation
Observing
System for Meteorology, Ionosphere and Climate measure the bending and
slowing
of microwave radio signals as they pass through Earth's atmosphere. The
signals
are transmitted from U.S.
global positioning system (GPS) satellites to Cosmic's GPS science
receivers,
which were designed by NASA's Jet Propulsion Laboratory. These bending
and
slowing events, referred to as occultations, occur when the GPS
satellite
signals are interrupted as the satellites rise or set on Earth's
horizon,
blocking their transmission.
By precisely measuring -- to a few
trillionths of a second
-- the time delay from this bending, scientists can infer information
on
atmospheric conditions such as air density, temperature, moisture,
refractivity, pressure and electron density. This makes GPS radio
occultation a
powerful new tool for weather and climate forecasting and space weather
research. Now, with Cosmic, the technology appears poised to take off.
A joint endeavor between several U.S.
and Taiwanese agencies and
institutions, Cosmic is currently feeding real-time, weather
balloon-quality
data on Earth's atmosphere every day, over thousands of points on
Earth.
Temperature and water vapor profiles derived from Cosmic will help
meteorologists improve many areas of weather prediction and observe,
research
and forecast hurricanes, typhoons and other storm patterns over the
oceans.
Over time, the mission should be a boon to scientists studying
long-term
climate change trends. Cosmic data will also help improve forecasting
of space
weather -- the geomagnetic storms in Earth's ionosphere (the part of
our
atmosphere filled with electrically charged particles). Those storms
can
disrupt communications around the world and affect electrical power
grids.
Cosmic blends with existing observing systems, filling in major gaps
and
enhancing computer-generated forecast models. "Through Cosmic, radio
occultation is at last emerging from the laboratory and is being
embraced by
the worldwide weather and climate community," said Tom Yunck, a
researcher
in JPL's Instruments and Data Systems Division and a key member of the
JPL team
behind Cosmic's radio occultation technology. "Nearly all developed
countries and major weather services are researching these data and
learning
how to ingest them into forecasts."
GPS occultation has many advantages.
It can probe Earth's
atmosphere from the top of the stratosphere (50 kilometers, or 30 miles
up)
directly to Earth's surface with extreme precision. It can operate in
all
weather conditions, penetrating the thickest clouds. And it is
relatively
inexpensive. GPS receivers, comparable in size and complexity to
notebook
computers, can be built for a fraction of the cost of traditional
spaceborne
sensors and placed unobtrusively on many low-orbiting spacecraft.
Since most Earth satellites already
carry such devices for
timing and navigation, upgrading those instruments for science purposes
might
ignite a revolution in Earth remote sensing. A single GPS receiver in
low Earth
orbit can acquire more than 500 measurements of atmospheric conditions
at
various heights every day, spread nearly uniformly across the globe.
This is
comparable to the number of weather balloons launched worldwide every
12 hours.
Unlike other remote sensing techniques, occultation measurements don't
require
calibration as the instruments age. This means measurements made today
can be
compared to measurements 20 to 30 years from now without any concern
about
measurement drifts and errors.
"There's an urgent need for very accurate, stable climate records that
can
be relied on for decades to come," said Tony Mannucci, supervisor of
JPL's
Ionospheric and Atmospheric Remote Sensing Group. "We expect
constellations such as Cosmic will continue to be deployed and that we
can
establish a long-term, stable and accurate atmospheric temperature
record that
the scientific community wants and needs."
Seven months after launch, all six
Cosmic spacecraft and
their payloads are healthy. Maneuvers continue to move the satellites
into
their final orbits 800 kilometers (500 miles) above Earth, in six
different
orbital planes. The satellites are averaging about 1,200 soundings a
day, in a
nearly uniform global distribution, providing independent data over
vast
stretches of ocean and ice where there are no weather balloons. As the
satellites approach their final positions, they will increase their
output to
about 2,500 soundings a day.
Cosmic data became available to the public on July 28. JPL and its
partners
have begun processing Cosmic data into temperature and water vapor
profiles of
the atmosphere and measurements of the electron content of the
ionosphere.
The capability of GPS to deliver high-quality measurements despite
thick cloud
cover is proving to be a major advantage. Preliminary results suggest
Cosmic
data will improve prediction of hurricane tracks, including where and
when they
will hit land.
It's too soon to say exactly how much impact Cosmic will have on daily
weather
forecasting, but the early evidence is that it will be large. "Even a
one-percent improvement would be considered a big impact," said George
Hajj, supervisor of JPL's Orbiter and Radio Metric Systems Group. "It's
expected that Cosmic-like data will become one of the main data streams
for
weather centers worldwide.” In fact, the U.S. National
Oceanic and Atmospheric
Administration will begin using Cosmic data in their operational
forecasts in
January 2007, and Canada
will follow next spring.
NASA has announced the availability of grants to advance GPS remote
sensing
science through the analysis of Cosmic and related data sets. JPL has
also
begun developing a third-generation Global Navigation Satellite System
receiver
that will use the vastly expanded signals expected in the next 10 years
from
the next-generation GPS satellites; the rejuvenated Russian Global
Navigation
Satellite System satellites, known as GLONASS; and the European Galileo
system.
For
more information and images, please visit:
http://www.nasa.gov/vision/earth/lookingatearth/cosmicf-20061130.html
For more
information
about the history of GPS occultation research, please visit:
http://www.nasa.gov/vision/earth/lookingatearth/cosmicf-20061130a.html
For
more information about Cosmic, please visit:
http://www.cosmic.ucar.edu/
For more information about GPS
occultation remote sensing, please visit:
http://genesis.jpl.nasa.gov/zope/GENESIS
##
Contact:
Alan Buis
NASA Jet Propulsion Laboratory
818-354-0474
alan.d.buis@jpl.nasa.gov
This text is
derived from:
http://www.nasa.gov/vision/earth/lookingatearth/cosmicf-20061130.html
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