A new, cost-effective technology based on the venerable global positioning
system (GPS) may soon revolutionize the way Earth’s atmosphere is monitored.

Scientists at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., are excited by
early analysis of data from their prototype instruments aboard two international scientific
spacecraft in low Earth orbit. Blackjack GPS receivers aboard the German Challenging
Minisatellite Payload, or “Champ,” and the Argentine Satelite de Aplicaciones
Cientificas-C (Scientific Applications Satellite), fitted with special antennas that focus on
Earth’s horizon, are tracking the radio signals broadcast by each of the 28 high-orbiting
global positioning system satellites as they rise and set on Earth’s horizon. The process is
called GPS limb sounding (also known as GPS occultation).

By measuring–to within a few trillionths of a second–the subtle changes in the
time it takes for the global positioning system signals to arrive at the spacecraft as they
travel through Earth’s atmosphere, scientists can derive a surprising amount of data.
These data include extremely precise profiles of atmospheric density, pressure,
temperature and moisture content. Additional analysis can yield global pressure contour
maps, critical climate variables and even the stratospheric wind fields that affect airline

Preliminary evaluations indicate this technology will be applicable to fields as
diverse as weather prediction and climate research, Sun-Earth interaction research, solid
Earth dynamics and oceanography. It may also be used to create the first 3-D images of
Earth’s ionosphere, a turbulent and mysterious shroud of charged particles that, when
stimulated by solar flares, can disrupt communications around the world.

“GPS occultation is a novel, cost-effective technology that augments current
methods of Earth remote sensing from space,” said Dr. Thomas Yunck, manager of JPL’s
GPS Observatories Office. “It offers accuracies and resolutions that rival those of
instruments aboard weather balloons, while filling in large global coverage gaps. Such
precise measurements of the lower atmosphere have never before been accomplished
from space. Our prototype instruments are serving as vital developmental test beds for
GPS remote sensing. NASA looks forward to a blossoming of this remote sensing
research as we continue to refine our knowledge of this new data source.”

Tens of thousands of occultation soundings have already been made. Dozens of
scientists worldwide are investigating how to best apply the data to Earth research.

Global positioning system limb sounding offers numerous attractions. It can
probe Earth’s atmosphere from the top of the stratosphere (50 kilometers, or 31 miles up)
directly to the Earth’s surface. It can operate in all weather conditions. It can calibrate
itself, resulting in stable measurements that can be compared between all occultation
sensors over time. Its fully independent measurement of pressure and height permits
atmospheric wind fields to be derived without external calibration or reference. In
addition, it has numerous applications outside of atmospheric science.

The technology’s biggest advantage may well be its low cost. GPS receivers,
comparable in size and complexity to a notebook computer, 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 possibly ignite a
revolution in Earth remote sensing.

Yunck says the potential from even a small array of such instruments is
impressive. A single GPS receiver in low orbit could acquire more than 500 soundings a
day, spread uniformly across the globe—comparable to the number of weather balloons
launched worldwide every 12 hours. When combined with Russia’s 24-satellite GPS-like
Global Navigation Satellite System and Europe’s planned 32-satellite GPS-like Galileo
system, a single GPS sensor could conceivably collect more than 2,000 soundings a day.

GPS limb sounding data from the Argentine Scientific Applications Satellite and
Champ are available through JPL’s GPS Environmental and Earth Science Information
System at http://genesis.jpl.nasa.gov . The database is one of a new generation of data
systems created under NASA’s Earth Science Information Partners program, which seeks
to create government-industry partnerships to advance Earth science. Additional
information on this program is available at http://www.esipfed.org .

This GPS limb sounding research was carried out as a part of NASA’s Earth
Science Enterprise, a long-term research effort dedicated to understanding and protecting
our home planet. Through the study of Earth, NASA will help to provide sound science
to policy and economic decision makers so as to better life here, while developing the
technologies needed to explore the universe and search for life beyond our home planet.

JPL is a division of the California Institute of Technology in Pasadena.