An experimental instrument, launched today from Vandenberg Air Force Base in Lompoc, Calif., could alter the way scientists monitor global atmospheric temperatures and climate change by using the worldwide array of Global Positioning System satellites.
The Global Positioning System Meteorology instrument -- or GPS Met -- was launched aboard an Orbital Sciences Corporation Microlab satellite on a Pegasus launch vehicle. The instrument, developed by the Jet Propulsion Laboratory, was successfully placed into orbit at 6:48 a.m. Pacific Daylight Time.
From its vantage point in low-Earth orbit, GPS Met will receive and track the radio signals broadcast by 24 high-orbiting satellites of the U.S. military's GPS network. Just before each GPS satellite passes out of view of the Earth, its signal, as seen by GPS Met, will slice through the atmosphere from the top of the stratosphere down to the Earth's surface. This process is known as atmospheric occultation or radio occultation.
"As the signal descends, the atmosphere acts as a lens, causing the signal's path to bend and its travel time to increase by a small, but perceptible amount," said JPL engineer Dr. Thomas Yunck, one of a team of experts who proposed the GPS technique in the late 1980s. "By precisely measuring the signal's increasing travel time and the fluctuating signal strength, we can recover highly accurate profiles of atmospheric density, pressure, temperature and, to some degree, turbulence and winds."
"The most obvious scientific application of this technique will be monitoring changes in climate by providing precise, stable and high-resolution profiles of atmospheric temperatures across the globe," added Rob Kursinski, a JPL scientist on the GPS Met team. "The GPS Met data represent the first, hopefully, in a long-term observation program, which will provide us with much needed, long-term information about how trace greenhouse gases may be modifying Earth's atmosphere and climate".
GPS Met will also be used to study the amount of water vapor in the lower atmosphere, the JPL science team said.
"Water vapor is extremely important to Earth's weather and climate system," Kursinski explained. "It is crucial to the operation of the Earth's atmospheric heat engine, which redistributes absorbed solar energy to higher latitudes. Water vapor is also the primary greenhouse gas in our atmosphere."
Using the technique of radio occultation to explore planetary atmospheres dates back to 1965, when scientists studied the signal sent back by Mariner 4 as it passed behind Mars. In the years since, this technique has been used to study other planets in the solar system and their moons.
"Studying Earth with this technique has been difficult because the observations require both a radio source and a suitable receiver located off the planet, outside the atmosphere," Yunck said. "Until now, we have not had such matched pairs in Earth orbit.
"Additionally, to be of use in studying Earth's atmosphere, whose nature we know quite well, such measurements must be continuous and comprehensive," he said. "We therefore need many transmitters and receivers aloft at once, densely sampling the global atmosphere every few hours. The cost of such an enterprise has generally made it impractical within Earth science programs."
The advent of microsatellites and small launcher technologies is changing all of that. GPS Met is the first "proof-of-concept" demonstration of the GPS occultation technique. If successful, it could lead to a future constellation of tiny microsatellites, each no larger than a small paperback book and weighing less than 2 kilograms (4.4 pounds), that would continuously survey the global atmosphere with unprecedented accuracy and spatial resolution, Yunck said.
The GPS observations from a constellation of tens or hundreds of receivers in low-Earth orbit may have a dramatic impact on weather forecasting because they would provide a number of unique and fundamental features which would complement the present suite of worldwide weather monitoring measurements, Kursinski explained.
Another important application of the GPS Met technology will be its ability to map the ionosphere -- the area of the upper atmosphere consisting of free electrons. Low frequency radio waves broadcast from Earth interact strongly with the ionosphere. This interaction allows the radio signals to be sent great distances as they are reflected off the ionosphere. Ionospheric disturbances have been known to cause violent solar flares, for example, which can disrupt radio communications worldwide.
With a large array of orbiting GPS receivers, it will be possible to create three-dimensional images of the ionosphere that will help scientists map its structure and could give them a near real-time picture of the ionosphere's often erratic behavior, Yunck said.
The GPS Met experiment is sponsored jointly by NASA's Office of Mission to Planet Earth, the National Science Foundation, the National Oceanographic and Atmospheric Administration and the Federal Aviation Administration.
The instrument was developed by NASA's Jet Propulsion Laboratory and is being managed by the University Corp. for Atmospheric Research in Boulder, Colo.
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