Using a technique originally developed in the 1960s for understanding the atmospheric properties of far-away planets, scientists around the world have been using radio signals from GPS satellites to learn more about the atmosphere of our own planet.
That technique, which is known as radio occultation, was pioneered by NASA's Jet Propulsion Laboratory and Stanford University. When a planet occults, or passes in front of, a star, the star's brightness decreases. The amount of the decrease can be used to approximate the height of the planet's atmosphere. When radio waves emitted by the star pass through the planet's atmosphere, they bend and change frequency, and this change can be measured to determine the atmospheric composition.
The GPS radio occultation (GPS-RO) technique works in a similar fashion. The 31 operational GPS satellites currently orbit at an altitude of 20,000 kilometers above the Earth's surface. Satellites with GPS-RO receivers, such as Taiwan's Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites, have a much lower orbit, around 800 kilometers above the Earth's surface.
As a GPS satellite begins to set below the horizon of Earth relative to a GPS-RO satellite, the radio waves it emits pass through the atmosphere on their way to the receiver. Molecules and electrons in the atmosphere cause the radio wave to bend and slow down, so a wave that has passed through the atmosphere will have a different frequency than one that has only traveled to the receiver through space.
By measuring the bend and signal delay relative to the unaffected radio waves, scientists are able to get readings on atmospheric temperature, humidity, pressure and electron density. The measurements are far more accurate than atmospheric readings taken from weather balloons or other space-based instruments and provide researchers with a good tool to predict hurricanes, measure climate change and assess space weather.
In what is widely accepted as a proof-of-concept mission for GPS-RO, the University Corporation for Atmospheric Research's (UCAR) GPS/Meteorology mission flew a GPS receiver aboard a microsatellite from 1995 to 1997.
In 2000, Germany's national geoscience research center, GFZ Potsdam, launched the Challenging Mini-Satellite Payload (CHAMP), which is still operational. CHAMP added the capability of measuring electron density in the ionosphere, which is helpful for detecting the space weather that causes degradation of satellite and terrestrial communications signals.
The Argentinian SAC-C mission was also launched in 2000, with contributions from the United States, France, Brazil, Italy and Denmark. It provided multi-spectral imagery of terrestrial and coastal environments and studied the Earth's magnetic field and ionosphere. SAC-C's dual antennas enabled it to take twice as many radio occultation measurements per orbit as CHAMP because it measured occultations as each GPS satellite was both rising and setting, relative to the receiving satellite.
The most advanced GPS-RO mission, Taiwan's six-satellite, $100 million COSMIC mission, was designed by UCAR and launched in April 2006. The satellites are now producing about 1,200 atmospheric profiles each day, about half of what they were designed to provide, because contact with one of the satellites has been lost and malfunctioning solar panels are limiting the power to two others.
The spacecraft are expected to perform for two years, but they carry enough fuel to last for five years. System support is being provided by the U.S. National Science Foundation, National Oceanic and Atmospheric Association, NASA and the Defense Department. In May, the National Oceanic and Atmospheric Association's National Centers for Environmental Prediction began using COSMIC operationally for its global weather forecast.
The European Organisation for the Exploitation of Meteorological Satellites launched the most recent radio occultation satellite in October 2006 aboard the Metop-A environmental satellite. It is operationally similar to COSMIC, but provides fewer atmospheric profiles per day as only one satellite is in orbit.
Robert Kursinski, an associate professor of atmospheric sciences at the University of Arizona, believes launching future GPS-RO missions is crucial to understanding climate change. Radio occultation provides temperature readings in the upper troposphere, where climate models predict the temperature should be heating up faster than the surface of Earth. But uncertainty in the temperature measurements of this region have been interpreted by some as an indication that the troposphere is not heating up as fast as the surface, he said.
Radio occultation may be the best method for studying temperature and humidity trends associated with atmospheric climate change because it provides more accurate data than other methods. Its frequency measurements are time-based, and its satellites are linked to atomic clocks. Time measurement is known to be the most accurate measurable physical quantity. Other instruments used to take atmospheric temperature readings may be unreliable for measuring climate change in the long term because the instruments' properties can drift over time, Kursinski said.
"That's the problem with a lot of the climate change data to date," he said. "You don't know if it's because the atmosphere is warmer or the instrument measurements are drifting."
No country in the world is currently planning to launch an operational GPS-RO satellite system. The United States' multi-billion dollar National Polar-orbiting Operational Environmental Satellite System was originally supposed to have GPS-RO receivers, but that plan fell victim to cost reduction efforts, Kursinski said.
One company has decided to take the business of radio occultation satellites into their own hands. GeoOptics, of Pasadena, Calif., aims to launch a constellation of 24 GPS-RO satellites by 2011, in hopes of expanding to 100 satellites by 2016. The 30-kilogram satellites will need no control from the ground once on orbit, and will last at least six years.
GeoOptics President Tom Yunck said the first 24 satellites will cost around $100 million to get into orbit and an average of $40 million a year to operate and maintain. The company is in negotiations with NOAA to fund part of the system and will attempt to get investment from international government agencies.
"With investments from countries around the world, the amount invested by each could be exceptionally small, while the return in terms of hurricane prediction and climate measurement will be enormous," Yunck said.
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