Listen to the noise.

One year earlier, the MicroLab 1 satellite had taken off from Vandenberg Air Force Base in California within the nose cone of a Pegasus rocket. The modest payload aboard the satellite included a shoebox-sized instrument. Its mission was to intercept signals from an array of 24 Global Positioning System (GPS) satellites deployed 20,000 kilometers (12,000 miles) above the earth by the U.S. military. The goal: to measure how much these signals were being distorted by noise from the earth's atmosphere--and to use that information to provide urgently needed global data on temperature, pressure, and moisture.

What? GPS/MET: meteorological applications of the Global Positioning System network of defense satellites Who? Principal investigators: Randolph Ware, Michael Exner, and Christian Rocken, UNAVCO; Benjamin Herman, University of Arizona; William Kuo, NCAR Mesoscale and Microscale Meteorology Division; Thomas Meehan, Jet Propulsion Laboratory (JPL) Why? To gain inexpensive vertical profiles of temperature and moisture across the globe with high spatial and temporal resolution How? By intercepting GPS signals with a satellite-based receiver and inferring the deviations in each signal's straight-line path caused by temperature and moisture gradients Where? Research and development based at UOP's GPS/MET Program Office and UNAVCO, with participation and support from NSF, NCAR, National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, Federal Aviation Administration, University of Arizona, and JPL; launch carried out at Vandenberg Air Force Base, California When? First data collected in April 1995, following several years of development; data processing and system refinement ongoing http://pocc.gpsmet.ucar.edu/

By the satellite's first birthday party, it had taken its first steps toward fulfilling the dream of its creators. Hundreds of meteorological profiles from near ground level to 60 kilometers (36 miles) had been sent to earth. The potential for global coverage was second to none. "I don't think there's any question," says Exner, "but that these data will have very high value for weather prediction and climate research."

It was a nationwide, interdisciplinary team that put together this experiment, dubbed GPS/MET for meteorological applications of GPS. Its nerve center is at UOP, headed by co-principal investigator Exner and spun off from the University NAVSTAR Consortium. (UNAVCO has been based at UOP since 1991.) Some of the know-how for GPS/MET came from UNAVCO's long-successful program, funded by the National Science Foundation, of sensing earthquakes and other geologic motions as small as a millimeter via GPS signals.

Like the Internet, GPS is a technology originally developed for U.S. defense goals that has spawned an unforeseen array of civilian applications. From precisely known points in space, the GPS satellites issue signals. If a receiving device catches four or more signals from these locations, it can calculate its own location in space or on earth. For example, some automobiles can now intercept GPS signals and overlay the car's position onto digitized road maps so that drivers can see exactly where they are. No matter how well the GPS machinery is engineered, the signals still have to pass through the ever-changing atmosphere. That poses a problem for navigation, but an opportunity for atmospheric science. As GPS signals pass through the atmosphere, they are affected by changes in density, a function of pressure, temperature, and moisture. The biggest changes occur near the ground, where moisture is most prevalent. Water vapor has an especially pronounced effect in bending the GPS signal path, which causes a measurable delay. The amount of water vapor above a given point on earth can vary more than tenfold, depending on local weather.

Meteorologists soon began to recognize the gold mine of data inherent in this "wet delay." By comparing the arrival times of signals sent to a ground-based receiver over a range of atmospheric conditions, scientists could infer the amount of water vapor present in a given column of air.

Even greater potential lay in the notion of using receivers in space. Researchers at the Jet Propulsion Laboratory (JPL) and Stanford University had studied the atmospheres of Mars, Venus, and Jupiter since the 1970s through radio occultation. In this technique, planetary-probe transmitters on a planet's far side sent signals to earth that brushed by the planet being studied so closely that they were bent and slowed as they passed in and out of the planet's atmosphere. The signal delay produced by the occultation was used to extract data on pressure and temperature.

Could the same approach work for our own atmosphere? In the early 1990s, Exner, UNAVCO director Randolph Ware, and a number of colleagues put together a plan to answer that question. If a receiver could be deployed in space, they figured, it could intercept a GPS signal that was occulted by the earth's atmosphere. With GPS transmitters stationed in space around the globe, occultations--and the atmospheric data they hold--could be gathered worldwide. The general concept had been suggested by JPL scientists as early as 1988 but had never been tested.

After its 1995 launch and a few days in orbit, the GPS/MET receiver came through. In its first full day of operation, the system successfully profiled the atmosphere above nearly 100 points from Greenland to Australia. The broad coverage brought cheer to the GPS/MET team, since it verified a key strength of the system. Radiosondes (instrument packages borne by balloon) have profiled the earth's atmosphere daily for more than 50 years, with hundreds launched across the globe every 12 hours. But few radiosondes can be deployed above the deep ocean or polar regions. "The value of GPS/MET," says Exner, "is that it gives you much of the information available from radiosondes, but with global coverage, and at a much lower cost per sounding." Moreover, weather balloons typically burst at heights of 25-30 kilometers (15-18 miles), while GPS/MET has obtained data from twice that high.

Other observing systems, such as rockets and spaceborne microwave sensors, each have their own limitations as well, making GPS/MET a useful complement to the existing mélange. Exner points out, "With a well-designed mix of observing systems, we could get more information for less money."

To assess their accuracy, the preliminary GPS/MET data were compared to readings from standard global analyses, rocket launches, radiosondes, and a microwave limb sounder from the Upper Atmosphere Research Satellite. Compared with the best available temperature analyses between 5 and 40 kilometers (3-25 miles), GPS/MET data differed less than 1°C on average. Below 5 km, it became more difficult for GPS/MET to separate the effects of temperature and moisture on the signal.

Mike Exner, GPS/MET program manager

However, if more GPS/MET receivers are launched, Exner foresees an improved ability to track and decipher signals closer to the earth's surface. Even now, computer models of the atmosphere are able to benefit from the data. Xiaolei Zou uses NCAR's primary mesoscale model to study midlatitude low-pressure systems and other phenomena. When she inserted the information from simulated GPS/MET signals during early tests, she found that it improved the model's overall picture of the atmosphere. Tests using actual data are now under way. The upper-level data from GPS/MET could be a boon in the model's handling of wind perturbations at high altitude that can work their way down and trigger the formation of storm systems days later.

Scientists across the world have been using GPS/MET data since the first promising signals arrived. There are now more than 200 investigators examining the data at universities and research centers in over a dozen nations. Their interests range from short-term weather forecasting to long-term climate monitoring.

High-altitude researchers may get a special boost. GPS/MET can gauge the number of electrons at different heights within the sparse atmosphere above 100 kilometers (60 miles). These measurements could prove vital as background data when, as Exner puts it, "the sun has a temper tantrum." Solar storms inject large numbers of high-energy particles into the upper atmosphere, jeopardizing power grids on earth and communications satellites in space. The GPS/MET data on electron density could provide baseline data for other satellite-based warning systems and help identify the best frequencies for various radio communications uses. By the turn of the century, billions of bytes of data could be telling us where our atmosphere is headed--a valuable spin-off from the signals that now tell us where we are.

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