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UNAVCO research group:
GPS/MET and more

GPS, a network of 24-plus satellites beaming radio signals toward the earth, brings the centuries-old skill of triangulation into the space age. A GPS receiver uses the time differences between signals from four or more of the satellites to compute its latitude and longitude within a few tens of meters. Even more accurate measurements, at the millimeter level, can be obtained by measuring the differences in location between two or more receivers.

Today, when cheap (and not research quality) GPS receivers can be found in cars and even in backpacks, it seems obvious that the technology would have wide appeal. But GPS was designed in the early 1980s strictly as a military tool by the U.S. Department of Defense. Its value to civilians for uses such as shipping and surveying was apparent from the start, but only a few visionaries foresaw any scientific uses. UOP's Randolph Ware, now the director of UNAVCO, was one of them.

"I've talked with many of the inventors and developers of the GPS system, and none of them anticipated that it could be used with such high accuracy," says Ware. "It was a complete surprise, and they're all delighted."

Before the GPS satellites went up, researchers pioneered a measurement technique using 30-meter radio telescopes that pick up signals from quasars at the edge of the known universe. By comparing the times the signals are received at antennas on opposite sides of the earth, they could estimate the distance from one antenna through the center of the earth to another, at the centimeter level of accuracy. They used repeated measurements to observe the movement of the earth's tectonic plates.

With the advent of GPS, scientists realized that they could use these strong, local radio signals the same way they had been using the quasar signals. What's more, the signals could be received by a tiny antenna instead of a large one. Although the Department of Defense degrades the GPS signals to baffle possible enemy users, the degraded signals are no harder to work with than the truly random quasar radio bursts.

Because GPS receivers cost about $150,000 in those days, interested university users banded together to create the University Navstar Consortium, along with a small GPS support facility in Boulder, under the sponsorship of NSF's Directorate for Geosciences. The facility has grown modestly over the intervening years, along with the applications of GPS, and offers a range of equipment and services to NSF-sponsored investigators. And with the development and demonstration by university and UCAR scientists of methods to sense the atmosphere using GPS receivers, a new world of atmospheric research and applications is coming into view.

A GPS receiver. (Photo courtesy of UNAVCO.)

The new methods take advantage of the fact that the time it takes a GPS signal to pass through the air depends on the density and moisutre content of that air. The greater these factors are, the more the signal is delayed. A delayed signal is a problem when using GPS to observe tectonic plate motion, but earth scientists began to recognize that this "problem" could be the source of valuable data for atmospheric scientists.

Working with Michael Bevis and Steven Businger (both of the University of Hawaii), Christian Rocken, head of UCAR's GPS Research Group, showed several years ago that accurate estimates of precipitable water vapor--the amount of water vapor in a column of air--can be obtained using GPS. Using this technique, Rocken's group is analyzing GPS data from the NOAA Forecast Systems Laboratory (FSL) network of receivers in Oklahoma, Kansas, Colorado, and New Mexico, to provide hourly estimates of precipitable water vapor. The estimates are now being posted on the World Wide Web with a two-hour delay.

Even more detailed observations of water vapor are possible with GPS. One receiver can observe eight or more GPS satellites at any given time. By measuring water vapor along the path of each satellite signal (called slant-path water vapor, or SWV), it is possible to estimate the water vapor distribution above the horizon. Ware believes that SWV measurements have the potential to significantly improve weather forecasting by improving the initial fields of water vapor in numerical models.

To simplify meteorological GPS uses, UNAVCO promoted the development of an innovative little package called CLAM, for Climate and Meteorological sensor. Available from several vendors, the CLAM turns a commercial GPS receiver into a mini-weather station, providing measurements of surface pressure, temperature, and precipitable water vapor.

Besides gleaning information from the hundreds of GPS receivers on earth, observers can gain much-needed global measurements from 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 occultations. In this technique, satellite-borne transmitters on, for example, the far side of Mars shot radio signals back to earth just past Mars' surface but well within its atmosphere. The Martian atmosphere bent and slowed the signal as it passed through, and the time delay was used to extract data on Mars' air pressure and temperature.

If occultation worked for other planets, researchers figured, it should work for ours. With 24 GPS satellites, plus spares, transmitting signals from around the earth, one spaceborne receiver could gather occultations for almost the entire global atmosphere.

UCAR initiated the GPS/MET Program in June 1993 under the leadership of Ware and Mike Exner to demonstrate the radio occultation method in earth's atmosphere using GPS. The GPS/MET receiver was launched in April 1995; it now has provided a year and a half of atmospheric profiles at thousands of locations around the globe. Comparisons of GPS/MET data with those obtained by other types of sounders have shown that the new sounding system is quite accurate: compared with the best available temperature analyses between about 7 and 40 kilometers, GPS/MET data differed less than 1°C on average. At lower altitudes, the current GPS/MET soundings encounter increasing difficulties. However, Exner believes these difficulties can be overcome using improved receivers and software.

But refining the atmospheric uses is only a part of UNAVCO's research agenda. The earth sciences applications of GPS are also spreading, as UNAVCO's member scientists develop techniques to reach greater and greater surveying precision. This work builds on the work of the International GPS Service, a consortium of universities and research institutions operating a truly global network, with receivers in such remote locations as Easter Island and Antarctica. Correcting the military's degraded signals, the International GPS Service determines GPS satellite orbits to within 5 cm. Using these orbit data, and other refinements in data collection and processing techniques, UNAVCO scientists have obtained astonishing submillimeter surveying accuracy over a baseline distance of 50 km.

Earthquake-prone Japan is particularly active in using GPS technology, with a network of 650 receivers, soon to expand to 900. The location of each receiver is recorded continuously. UNAVCO's GPS Research Group In response to a request from Japan's Geographical Survey Institute, UNAVCO's GPS Research Group is assisting with operational analyses of these data. Changes in receiver locations indicate that the four tectonic plates that underlie the Japanese islands are moving, creating tremendous strains that may presage earthquakes. The sturdy Japanese receiver stations have survived several large earthquakes in the last few years, recording previously unknown phenomena in connection with the quakes themselves. Unfortunately, they have found no significant precursors that might lead to prediction.

Summing up the reasons GPS technology has been so successful, Ware says, "The GPS system is based on atomic clocks providing the most accurate measurements that humans know how to make. Taxpayers have already invested more than $10 billion in the orbiting GPS satellite beacons, and manufacturers have probably invested another billion in GPS receiver development. This provides a very powerful worldwide system that doesn't need to be calibrated. It's poised to have a huge impact on science."

For further information, contact Ware (303-497-8005 or ware@ucar.edu) or visit UNAVCO on the World Wide Web.

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 10:18:15 MDT 2000