This newsletter is a joint project of SIRS Publishing, Inc. and the University Corporation for Atmospheric Research

SPRING 1997 — VOLUME 4, NO. 3

Copyright | Note to Teachers

[Story] [Resources]

GPS/MET Maps an Ever-Changing Atmosphere   Earth's atmosphere holds a wealth of information that scientists want to analyze so they can better understand how the atmosphere at all levels affects climate, weather and radio communications. GPS/MET provides scientists with atmospher ic data on temperature, pressure, and moisture. Deployed 730 kilometers (450 miles) overhead, the GPS/MET receives signals from 24 Global Positioning Satellites orbiting the earth. Creating a map can be as easy as drawing lines from Point A to Point B, or as complicated as plotting the minute geographic detail of a topographic map, but how can anyone map the temperature and moisture of the air from the ground up to dozens of kilo meters into space? The answer lies in the abilities of earth-orbiting satellites that send signals through earth's atmosphere. Researchers at the University Corporation for Atmospheric Research devised a plan to use satellite signals to measure certain el ements of the atmosphere to better understand how atmospheric conditions affect climate, weather and even radio signals. A special laptop-sized instrument called GPS/MET (Global Positioning System/ Meteorology) is now flying 730 kilometers (450 miles) overhead, circling the globe every 100 minutes on the MicroLab-1 satellite that took off from Vandenberg Air Force Base in C alifornia, tucked into the nose cone of a Pegasus rocket. A Lockheed L1011 aircraft carried the rocket under its fuselage, launching it at 40,000 feet. The mission of the special instrument is to intercept signals from an array of 24 GPS satellites deployed 20,000 kilometers (12,000 miles) above the earth by the U.S. military. The goal: find out how these signals are distorted by the earth's atmosphere - and use that information to provide urgently needed global data on temperature and moisture. Thousands of meteorological profiles from ground level to 60 kilometers (37 miles) are sent to earth. "I don't think there's any question," says GPS/MET Project Manager Mike Exner, "but that these data will have very high value for weather prediction and climate research." Like the Internet, GPS is a technology originally developed for U.S. defense goals that has spawned an unexpected variety of non-military uses. From precisely known points in space, the GPS satellites transmit signals. If a receiving device catches four o r more signals from these known locations, it can calculate its own location in space or on earth. For example, many commercial truck drivers now intercept GPS signals and overlay a position computed from them onto digitized road maps so that drivers can determine their current location. Sailors also use GPS technology to determine their precise location on the open ocean.   A special laptop-sized instrument called GPS/MET (Global Positioning Satellites for Meteorology) is now flying 730 kilometers (450 miles) overhead, circling the globe every 100 minutes on the MicroLab-1 satellite that took off from Vandenberg Air force Base in California, tucked into the nose cone of a Pegasus Rocket. A Lockheed L1011 aircraft carried the rocket under its fuselage, launching it at 40,000 feet. Researchers above are pictured with the instrument, getting ready for launch. The rocket is in the background. 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, which is a function of temperature, pressure, and moisture. The biggest changes occur near the ground, where moisture is one of the most common elements. Water vapor has an especially pronounced effect in bending the GPS si gnal path, which causes a measurable delay. The amount of water vapor above a given point on earth can vary more than tenfold, depending on weather. (The signals are refracted, or bent, by the water vapor in the atmosphere in the same way that light is re fracted or bent at the surface of a pool of water. Objects placed in water appear to be in a different location than they truly are. Stop and look at a creek or an aquarium and see if you can notice this phenomenon next time you have the opportunity. Meteorologists soon began to recognize the gold mine of data 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. Water vapor is a key element of accurate weather prediction because it affects temperature as well as rainfall. Even greater potential lay in the idea 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 with a unique radio signal technique. 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. Researchers used this signal delay to extract data on and temperature. Could the same approach work for our own atmosphere? Exner and a number of colleagues formulated a plan to answer that question. If a receiver could be deployed in space, they figured it could intercept a GPS signal that was bent and slowed as it was bloc ked by the earth's atmosphere. With GPS transmitters stationed in space around the globe, occultations (the blocking of the signals by the atmosphere) and the data they hold could be gathered worldwide. JPL scientists suggested the concept as early as 19 88 but they never tested the concept. Exner and his colleagues tested the idea and found that it worked. After its 1995 launch and a few days in orbit, the GPS/MET receiver successfully profiled the atmosphere above nearly 100 points from Greenland to Australia. The broad coverage is a key st rength of the system because it is an improvement over the use of radiosondes. Radiosondes, which are instrument packages carried by balloons, have profiled the atmosphere daily for more than 50 years, with hundreds launched across the globe every 12 hours. However, few radiosondes are 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." Als o, weather balloons typically burst at heights of 25 - 30 kilometers (12 - 18 miles), while GPS/MET has obtained data from twice that high. Other observing systems, such as rockets and spaceborne microwave sensors, each have limitations as well, making GPS/MET a useful addition to instruments that are already in use. Exner points out, "With a well-designed mix of observing systems which inclu ded GPS/MET, we could get more information for less money." To be valuable, the data obtained from GPS/MET had to be accurate. To determine its accuracy, preliminary data was compared to readings from standard global analyses, rocket launches, radiosondes and an instrument from the Upper Atmosphere Research Satell ite. Compared to the best available temperature analyses between 5 and 40 kilometers (3 - 25 miles), GPS/MET data differed less than 1 degree Celsius on average. The system was accurate. Below 5 kilometers however, it was more difficult for GPS/MET to se parate the effects of temperature and moisture. "GPS/MET can provide high accuracy - better than any other space sensor, under most (but not all) conditions. But if accuracy were the only issue, GPS/MET would not be so interesting. It is the combination of high accuracy, high vertical resolution, all weather operation, global coverage and low cost that makes it so interesting," says Exner. 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. Researchers anticipate that the wealth of data will improve weather forecasting. Since forecasts are run on numerical computer models using equations that describe the motion of the atmosphere, ac curate numbers for current conditions will generate more accurate forecasts. GPS/MET data also has applications beyond weather and climate because atmospheric conditions significantly impact radio communications. GPS/MET can measure 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 communicati ons satellites in space. GPS/MET data on the density of electrons in the upper atmosphere could help identify the best frequencies for various radio communications and avoid power surges or radio blackouts." By the turn of the century, billions of bytes of data could be telling us where our atmosphere is headed - a valuable spinoff from the signals that now tell us where we are. RESOURCES [Story] [Credit and Copyright Information] Do you want to learn more about the atmosphere and satellites? Use the following keywords to research topics in your library or on the Internet: weather forecasting climate modeling Global Positioning Satellites radio communications weather satellites radiosondes weather balloons   You can also try the following Internet addresses. - For more information about GPS/MET: http://pocc.gpsmet.ucar.edut/ - The International Weather Satellite Imaging Center: http://www.t-e.k12.pa.us/~~dbaron/satellite/ - Skyviews - An Atmospheric Field Trip: http://162.127.88.3/skyviews/ - The Upper Atmosphere Research Satellite: http://uarsfot08.gsfc.nasa.gov/

Science Now is jointly published by the Walter Orr Roberts Institute at the University Corporation for Atmospheric Research and SIRS Publishing, Inc. (Social Issues Resources Series.) Science Now is published three times during the school year and is distributed to SIRS subscribers. Comments and questions should be directed to Joyce Gellhorn via Internet at jgellhorn@sprynet.com. You can also contact your SIRS representative or write to:

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Editor:
Caroline Hanson

Scientific Editor:
Mike Exner, GPS/MET

Contributors:
Bob Henson, UCAR Communications

UCAR is a consortium of over 60 universities in the U.S. and Canada with doctoral programs in atmospheric and related sciences. UCAR manages and operates the National Center for Atmospheric Research under the sponsorship of the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Anyone who undertakes any of the activities described herein shall do so at their own risk; UCAR and SIRS Publishing, Inc. assume no liability, whatsoever, for any injury or harm, which may result therefrom.

© COPYRIGHT 1997 UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH. ALL RIGHTS RESERVED.

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