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With COSMIC and company, radio occultation comes of age |
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Like a youth reaching adolescence, a new type of atmospheric monitoring has
kicked into high gear. Several low-orbiting satellites, all launched in the
last three years, have been intercepting radio signals from Global Positioning
System (GPS) satellites and inferring the atmospheric state along their paths.
Together, these systems produce several hundred profiles a day that slice through
the depth of Earth’s atmosphere, from the surface to the stratosphere and
beyond. Most of these data can be accessed through a UCAR-based Web site (see
sidebar). They’re being applied to everything from weather-prediction
models to the detection of climate change and solar storms.
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| This image compares the current global coverage of instruments launched via radiosondes each day (in [COLOR 1]) with the expected coverage from the COSMIC satellite network in a 24-hour period (in [COLOR 2]). |
Scientists involved in this far-flung discipline—nearly 100 in all, from a dozen countries—met at UCAR on 21–23 August to discuss the state of their science. The meeting was sponsored by NSF and Taiwan’s National Science Council. The two agencies are part of an eagerly awaited entry in the radio occultation fleet: the $100-million, six-satellite Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC, called ROCSAT-3 in Taiwan). The system will be launched late in 2005.
“After several years of planning and negotiations, COSMIC is on the go,” says Ying-Hwa “Bill” Kuo. The NCAR senior scientist and mesoscale modeling specialist heads up the COSMIC Project Office, which is based in the UCAR Office of Programs. Primary support for COSMIC is coming from Taiwan, while U.S. partners are contributing roughly $20 million plus critical in-kind support.
Originally set for launch in 2001, COSMIC found itself with a number of hurdles to clear, many related to the delicate politics of international satellite teamwork. By this spring, though, the final agreements had been signed and the players were all in place (see sidebar).
Birth of a discipline
Radio occultation was pioneered by the Jet Propulsion Laboratory (JPL) in the 1960s for studying the atmospheres of other planets, but it wasn’t applied to our own atmosphere until 1995. In April of that year, a MicroLab-1 satellite carried a low-cost GPS experiment into low Earth orbit (at about 750 kilometers or 450 miles). The satellite captured GPS signals after they’d passed through the atmosphere just above Earth’s horizon. From these occultations, scientists could infer the density of the atmosphere at each ray's tangent point to Earth and, with other data, the moisture, temperature, and pressure at various heights. UCAR’s Randolph “Stick” Ware was the principal investigator for the GPS/Meteorology project, with participation from JPL, the University of Arizona, and other partners.
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| Bill Kuo. (Photo by Carlye Calvin.) |
“GPS/MET was very successful,” says Kuo. “It showed that the radio occultation technique really works, especially in the upper atmosphere.” The system was particularly adept at spotting temperature variations through heights of roughly 5 to 25 km (3–15 mi). Analysis of GPS/MET soundings over China and elsewhere showed that the sharp temperature minimum that occurs at the tropopause may be profiled more crisply through radio occultation than by radiosondes.
GPS/MET soon attracted interest from Taiwan’s National Space Program Office. Eager to hone its expertise in satellite technology in an area of global importance, NSPO forged an agreement with UCAR in 1997 to plan COSMIC.
Meanwhile, emboldened in part by the success of GPS/MET, several other radio occultation projects got rolling. Germany’s Challenging Minisatellite Payload for Geophysical Research (CHAMP) went into space in July 2000, followed by Argentina’s Scientific Applications Satellite (SAC-C) in November 2000. Both of these single-satellite systems were developed in partnership with NASA, and both include a new generation of receivers that allow high-quality data to be retrieved from the stratosphere to near Earth’s surface.
Another new arrival is the Gravity Recovery and Climate Experiment (GRACE), a pair of satellites launched in May 2002 by the German Aerospace Center (DLR) and NASA. Although GRACE also collects atmospheric soundings, its main goal is to make ultra-precise measurements of Earth’s gravity field. Global warming and other aspects of climate change are expected to shift the mass balance of the atmosphere, as well as the ocean and even the land. Both COSMIC and GRACE will shed light on this process: COSMIC’s frequent observations of the shifting components of gravity should complement the high-precision baseline of GRACE. Already planning to analyze COSMIC’s gravity data is a team drawn from NASA Goddard Space Flight Center, National Chiao Tung and National Cheng-Kung Universities (both in Taiwan), and Ohio State University.
For all of these other systems, it takes a few days before researchers have access to data. COSMIC, in contrast, will be the first radio occultation system to make its products available in near–real time. Data will reach ground stations in Fairbanks, Alaska and Kiruna, Sweden every 15 minutes before being processed in Boulder and forwarded to data centers in Taiwan and elsewhere. The final products should reach users within three hours of observation—promptly enough so that day-to-day weather predictions could benefit as data are assimilated into computer forecast models.
What to do with an observational windfall
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| The constellation of six COSMIC satellites in low Earth orbit is surrounded by satellites from the Global Positioning System whose signals COSMIC will use. (Images courtesy COSMIC.) |
In some ways, COSMIC and its brethren provide an embarrassment of riches. For example, NCAR scientist William Randel and others have found evidence in the GPS/MET data for gravity waves kicked up by tropical thunderstorms. As these waves propagate upward, they appear to help trigger ionospheric disturbances that can affect satellite communications. COSMIC data have the potential to spot both the gravity waves and the ionospheric effects that follow—but the waves are too small to be individually tracked in today’s weather and climate models. Thus, researchers at NCAR, Kyoto University, and elsewhere will be working on process studies to create models that can handle these and other new types of detail from radio occultation.
Other projects will examine the deficiencies that remain in current and expected data. One of the biggies is “super refraction,” an error induced as GPS signals pass through a sharp gradient in air density (as when a moist boundary layer is overlaid by a dry upper layer).
The largely uncharted terrain of COSMIC should prove rewarding territory for
adventurous faculty and students. “This is the kind of thing universities
need to start thinking about now, so they can start writing proposals for 2004
and 2005,” says UCAR president Richard Anthes. In the same vein, NSPO chief
scientist Chin Lin told the workshop, “We’d like to encourage more
collaboration and send more graduate students to the U.S. We really appreciate
this opportunity.”
Participants left the Boulder workshop with a mandate to stay in touch in a
variety of ways: newly formed working groups, e-mail exchanges, special journal
issues, and possible sidebar meetings at the American and European Geophysical
Union conferences. Kuo noted that six other radio occultation meetings had been
been scheduled for the last half of 2002 and early 2003 in Austria, China, Denmark,
and Japan. A sign of burgeoning interest, or a red flag calling for better coordination?
Probably both, Kuo said. “There’s plenty of interest in this technology,
but more collaboration is needed—not just between Taiwan and the U.S.,
but globally.”
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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised:
Thursday, October 17, 2002 12:51 PM