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New type of weather satellite to improve marine forecasts

Jan 1, 2003

If there were a public relations competition between weather satellites and weather balloons, the balloons would lose … badly. Weather satellites are seen as high-tech, state-of-the-art eyes in the sky, the ultimate weather instrument. Satellites are even on TV — or the pictures taken by them are anyway. When was the last time you heard a weather person on TV news say, "And now let's look at today's weather-balloon picture"?

Yet, the case can be made that balloons are easily as valuable to weather forecasting as satellites, given their ability to gather vertical profiles of the atmosphere as they rise from the surface to high altitude. By contrast, most weather satellites generally capture a picture of conditions at a particular altitude or level of the atmosphere. Imaging satellites aren't good at obtaining vertical "cores" from high altitudes all the way to the ground. And as valuable as satellite pictures are to meteorologists, in many ways the bread-and-butter data is that information picked up by the weather balloons.

But now a satellite system called the Constellation Observing System for Meteorology, Ionosphere and Climate, which is currently under development via a U.S.-Taiwan partnership, will be able to do the same sort of vertical profiling that weather balloons do best. For sailors, this means more weather data and, it is hoped, better weather forecasts. A test satellite has been in orbit since 1995 and proved that the COSMIC technique works. "We know we can get highly accurate profiles of the atmosphere," said COSMIC's chief scientist, Christian Rockens. "We expect it to have a positive impact on weather predictions."

Unlike balloons, which carry weather instruments aloft, COSMIC will do its vertical profiling using radio waves. COSMIC will have six satellites in low earth orbit that will tune into the radio signals from GPS satellites, which are in much higher orbits more than 9,000 miles above them. By noting how the GPS radio waves are affected by the atmosphere, it is possible to get a narrow profile of atmospheric conditions.

The big advantage of such a system is that because there are no balloons to ready or release — because the measurements can be taken quickly and automatically — it will be possible for this constellation of COSMIC spacecraft to gather huge amounts of data, and maybe more important, COSMIC will provide more than 3,000 observations a day about the condition of the atmosphere over the oceans. Right now, weather balloons are released twice a day at hundreds of locations worldwide. Almost all those spots, however, are located on land. Vast areas of the world's oceans are never profiled by weather balloons. COSMIC offers the chance to gather data over these areas and to do so more than twice a day. "The effect will be most pronounced for marine areas, the Southern Hemisphere and the Arctic and Antarctic," Rockens said.

COSMIC data won't replace weather balloons, but it will be melded with the data gathered by the regularly released balloons. Weather balloons with their attached instrument packages, called radiosondes, have been sent aloft for decades, with the first used by the U.S. Weather Bureau in 1936. "The radiosondes are still the anchor for the global weather (data gathering) system," said Rockens, who didn't see the useful balloons being replaced anytime soon. Weather balloons are released at 70 stations in the United States every day at 0000 and 1200 Greenwich time. As a weather balloon rises, a basic radiosonde package measures temperature, humidity and pressure (as well as other optional parameters, such as ozone and radioactivity) and transmits this data to the ground station. (The balloons themselves undergo a fatal expansion — starting out 6 feet in diameter when released at the surface and swelling fivefold to more than 30 feet at their bursting point, typically at around 95,000 feet.) Many radiosondes can now be equipped with integral GPS receivers for determining the unit's lat/long and altitude.

While weather balloon radiosondes measure atmospheric conditions directly, the satellites of the COSMIC constellation will measure the behavior of radio waves as they pass through varying areas of electron density, air density, temperature and moisture. If the precise positions of the transmitting and receiving units are known, then the data can be derived. In this case, the GPS constellation becomes a doubly effective tool since the positions of the 24 GPS satellites are well known, and the positions of the COSMIC satellites can be determined very precisely by using GPS signals for navigation. This technique is called radio occultation (limb sounding) and it was developed by the Jet Propulsion Laboratory and Stanford University in the late 1960s to study planetary atmospheres. By analyzing this data, it is possible determine the conditions in the atmosphere. To accentuate the gyrations of the GPS signals as they pass through the atmosphere, these radio measurements are performed not when the GPS satellites are overhead, but at the horizon. This way, the radio waves travel through the thickest slice of atmosphere, yielding the maximum amount of data.

Unfortunately, sailors will have to wait a while before COSMIC can improve offshore weather forecasts. The system is not scheduled to be launched until spring of 2005. The satellites will be put in 216-nm orbits and eventually boosted to 430-nm orbits.

COSMIC has been developed by the University Corporation for Atmospheric Research, which is based in Boulder, Colo., and Taiwan's National Science Council and National Space Program Office. Other agencies involved in the development work include the National Science Foundation, the JPL, the Naval Research Laboratory, the University of Arizona, Florida State University, the University of Texas and the Orbital Sciences Corporation.