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CALIPSO Takes Its One-Billionth Measurement

On Sunday morning at about 8:17 Eastern Time, a laser pulse beamed down from 700 kilometers above Ivory Coast and took a 100-meter-wide picture of the atmosphere. The light reflected back to the satellite from the tiny particles in the air or clouds offers information on how they warm or cool the Earth.

It was Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) doing its job.

For the one billionth time.

CALIPSO uses a laser technique called lidar to measure clouds and aerosol layers

Image above: CALIPSO uses a laser technique called lidar to provide data on the thickness and composition of clouds and aerosol layers and the altitudes at which they reside. Credit: CNES.

The occasion was on CALIPSO's 9,491st orbit of Earth. Since its launch, on April 23, 2006, the satellite has traveled 260,198,685.3 miles. It has generated data that would fill almost 3,500 DVDs or more than 24,000 CDs, delighting scientists and researchers from 35 countries who are using the information to prove any number of theories about the atmosphere, about weather, about the past and how it will relate to the future.

"We're beginning to be able to quantify effects of aerosols on the climate," says Dave Winker, principal investigator of CALIPSO at Langley Research Center.

Those measurements are answering questions on the Earth's warming and cooling patterns, and they are prompting scientists to ask questions about weather. The answers and the questions are products of understanding where aerosols are hindering sunlight, where high ice clouds actually warm the atmosphere and where lower stratus clouds cool the earth.

The key is CALIPSO's unique vertical measurement capability, which offers data on the thickness and composition of clouds and aerosol layers and the altitudes at which they reside. It is the product of space-borne Light Detection and Ranging (LIDAR), similar to radar but using light instead of microwave energy.

CALIPSO has proved its value.

"Before, we had ideas and you could run a model and predict certain things, but you didn't know how close to reality that was," said Winker. "Now, with those three-dimensional measurements of aerosols, we're beginning to really be able to quantify these impacts where we couldn't before."

Unlike satellite cameras, CALIPSO's lidar isn't hindered by darkness.

"What we're doing is looking at parts of the globe that have been kind of hidden to other satellites," says Chip Trepte, CALIPSO's project scientist. "During the winter, for example, the North Pole is dark. Other satellites just can't make reliable measurements there."

A collection of lidar profiles from CALIPSO

Image above: A collection of lidar profiles from CALIPSO gives scientists a better understanding of the polar atmosphere. Credit: NASA

That makes CALIPSO's data particularly timely because of the reported effects of global warming, particularly at the Earth's poles.

"We're seeing changes in particle and cloud distribution that haven't been seen," Trepte says. "We're doing new things, pioneering activities. Opening up new vistas, so to speak."

While still learning to use the new data, scientists and researchers are beginning to take those vistas to conclusions that have become more logical. Pollution over Asia was tied to climate effects worldwide in a publication from the University of Western China.

Hurricane formation could be hindered by dust from North Africa blowing into the South Atlantic and scattering sunlight.

A colleague at Langley, Yong xiang Hu, used CALIPSO data to determine wind speed on an ocean's surface.

"We didn't design CALIPSO to do this, and we weren't sure that we'd be able to reflect the returns from the ocean surface or if they would be too big to measure," says Winker. "It turns out that we can."

It's an example of the evolution of uses of CALIPSO's data. It's keeping Winker, Trepte and other members of the satellite's team busy.

"Because this is a new kind of data, new satellite data that nobody's seen before, there isn't a general understanding of how to interpret it," Winker says. "There's a chance that it can be misinterpreted, so we try to stay in touch with the research as much as we can."

But even the people who conceived CALIPSO are being surprised.

"We know how the data is intended to be used, but we're still exploring the limitations," Winker says. "There's an area where you've got signal and you know what you're looking at. And there's an area where you've got noise, and you don't pay any attention to that. But there's always a gray area, and people always try to push into that gray area and that's where it gets tricky.

"You try to figure out: can you really believe this or is it just something odd going on that nobody can explain?"

An even more complicated area is on the data horizon. CALIPSO is part of the "A-Train," a constellation of five satellites with more to come. CALIPSO flies only 15 seconds behind CloudSat, which produces complementary data with its cloud-profiling radar.

But combining the data of two or more satellites is also an evolutionary process.

"At this point, people aren't doing a lot of that," says Winker. "They're just focusing on CALIPSO. But they have to start with one instrument and understand that. After you've done that, you can start combining measurements of different instruments. We're still in the process of doing that."

It's part of the process of learning to use new tools.

And while they are being used, more tools are being added. The one billionth pulse of LIDAR is one benchmark. The second anniversary of CALIPSO's producing data is June 7 of this year.

"The first big milestone was a year, because you want to be able to measure all four seasons," Winker says. "After two years, then you start to see how things vary from year to year. … You want at least two years, and hopefully three or four to get a sense of what's normal and what's not normal."

CALIPSO has almost two years with the original LIDAR still pulsating. A backup LIDAR has not yet been used, and hope is strong that the satellite will continue its work long past its originally forecasted three years.

For now, though, "the billionth is a metric," Trepte said. "It's a hallmark of a lot of hard work that went into it and deserves recognition. It was the collective efforts of many, several of the NASA centers, leadership at headquarters and certainly our international, university and industrial partners, namely the Centre National d’Etudes Spatiales, Ball Aerospace, and Fibertek."

And the one billionth is a benchmark for scientists who are just beginning to understand the answers that CALIPSO is giving them.

Jim Hodges
NASA Langley Research Center