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Validation Exercise Asks, "May I Have This Dance?"


The satellites are in orbit. The instruments are turned on. The scientists kick back and watch the data flow in.


NASA's CloudSat and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) spacecraft launched April 28 from Vandenberg Air Force Base, Calif. Together, they're returning never-before-seen images of clouds and aerosols, tiny particles suspended in the air.


Information from the CALIPSO and CloudSat satellites is coming in -- loud and clear -- and scientists from both missions report they are very pleased with the initial results.


If both satellites are looking at clouds and aerosols from 438 miles above Earth, how do the researchers know these long-distance observers are getting it right?

CALIPSO CloudSat Validation Experiment Logo

First, NASA and its partners spend years making sure all of the hardware, software, solar panels, batteries, -- in other words everything -- works before launch. After that, it's the calibration-validation two-step also known as the CALIPSO and CloudSat Validation Experiment. The experiment ensures that the data collected from the satellites' observations is correct. It's this information that will provide new insight into the global distribution and evolution of clouds and aerosols that will lead to improvements in weather forecasting, air quality assessments and climate prediction.

Image right:The CALIPSO and CloudSat Validation Experiment is a coordinated effort among researchers around the world seeking to calibrate and validate the data from these two new satellite missions.

Closer to the surface of our home planet are three aircraft: the NASA's Dryden Flight Research Center's ER-2, NASA's Langley Research Center's B-200, and a Learjet leased from Weather Modification, Inc. With support crews and CALIPSO and CloudSat research teams, the planes headed to Robins Air Force Base, Ga., for the validation experiment. All were outfitted with a suite of instruments similar to those on the satellites.

The high-flying ER-2 (a civilian version of the U-2 spy plane) flies above the clouds around 65,000 feet while the B-200 and the Learjet fly closer to ground -- in and around the clouds and aerosols.

At some point, comes a well-choreographed aerial ballet of planes and satellites.

Chip Trepte, CALIPSO mission scientist at NASA's Langley Research Center, Hampton, Va., explains the dance this way, "The reason we need aircraft is because clouds and layers of aerosols can change within minutes and so synchronized measurements are needed between instruments."

In addition, aircraft can fly directly along the satellite track. That comes in handy considering that CloudSat's super sensitive Cloud-Profiling Radar which is more than 1,000 times more sensitive than typical weather radar and can distinguish between cloud particles and precipitation, has a relatively narrow ground "footprint" … just about a mile wide. And CALIPSO's lidar has even a smaller footprint -- the length of a football field. A lidar is a device similar to radar but it emits pulsed laser light instead of microwaves to collect vertical profiles of aerosol and clouds.

"We also fly to areas along the track where interesting weather occurs," Trepte added. "With ground observing sites we would have to wait until the appropriate weather or clouds and aerosols were co-located with the satellite overpass. Plus, we need an aircraft -- the ER-2 -- that could fly above the highest clouds so that our measurements would have a similar viewing geometry -- sharing a down-looking view."

So following CloudSat's and CALIPSO's lead, their jet and propeller-driven partners fly through the same aerosols and clouds the satellites are looking at; then the scientists and engineers compare, validate and calibrate from the results.

CALIPSO CloudSat Validation Experiment -- Data from CALIPSO and the ER-2 Image above:This graphic shows a flight comparison between CALIPSO data (middle) and data taken from the cloud physics lidar on the ER-2 (bottom) from the same time a location on August 12. The image at the top is infrared data from a GOESS satellite illustrating the cloud cover that both the CALIPSO and the ER-2 lidars observe. The flight path of the ER-2 and the CALIPSO track can be seen at the right of the image as a solid line.

In some cases, the instruments on the aircraft may be superior to their satellite counterparts in orbit. The High Spectral Resolution Lidar (HSRL) on the B-200 is considered to be more technologically advanced than CALIPSO's lidar, according to Chris Hostetler, HSRL science lead at Langley.

"In addition to being a valuable science and validation instrument, HSRL is a prototype for the next generation of aerosol and cloud lidar," says Hostetler. "Because the HSRL is based on a more advanced technique, it acquires the 'truth' data set needed to validate the data products from CALIPSO."

Before any of the aircraft take-off, there are the real-world considerations of forecasting weather, airspace coordination, instrument preparation, aircraft pre-flight activities, and numerous other issues requiring prompt decision-making and difficult priority calls. So says Mike Wusk, B-200 flight test engineer from Langley.

"The task of putting two aircraft in the air over the same spot may appear to be easy, but the reality is that it takes a lot of teamwork and coordination to make the mission a success," says Wusk. "The satellite has a set path and speed, but the aircraft must deal with differing winds, weather, traffic, performance issues, and other operational constraints."

But back to the question: why are satellites important to NASA's Earth observing program to begin with? Balloons and ground-based instruments require a lot of labor and are only point measurements. Aircraft have operational limits, and there's the human factor.

"Satellites can observe most of the globe with the same instrument," Trepte said. "This eliminates many uncertainties that can occur with different instruments and how they are measured. People can take measurements slightly differently from each other that can introduce artifacts -- that is -- something that 'appears' to exist in the collected datasets."

Satellites also have the advantage of constant, comprehensive, routine measurements -- day and night, seven days a week, 12 months a year. But again, the validation and calibration exercises like CALIPSO-CloudSat's are necessary and don't end in Georgia. The data will have to be analyzed to determine the sensitivity of the satellites' instruments and the accuracy of products.

the CALIPSO CloudSat Validation Experiment team Image left:Some team members from the CALIPSO and CloudSat Validation Experiment pose in front of the hanger at Warner Robins Air Force Base, Ga.

"We've satisfied almost all of our pre-mission objectives for both CALIPSO and CloudSat validation," Trepte said. "We've sampled air with enhanced aerosols - possibly dust from the Saharan desert or urban aerosols from southeastern U.S. states. We've seen simple cirrus cloud structures and complex multilayer clouds composed of water drops, ice crystals, and a mixture of both. We've sampled fair weather cumulus clouds and thunderstorms -- the mission went well."

Mike Wusk agrees. "The aircraft missions out of Warner Robins AFB were great thanks to the wonderful support of the Air Force and Georgia Air National Guard personnel assisting us," added Wusk. "The FAA Air Traffic Centers were extremely helpful in coordinating our flights along the required paths and altitudes, in spite of the large geographic area we needed to cover."

Next up for Wusk and the B-200 is participation in the multi-aircraft campaign supporting the Texas Air Quality Study over Houston. For Trepte and the other mission scientists, they'll blend the various data sets and make scientific sense of the information collected.

And at some point down the validation road, there'll be another data-dance party for the planes, satellites, aircrews, and scientists.

Related Links:

+ NASA's CALIPSO Mission
+ NASA's CloudSat Mission

Chris Rink, 757-864-6786
NASA Langley Research Center, Hampton, Va.

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