The extended data set will help capture the pattern of year-to-year variations in cloudiness and the distribution of elevated dust layers that have been difficult to predict in climate models.

The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission is now operating on its redundant laser, which was designed as a contingency in case of a problem with the primary laser.

A slow pressure leak in the canister that housed the primary laser, which the CALIPSO team had been aware of since launch, made the switch necessary.

The backup laser, which stood ready for nearly three years, sent back its "first light" image on March 12. The instrument is now operational and undergoing a review of its calibration.

The release of standard data products from CALIPSO should resume in late April. Eventually data from mid-March on will be processed and the total gap in data products will be limited to about 10 days in early March.

CALIPSO makes unique vertical profile measurements of clouds and aerosols. Aerosols are suspended particles from fires, industrial activities and natural processes that are one of the least understood weather and climate variables. CALIPSO's lidar instrument measures the altitude and thickness of aerosol and cloud layers in the atmosphere.

It also measures particles' size, whether they are spherical or non-spherical and whether they are composed of water or ice. CALIPSO's observations complement data recorded by four other satellites flying in formation called the A-Train, which provide an unprecedented comprehensive global view of atmospheric chemistry and composition within eight minutes of one another over the same ground track.

First operational in June 2006, CALIPSO's primary laser generated more than 1.6 billion pulses of light and 20 terabytes of data that scientists from around the world are using to investigate our global integrated Earth system.

More importantly, the CALIPSO data are giving scientists new insight into processes that control how aerosols are formed and dispersed, how clouds form and dissipate, and how aerosols and clouds interact.

The CALIPSO team became aware before launch that the pressure canister that housed the primary laser had a slow leak. The decision to launch was made because it was expected the primary laser could still complete the three-year prime mission.

If it could not, the back-up instrument could take over. Early this year the laser showed unstable behavior consistent with low canister pressure. An investigation determined that turning on the backup laser now was the best solution.

"We designed the system with the ability to change to a back-up laser," said Chip Trepte, CALIPSO's project scientist at NASA's Langley Research Center. Trepte compared building in the second laser to carrying an extra flashlight on a long camping trip.

Very few lasers of the type used in CALIPSO had been flown in space before. It was unknown how it would perform in the hostile space environment. The back-up laser provided assurance that the mission would meet its objectives.

"The good news is we turned on the second laser that had been idle for three years, and it's working as well as the primary laser did early in the mission," Trepte said. "The pressure in the second laser canister is quite high, and it should be able to operate for many more years."

Throughout the CALIPSO mission, NASA and CNES have worked closely, from developing the mission, building the satellite platform and integrating the payload to monitoring the instrument and processing its valuable measurements.

"Even though we are on each side of the Atlantic, we work as a single, integrated NASA-CNES team," said Nadege Queruel, mission operations manager with the CNES team.

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