Early results from the lidar in space technology experiment (LITE) conducted by the US National Aeronautics and Space Administration (NASA Langley Research Center, Hampton, VA) highlight pollution plumes from 40 major urban centers worldwide, including 17 in North America. Two major plumes over California show the particulate emissions from the San Francisco Bay Area and from Los Angeles. Flown for the first time in space aboard the space shuttle Discovery last September (mission STS-64), the LITE experiment also provided a birds-eye view of typhoon Melissa as it moved across the Pacific Ocean (see Laser Focus World, April 1995, p. 92).
The spaceborne lidar system incorporates a laser transmitter module, receiver, and boresight assembly. Two flashlamp-pumped Q-switched Nd:YAG lasers comprise the transmitter module; both lasers are configured to emit simultaneously in the IR (1064 nm), green (532 nm), and UV (355 nm). Only one laser operates at a time, however; the other provides redundancy. The receiver includes a 1-m telescope for collecting return signals and a wavelength-selective optics package. Photomultiplier tubes are used to detect the 532- and 355-nm returns, while a silicon avalanche photodiode monitors the 1064-nm signals. The boresight assembly uses a two-axis motor-driven prism to align the laser beam to the telescope field of view so both point at the same column of atmosphere.
The LITE mission was designed as a technology test to validate the spaceborne lidar system by comparing its results with "ground-truth" data collected simultaneously from conventional airborne lidar equipment flying over the same terrain. During the mission, five aircraft in various locations made numerous flights synchronized with shuttle orbits. The tests involved scientists from all over the world. A total of 53 hours of lidar data was collected in 11 separate sessions. The data include measurements of various atmospheric phenomena, including cloud cover, volcanic particles in the atmosphere, and aerosols from pollution sources.
To obtain the California results, a team of scientists from the Canadian Atmospheric Environment Service (AES, Egbert, Ontario, Canada) working with National Research Council of Canada was based in Palm Springs, CA. The team conducted the conventional lidar "truth" experiments from a Convair 580 aircraft. As the Discovery traveled overhead, the aircraft flew under its path and collected data similar to that being gathered aboard the shuttle. Both sets of measurements were made during nine shuttle orbits over an eight-day period.
Other data obtained during the LITE mission showed a 200-mile-wide pollution plume of industrial haze stretching from eastern North America across the Atlantic Ocean along the "Gulf Stream highway" to the Azores (islands off the coast of Spain). While such long-range transport of pollution has been suspected for some time, the LITE experiment allowed definitive proof of such transport, according to Ray Hoff of the AES.
At a Baltimore, MD, meeting of the American Geophysical Union in May, Hoff said that LITE has given the scientific community, for the first time, the opportunity to observe large-scale hazy air masses in three dimensions. He characterized such results as extremely important in assessing the potential climate impacts of haze masses over wide areas of the globe.