One key to understanding the atmosphere is the ability to study its components, including clouds (liquid), aerosols (suspended particles), and ozone and water vapor (gases). Researchers at NASA Langley use laser-based systems called lidars (light detection and ranging) to study the atmosphere with high precision. A lidar can penetrate thin or broken clouds in the lower atmosphere, where humans live, letting researchers "see" the vertical structure of the atmosphere. A space-based lidar can provide global measurements of the vertical structure of clouds and atmospheric gases. Both ozone and water vapor are involved in many important atmospheric processes that can affect life on Earth, climate change, weather, the Earth's energy budget, and regional and global pollution levels.
Perhaps the greatest value of lasers as remote sensing tools is the unprecedented accuracy with which they can measure clouds. The latest advancements in laser remote sensing can fill the gaps we have in our understanding of how clouds reflect and absorb solar energy, and how heat and moisture are exchanged between the air, ocean and earth.

How Does A Lidar Work?
A lidar is similar to radar, which is commonly used to track everything from airplanes in flight to thunderstorms. Instead of bouncing radio waves off its target, however, a lidar uses short pulses of laser light to detect particles or gases in the atmosphere. Traveling as a tight, unbroken beam, the laser light disperses very little as it moves away from its origin -- such as from space down to the Earth's surface. Some of the laser's light reflects off of tiny particles -- even molecules -- in the atmosphere. The reflected light comes back to a telescope and is collected and measured. By precisely timing the collected light, and by measuring how much reflected light is received by the telescope, scientists can accurately determine the location, distribution and nature of the particles.
A lidar carries its own source of laser light, which means it can make measurements both in the daytime and at night. The result is a revolutionary new tool for studying what's in our atmosphere - from cloud droplets to industrial pollutants - many of which are difficult to detect by other means.
Remote Sensing Lasers in Space
In September 1994, NASA launched the Lidar In-Space Technology Experiment (LITE). LITE was the first use of a lidar system for atmospheric studies from space. LITE orbited the Earth while positioned inside the payload bay of the Space Shuttle Discovery (STS-64). During the ten-day mission, LITE measured the Earth's clouds and various kinds of aerosols in the atmosphere for 53 hours. 
Because this type of lidar had never flown in space before, the LITE mission was primarily a technology test. Scientists and engineers wanted to verify that the entire system worked as plannedwhile on orbit.
An important secondary goal of the LITE mission was to explore the applications of space-based lidars and gain operational experience for a future satellite-based lidar system. Such a satellite could provide continuous global atmospheric data.
NASA Langley researchers are now exploring the feasibility and potential advantages of using lidar instruments on Earth-observing satellites.

The Future of Laser Remote Sensing
LITE and LASE collected data on a wide range of phenomena, from aerosols in the upper atmosphere, to cloud droplets, pollutants and ozone in the lower atmosphere. Future lidar instruments will be tailored to more specific purposes. While one instrument studies the vertical structure of clouds, another will track urban smog or desert dust storms; all of which affect Earth's atmosphere, and, in turn, its weather and climate.
Only by gathering more accurate information can scientists improve their understanding of the atmosphere to the point where they can confidently predict its behavior, and determine how it is being affected by human activities. This improved understanding would enable us to prepare for natural phenomena and take stronger measures to protect Earth's irreplaceable atmosphere. LITE, LASE and their successors will continue to make unique and valuable additions to our understanding of the Earth's atmosphere.