In the 1970s, NASA put together groups to study the capabilities of lidar on satellite platforms. Because of the heavy-weight and high-power requirements for these early lidars, the obvious platforms for demonstrating lidar's capabilities were Spacelab and Shuttle. Many other workshops were held in the subsequent years of the late 1970s and early 1980s. Finally, after some delays in its development, primarily due to the Shuttle Challenger mishap, the Shuttle flight of LITE (Lidar In-space Technology Experiment) took place for 11 days in September 1994. This flight was truly a pathfinder mission for future space lidars, and ushered in a new era of remote sensing from planetary orbit. This flight showed the science community the exceedingly important data that a spaceborne lidar can provide.
Previously, lidar active remote sensing techniques in the U.S. were not able to receive approval and funding for flight in competition with passive sensors, which have been used since satellites first orbited Earth. Passive sensors, however, have great difficulty with vertically resolving and uniquely determining tropospheric species. It was obvious that the innate characteristics of lidars would provide a small footprint on the ground, i.e. high horizontal resolution, very high vertical resolution, a high sensitivity to aerosol measurements, and an excellent discrimination against noise because of laser spectral purity. Perhaps most importantly, these characteristics allow lidars to probe between clouds and penetrate through optically thin clouds and, therefore, profile the troposphere. Technology, however, held the lidars back from successfully obtaining long duration Earth-orbiting flights in the 1970s through the 1990s. Long-lifetime, laser power efficiency, cooling and weight issues had to be solved if lidars were to fly for long-duration on Earth-orbiting spacecraft. In the late 1980s and 1990s diode-pumped and long-lived ND-YAG lasers, and light-weight optics and structures, changed significantly the feasibility for lidar flights.
Coupled with the successes of LITE, and the successful laser altimeter flights of SLA (Shuttle Laser Altimeter) and MOLA (Mars Orbiting Laser Altimeter), lidars became competitive for spaceborne missions. Therefore, when new flight opportunities presented themselves, GLAS (Geoscience Laser Altimeter System) aboard Ice-Sat and CALIPSO (Cloud Aerosol Lidar and Infrared Pathfinder Observations) were accepted for flight through the proposal process, as was ESA's Aeolus Mission with the atmospheric doppler lidar ALADIN aboard. Ice-Sat was launched in January 2002, and CALIPSO in April 2006. ICESat's primary goal is to quantify ice sheet mass balance and understand how changes in the Earth's atmosphere and climate affect polar ice masses and global sea level. The GLAS instrument, unlike other altimeters, does have a lidar channel for height-resolved data and, therefore, is designed to make aerosol and cloud measurements. The Mercury Laser Altimeter (MLA), which is part of the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission is on its way to Mercury where it will make measurements of the topography of Mercury.
CALIPSO is flying in formation in a constellation of satellites called the A-train. The addition of data from the Aura constellation (the A-train), will be a challenge and an attribute. The challenge is to incorporate these data into a more complete and understandable data set, and to use the data for various modeling studies. Thus lidar has come of age and it is hard to imagine solving the myriad of atmospheric and climate problems without including lidar remote sensing alongside passive sensors.
This paper will describe present and future spaceborne lidars, and incorporate data applications to illustrate the steady progress and the important place lidar has become in today's remote sensing research.