From LITE to CALIPSO and Beyond: A Brief History of NASA Langley Research Center Spaceborne Lidar Missions and Measurements

NASA’s Lidar In-space Technology Experiment (LITE) operated on the Space Shuttle Discovery during September 1994. LITE was the first spaceborne lidar mission to study Earth’s atmosphere and provided the first global view of multilayer cloud structures, distributions of desert dust and smoke, and the planetary boundary layer. Prior to LITE, the knowledge gleaned from lidar measurements was limited to data acquired by airborne and ground-based systems during field campaigns and occasional site-specific extended-time operations. With LITE, the scope of the data collection shifted from local to global scales, and the instrument operations moved from hands-on interaction by an expert instrument scientist to a semi-autonomous approach commanded from the ground. This required the LITE teams to develop software techniques and specialized hardware to support mission operations and data processing/analysis techniques suitable for space-based lidar measurements. The LITE team succeeded and the results exceeded expectations. Not only did the LITE mission demonstrate that a lidar can successfully operate in space, it also showed that most high altitude clouds are optically thin and do not prevent the lidar from observing features closer to the Earth’s surface. LITE observations were used to study the atmosphere across important spatial scales, from small-scale aerosol and cloud interactions to large-scale cloud convective systems. Following LITE, NASA launched three additional spaceborne lidar missions: ICESat (Ice, Cloud, and land Elevation Satellite), which operated from 2003 to 2010; CATS (Cloud-Aerosol Transport System), which operated between 2015 to 2017; and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation), which was launched in 2006 with a forecast mission lifetime of 3 years and is still operational as of this writing in mid-2018. The ICESat mission produced measurements of ice sheet elevations and height profiles of clouds and aerosols. CATS operated on the International Space Station and extended the profile measurements of clouds and aerosols. The CALIPSO measurements have and continue to provide the vertical distribution of clouds and aerosols, cloud phase, aerosol type, and many other higher-level products derived from the lidar data alone or in combination with that from other instruments in the A-Train. Lessons learned from LITE were invaluable for the initial development of CALIPSO. While the CALIPSO team was able to reuse many of the LITE retrieval algorithms, the CALIPSO mission design presented several new challenges. Among the challenges in going from LITE to CALIPSO were (a) limited on-board power and data downlink bandwidth, which affected both the instrument and retrieval algorithm designs; (b) 24-7 data acquisition, which required the development of fully automated level 1 and level 2 retrieval schemes; and (c) the addition of depolarization ratio measurements for the 532 nm channel. Major benefits to the CALIPSO mission were the international partnership with CNES, the combination of active and passive sensors with the addition of the Imaging Infrared Radiometer and Wide Field-of-view Camera, and flying in formation with the A-Train. With over 2,000 publications using CALIPSO data, the mission has been a huge success. Work is currently underway in planning the next-generation spaceborne lidar. The 2017-2027 Decadal Survey for Earth Science and Applications from Space recommends cloud and aerosol lidar observations to achieve the objectives of the “Aerosols” mission, which nominally also includes a polarimeter, and is highly complementary with the “Clouds, Convection, and Precipitation” mission, which nominally includes a cloud radar and microwave radiometer. This presentation will highlight some of the many of the challenges encountered with the LITE and CALIPSO missions and will show how the lessons learned from early missions has been helpful in developing science objectives, instrument concepts, and retrieval approaches for NASA’s next-generation spaceborne lidar. It will also present an inside look at what it is like to support all stages of a spaceborne lidar mission from pre-planning to mission implementation.