After more than 16 years of operation, NASA’s Earth Observing–1 (EO-1) spacecraft was decommissioned on March 30 of this year. The Advanced Land Imager (ALI) developed by MIT Lincoln Laboratory was aboard as an alternative to the land-imaging sensor that was used by the Landsat Earth-observing program.
ALI not only achieved higher image resolution and quality, it also exhibited greater sensitivity and dynamic range, and realized higher radiometric accuracy. Moreover, compared to the Landsat imager, ALI weighed 24 percent less, occupied one-third less space, consumed 80 percent less power, and cost less to build.
ALI has collected more than 90,000 images, many of which were groundbreaking: the first mapping of a lava flow from space and the first tracking of regrowth of an Amazon forest as seen from space. During its lifetime, ALI has captured many dramatic scenes — depictions of the ash deposits left by the 2001 World Trade Center attacks, the flooding caused by Hurricane Katrina in 2005, and the eruption of Momotombo volcano, Nicaragua, in December 2015.
“A most significant Lincoln Lab effort was the optomechanical redesign of the telescope structure using three pieces of Invar. The initial intent of an outside vendor was to use an all-silicon carbide design that was found could not be implemented. In a very short period, so as not to compromise a very demanding schedule, Vin Cerrati and Keith Doyle of the [then] Optical Systems Engineering Group redesigned and analyzed the structure to efficiently support the optics and focal plane,” said Steven Forman of the Laboratory’s Engineering Division.
Five days after EO-1’s launch on November 21, 2000, ALI captured its first images of land. Those images showed remarkable detail of Sutton, Alaska, a small town wedged in a dark valley. Later that day, ALI collected imagery of east Antarctica, the Marshallese island of Roi-Namur, and north-central Australia.
One of the objectives of the ALI demonstration was to evaluate its imagery against that of the Landsat 7 instrument. Thus, EO-1 was maneuvered into orbit to trail Landsat 7 by one minute as it completed 14 orbits each day and repeated the collections every 16 days. Comparison of the ALI and Landsat performances on imaging the same regions at virtually the same times confirmed that the new imager could image Earth at the same level of detail (30 meters per pixel) as the Landsat sensor; however, ALI’s set of sensors enabled sharper, photo-like images once the data were processed at the ground station.
“The Advanced Land Imager employed a new architecture that eliminated the Landsat scan mirror and implemented new technologies, such as large, modular focal plane arrays and wide-field-of-view optics,” said William Brown, the head of the Aerospace Division at the time of ALI’s development. To reduce the optical diameter of the sensor, and thus its weight, the Laboratory’s researchers increased the number of detectors in the focal plane array. This choice allowed a “push-broom” approach to scanning a wide swath of Earth each day. The Landsat system had employed a sensor that collected data in a “whisk broom” mode, i.e., using a single camera that focuses on narrow section of a scene. Such a whisk broom sensor is heavy and expensive, requiring large moving parts that are difficult to stabilize. “By building a focal plane that could be used as a ‘push broom’ to collect the data as the satellite flies along the ground track, the ALI team demonstrated that the necessary data could be acquired with an instrument that had no moving parts This was a groundbreaking technology advance,” said Grant Stokes, head of the Laboratory’s Space Systems and Technology Division. In addition, ALI used detectors fabricated from different materials to enable the use of several spectral bands for comprehensive imaging of objects and topography, and the ground data system was automated to permit one operator to quickly acquire and process ALI data.