In 1972, National Aeronautics and Space Administration (NASA) and the United States Geological Survey (USGS) launched the first satellite of their joint ‘Landsat’ program, ERTS-1 (later renamed Landsat I). This satellite was the first of its kind: a satellite specifically designed to image the physical Earth. On board the satellite were two sensing instruments that would be used to accomplish the task: a Return Beam Vidicon (RBV) and a Multi-Spectral Scanner (MSS). The RBV was presumed to be the primary sensor, with the MSS serving as a back-up, experimental sensor. However, it was the MSS, designed by Virginia Norwood and her colleagues at the Hughes Aircraft Company, that would change earth science.
This post will discuss the key component of the MSS: its oscillating scan mirror. This mirror and its design is the subject of a new Geography & Map Division collection, the “Virginia T. Norwood Papers,” which is comprised of a physical 3D prototype of the oscillating scan mirror and a collection of working papers written between 1968-1970, documenting the design of the mirror. The collection contains Hughes Aircraft Company memos, progress reports, notes, and diagrams, all of which was circulating between Norwood and her colleagues.
The oscillating scan mirror needed to perform a critical function: as the Landsat satellite moved around Earth, the mirror would swing back and forth, scanning the Earth in careful lines on a track perpendicular to the flight path of the satellite. To keep up with the rate at which the satellite was orbiting Earth, six lines at a time would be scanned. The mirror itself would reflect light from the Earth into an optical sensor mechanism, which would then split the light into four spectral bands (distinct wavelengths of light that can be measured by sensors). Visible red, visible green, and two infrared bands were collected, which are useful for visualizing vegetation and the boundaries between land and water. Energy was also directed into a multiplexer which created 6-bit digital data, sent back to receiving stations on Earth that were capable of translating that data back into four spectral band images. All of this depended on a mirror moving back and forth at a carefully calibrated rate to account for the motion of the satellite itself, ideally making sure that each “pass” of scanning the earth would overlap (just slightly) with the one before. Norwood enlisted Hughes Aircraft Company employee Webb Howe to help design the mirror.
Much of the documentation contained in the “Virginia T. Norwood Papers” details the incredible scientific and engineering work that went into achieving a functioning mirror scan mechanism. One of the biggest challenges detailed across the collection is the mirror’s “jitter problem” – making sure that the mirror moved back and forth in a smooth motion, at just the right rate. Another challenge was combating any distortion from the mirror. Years of testing, design, and development were required. The papers include detailed design schematics and reflect the intensely detailed scientific process required to produce a working mechanism.
According to NASA, the final scan mirror design was 9×13 inches (much larger than the prototype model). It operated at 13.62 Hz, moving back and forth “more than 13 times per second,” creating a loud banging sound as it operated. After the launch of Landsat I, the first images returned from the multi-spectral scanner outperformed the Return Beam Vidicon. Crucially, Norwood’s decision to encode the multi-spectral scanner digitally meant that Landsat data could be processed by computers, opening the door for radiometric calibration, quantitative analysis, and the modern field of remote sensing.
It’s hard to overstate the impact of the Landsat program: Landsat data allows scientists to understand land use and land cover change, including processes such as urbanization, deforestation, crop monitoring, monitoring and management of natural disasters (such as floods and wildfires), and more. Landsat 9 launched in 2021 with an Operational Land Imager (OLI) and Thermal Sensor (TIRS) instruments on board, and Landsat Next is planned to launch in 2030 or 2031, with a plan to acquire 26 spectral bands.
For more information on the images captured by the MSS, see a previously published Worlds Revealed blog post, “The World From ERTS-1.”
Sources Consulted/Further Reading:
- “Virginia T. Norwood: The Mother of Landsat,” by Laura E.P. Rocchio, National Aeronautics and Space Administration, 2020
- “Multi-Spectral Scanner” by the National Aeronautics and Space Administration
- “Landsat I” by the United State Geological Survey
- “Earth as Art 3: A Landsat Perspective,” Library of Congress