At 11:36 am on April 15, 1999 the latest member of the Landsat family was Launched into orbit. Landsat 7 will continue the flow of global change information to users worldwide. Scientists use Landsat satellites to gather remotely sensed images of the land surface and surrounding coastal regions for global change research, regional environmental change studies and other civil and commercial purposes.
No other remote sensing system, public or private, fills the role of Landsat in global change research or in civil and commercial applications. Landsat 7 will fulfill its mission by providing repetitive, synoptic coverage of continental surfaces; spectral bands in the visible, near-infrared, short-wave, and thermal infrared regions of the electromagnetic spectrum; spatial resolution of 30 meters (98-feet); and absolute radiometric calibration.
The continuation of the Landsat mission is important for several reasons. The repetitive, broad-area coverage is needed for observation of seasonal changes on regional, continental and global scales. Other systems afford frequent global coverage, but none provide this global coverage at the 30-meter (98-feet) spatial resolution of the Landsat Thematic Mappers. Unlike the ocean and atmosphere, characterizing the land surface is distinguished by high spatial frequency processes that require a high spatial resolution. Both man-made (deforestation) and natural changes (glacial recession) are often initiated at scales requiring high resolution for early detection.
The Landsat 7 system offers the unique capability to seasonally monitor important small-scale processes on a global scale, such as the inter- and intra-annual cycles of vegetation growth; deforestation; agricultural land use; erosion and other forms of land degradation; snow accumulation and melt and the associated fresh-water reservoir replenishment; and urbanization. The other systems affording global coverage do not provide the resolution needed to observe these processes in detail and only the Landsat system provides a 26-plus year record of these processes.
Landsat History
Images acquired by Landsat satellites were used to produce the first composite multi-spectral mosaic of the 48 contiguous United States. Landsat imagery has provided critically important information for monitoring agricultural productivity, water resources, urban growth, deforestation, and natural change due to fires and insect infestations. The data have also been used successfully for mineral exploration, to measure forest cover at the state level, and to monitor strip mining and strip mine reclamation.
The first Landsats, originally called ERTS for Earth Resources Technology Satellite, were developed and launched by NASA between July 1972 and March 1978. During that time, a second generation of Landsat satellites was developed. Landsat 4 was launched in July 1982 and Landsat 5 in March 1984. Landsat 5 is still transmitting images. Landsat 7 was authorized by a Presidential Directive signed by President Bush in 1992.
The 1992 Land Remote Sensing Policy Act identifies data continuity as the fundamental goal of the Landsat program. The scientific mission of Landsat 7 is entirely consistent with this legislated goal. The mission is to extend and improve upon the more than 26-year record of images of the Earth s continental surfaces provided by the earlier Landsat satellites. The continuation of this work is an integral component of the U.S. Global Change Research Program. Landsat 7 also will continue providing essential land surface data to a broad, diverse community of civil and commercial users.
Mission Facts
Landsat 7 was launched on April 15, 1999 from the Western Test Range at Vandenburg Air Force Base on a Delta-II expendable launch vehicle. At launch, the satellite weighed approximately 4,800 pounds (2,200 kilograms). The spacecraft is about 14 feet long (4.3 meters) and 9 feet (2.8 meters) in diameter. It consists of a spacecraft bus, built by Lockheed Martin Missiles and Space in Valley Forge, Pa., and the Enhanced Thematic Mapper Plus (ETM+) instrument, developed by Raytheon Santa Barbara Remote Sensing in Santa Barbara, Calif.
The ETM+ instrument is an eight-band multispectral scanning radiometer capable of providing high-resolution imaging information of the Earth s surface. It detects spectrally-filtered radiation at visible, near-infrared, short-wave, and thermal infrared frequency bands from the sun-lit Earth. Nominal ground sample distances or "pixel" sizes are 49 feet (15 meters) in the panchromatic band; 98 feet (30 meters) in the 6 visible, near and short-wave infrared bands; and 197 feet (60 meters) in the thermal infrared band.
The satellite will orbit the Earth at an altitude of approximately 438 miles (705 kilometers) with a sun-synchronous 98-degree inclination and a descending equatorial crossing time of 10 a.m. The orbit will be adjusted upon reaching orbit so that its 16-day repeat cycle coincides with the Landsat Worldwide Reference System. This orbit will be maintained with periodic adjustments for the life of the mission. A three-axis attitude control subsystem will stabilize the satellite and keep the instrument pointed toward Earth to within 0.05 degrees. The Landsat World-Wide-Reference system catalogues the world s landmass into 57,784 scenes, each 115 miles (183 kilometers) wide by 106 miles (170 kilometers) long. The ETM+ will produce approximately 3.8 gigabits of data for each scene, which is roughly equivalent to nearly 15 sets of encyclopedias at 29 volumes per set.
Landsat Ground System
The Landsat ground system includes a spacecraft control center, ground stations for uplinking commands and receiving data, a data handling facility and a data archive. These facilities will communicate with Landsat 7, control all spacecraft and instrument operations, and will receive, process, archive, and distribute ETM+ data. The primary ground station, the data handling facility and archive are located at the USGS/EROS Data Center in Sioux Falls, SD. NASA will manage flight operations from the control center at the Goddard Space Flight Center until October 1, 2000, when responsibility for flight operations transfers to the USGS. The ground system will be able to distribute raw ETM+ data within 24 hours of its reception at the EROS Data Center.
The ground system at the data center will be capable of capturing and processing 250 Landsat scenes per day and delivering at least 100 of the scenes to users each day. All 100 of these scenes can be radiometrically corrected to within five percent and geometrically located on the Earth to within 820 feet (250 meters). Uncorrected data that is ordered will contain sufficient information to allow a user to do the correction. Data captured will routinely be available for user ordering within 24 hours of its receipt at the EROS Data Center. The user will be able to query metadata and image browse data from the archive electronically to determine if it contains suitable information. If so, the data can be ordered and delivered either electronically or in a digital format by common carrier.
This article was derived from a NASA Public Affairs Fact Sheet. http://mtpe.gsfc.nasa.gov/landsat/default.htm