History of the Landsat Program
Seven Landsat satellites have been launched since 1972, although the rocket carrying Landsat-6 failed shortly after launch in October 1993. Since the loss of this satellite, the Landsat program has rebounded and recently launched Landsat-7.
Landsats 1,2, and 3 each carried two remote sensing systems: a three-channel, or three-band, Return Beam Vidicon (RBV) and a four channel MSS system. The RBV consisted of three television-like cameras aimed at the same ground area (185x185km, with a ground resolution of 80m), the channels calibrated to roughly approximate the red-green-blue spectral characteristics of color infrared film. The MSS system consisted of sensors that recorded two bands of reflected light in the visible spectrum (green and red) and two in the near infrared spectrum. This sensor scanned a continuous 185km-wide swath with a ground resolution of 79m. An additional, thermal band was included on Landsat 3 but it failed shortly after the satellite was placed into orbit. Landsats 1,2, and 3 were decommissioned by 1983.
Landsats 4 and 5 were launched into similar orbital geometries as 1,2, and 3 (repetitive, circular, sun synchronous and near-polar), although the orbit altitude was lowered from 900 to 705km. This improved ground resolution of the images because the altitude of the satellite sensors was almost 200 kilometers closer to earth's surface, and brought the satellites within range of the space shuttle for maintenance or retrieval. Landsats 4 and 5 were launched in 1982 and 1984, respectively, and Landsat 4 was decommissioned in 1993.
The biggest technological improvement in the post-Landsat 3 imaging system is the incorporation of the Thematic Mapper (TM), an advanced sensor that includes spectral, radiometric, and geometric design improvements. Data is collected in seven bands, as opposed to four previously, including three new bands in the visible, mid-infrared, and thermal part of the electromagnetic spectrum. The wavelength range and location of the bands has also been calibrated to improve the spectral distinction of earth surface features. Landsats 4 and 5 did not include the older RBV cameras.
Equipped with higher resolution instruments, Landsat-7 was successfully launched on April 15, 1999. This satellite carries the Enhanced Thermal Mapper Plus (ETM+) which is an eight-band, multispectral scanning radiometer. The ETM+ is capable of resolving distances of 15 meters in the panchromatic band; 30 meters in the visible, near and short-wave infrared band; and 60 meters in the thermal infrared band.
Data acquired during the first three missions is archived according to their location in the Worldwide Reference System (WRS), which designates each 103 minute orbit within the 18 day orbital cycle as a path, and each image frame center as a row. The path number, the row number, and the date of acquisition form a unique designation for each scene. Landsats 4 and 5 were set to orbit the earth every 99 minutes and make a complete cycle every 16 days. The orbits of Landsats 4 and 5 were established 8 days out of phase, so that an 8 day coverage cycle would be maintained with alternating coverage by each satellite. Images are catalogued according to the WRS, modified to account for the fewer number of paths (233 vs. 251) required to cover the earth in one 16 day cycle. Landsat-7 data is being catalogued according to the same system. Its orbit is like that of missions 4 and 5, and it is 8 days out of phase with Landsat 5, which is the only other Landsat still transmitting images.
Ground Coverage Provided by Landsat
A single Landsat scene covers a ground area of about 34,000 square kilometers, although smaller sub-scenes are often selected to speed up intensive analysis of particular areas or features. By comparison, it would take 1600 aerial photographs at a scale of 1:20,000 to cover the same area. Thus it is important to understand the implications that scale of imagery has in the analysis of earth features. Common applications of MSS and TM data include geologic mapping and exploration, vegetation mapping and change detection, cartography, civil engineering, oceanography, and assessment of the effects of agriculture, logging, and urbanization on different parts of the earth's surface.
Additional Information
Details of the Landsat-7 mission and how it compares to past missions are available on the Landsat 7 Project Page. General information on Landsat-7 ground stations and a real-time map of the satellite's location are available on the USGS Landsat 7 website. Also, Canada's Centre for Remote Sensing offers an excellent outline of many international satellite missions.
ESRC publishes a list of their ERSC archived Landsat data and other Wisconsin-specific Landsat resources at www.ersc.wisc.edu/resources/landsat/.
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