Landsat program
The Landsat program is the longest running enterprise for acquisition of satellite imagery of Earth. On July 26, 1972 the Earth Resources Technology Satellite was launched. This was eventually renamed to Landsat.[1] The most recent, Landsat 7, was launched on April 15, 1999. The instruments on the Landsat satellites have acquired millions of images. The images, archived in the United States and at Landsat receiving stations around the world, are a unique resource for global change research and applications in agriculture, cartography, geology, forestry, regional planning, surveillance, education and national security. Landsat 7 data has nine spectral bands with spatial resolutions ranging from 15 to 60 meters; the temporal resolution is 16 days. [2]
History
Hughes Santa Barbara Research Center initiated design and fabrication of the first three MSS Multispectral Scanners in the same year man landed on the moon, 1969. The first prototype MSS was completed within nine months by fall of 1970 when it was tested by scanning Half Dome at Yosemite National Park.
The program was called the Earth Resources Technology Satellites Program when it was initiated in 1966, but the name was changed to Landsat in 1975. In 1979, Presidential Directive 54 under President of the United States Jimmy Carter transferred Landsat operations from NASA to NOAA, recommended development of long term operational system with four additional satellites beyond Landsat 3, and recommended transition to private sector operation of Landsat. This occurred in 1985 when the Earth Observation Satellite Company (EOSAT), a partnership of Hughes Aircraft and RCA, was selected by NOAA to operate the Landsat system under a ten year contract. EOSAT operated Landsats 4 and 5, had exclusive rights to market Landsat data, and was to build Landsats 6 and 7.
In 1989, this transition had not been fully completed when NOAA's funding for the Landsat program was due to run out (NOAA had not requested any funding, and Congress had appropriated only six months of funding for the fiscal year)[3] and NOAA directed that Landsats 4 and 5 be shut down.[4] The head of the newly formed National Space Council, Vice President Dan Quayle, noted the situation and arranged emergency funding that allowed the program to continue with the data archives intact. [3][4][5][6]
Again in 1990 and 1991, Congress provided only half of the year's funding to NOAA , requesting that agencies that used Landsat data provide the funding for the other six months of the upcoming year.[3] In 1992, various efforts were made to procure funding for follow on Landsats and continued operations, but by the end of the year EOSAT ceased processing Landsat data. Landsat 6 was finally launched on October 5, 1993, but was lost in a launch failure. Processing of Landsat 4 and 5 data was resumed by EOSAT in 1994. NASA finally launched Landsat 7 on April 15, 1999.
The value of the Landsat program was recognized by Congress in October 1992 when it passed the Land Remote Sensing Policy Act (Public Law 102-555) authorizing the procurement of Landsat 7 and assuring the continued availability of Landsat digital data and images, at the lowest possible cost, to traditional and new users of the data.
Satellite chronology
- Landsat 1 (originally named Earth Resources Technology Satellite 1) - launched July 23, 1972, terminated operations January 6, 1978
- Landsat 2 - launched January 22, 1975, terminated January 22, 1981
- Landsat 3 - launched March 5, 1978, terminated March 31, 1983
- Landsat 4 - launched July 16, 1982, terminated 1993
- Landsat 5 - launched March 1, 1984, still functioning. [7] [8]
- Landsat 6 - launched October 5, 1993, failed to reach orbit
- Landsat 7 - launched April 15, 1999, still functioning, but with faulty scan line corrector (May 2003) [9]
Technical details
The Multispectral Scanner had a 9" fused silica dinner-plate mirror epoxy bonded to three invar tangent bars mounted to base of a Ni/ Au brazed Invar frame in a serreuire truss that was arranged with four "Hobbs-Links" (conceived by Dr. Gregg Hobbs) crossing at mid truss. This construct ensured the secondary mirror would simply oscillate about the primary optic axis to maintain focus despite vibration inherent from the 14-inch (360 mm) Be scan mirror. This engineering solution allowed the US to develop LANDSAT at least five years ahead of French SPOT which first used CCD arrays to stare without need for a scanner.
The MSS FPA, or Focal Plane Array consisted of 24 square optical fibers extruded down to .0002"square fiber tips in a 4x6 array to be scanned across the Nimbus spacecraft path in a +/-6 degree scan as the satellite was in a 10:30 polar orbit, hence it had to be launched from Vandenberg AFB. The fiber optic bundle was embedded in a fiber optic plate to be terminated at a relay optic device that transmitted fiber end signal on into six photodiodes and 18 photomultiplier tubes that were arrayed across a 0.30-inch (7.6 mm) thick aluminum tool plate, with sensor weight balanced vs the 9-inch (230 mm) telescope on opposite side. This main plate was assembled on a frame, then attached to the silver-loaded magnesium housing with helicoil fasteners.
Key to MSS success was the scan monitor mounted on the underbelly of the Mg housing. It consisted of a diode source and sensor mounted at ends of four flat mirrors that were tilted so that it took 14 bounces for a beam to reflect length of the three mirrors from source to sender striking Be scan mirror seven times as it reflected seven times off the flat mirrors. It only sensed three positions, both ends of scan and the mid scan, but that was all that was required to determine where MSS was pointed and electronics scanning could be calibrated to display a map.
Future
The Landsat Data Continuity Mission, scheduled to be launched in 2013, will be the next satellite in the Landsat series. The new satellite is being built in Arizona by Orbital Sciences Corporation. [10]
References
See also
External links
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