Landsat 7, launched on April 15, 1999, is the latest satellite of the
Landsat programThe 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. The most recent, Landsat 7, was launched on April 15, 1999. The instruments on the...
. Landsat 7's primary goal is to refresh the global archive of satellite photos, providing up-to-date and cloud-free images. The Landsat Program is managed and operated by the USGS, and data from Landsat 7 is collected and distributed by the USGS. The
NASA World WindWorld Wind is an open-source virtual globe developed by NASA and the open source community for use on personal computers. Old versions need Microsoft Windows but the more recent Java version, , is cross platform and provides a suite of . The World Wind Java version was awarded in November 2009...
project allows 3D images from Landsat 7 and other sources to be freely navigated and viewed from any angle. The satellite's companion,
Earth Observing-1The Earth Observing-1 Mission is part of NASA's New Millennium Program , to develop and validate a number of instrument and spacecraft bus breakthrough technologies designed to enable the development of future earth imaging observatories that will have a significant increase in performance while...
, trails one minute following the exact orbital characteristics.
Satellite specifications
Landsat 7 was designed to last for five years, and has the capacity to collect and transmit up to 532 images per day. It is in a
polarA polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. It therefore has an inclination of 90 degrees to the equator...
,
sun-synchronous orbitA Sun-synchronous orbit is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given point of the Earth's surface at the same local mean solar time. The surface illumination angle will be nearly the same every time...
, meaning it scans across the entire earth's surface. With an altitude of 705 kilometres +/- 5 kilometres, it takes 232 orbits, or 16 days, to do so. The satellite weighs 1973 kg, is 4.04 m long, and 2.74 m in diameter. Unlike its predecessors, Landsat 7 has a solid state memory of 378 gigabits (roughly 100 images). The main instrument on board Landsat 7 is the Enhanced Thematic Mapper Plus (ETM+).
Main features
- A panchromatic band with 15 m (49.2 ft) spatial resolution (band 8)
- Visible
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
(reflected light) bands in the spectrum of blue, green, red, near-infrared (NIR), and mid-infrared (MIR) with 30 m (98.4 ft) spatial resolution (bands 1-5, 7)
- A thermal infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
channel with 60 m spatial resolution (band 6)
- Full aperture, 5% absolute radiometric calibration
Scan Line Corrector failure
On May 31, 2003 the Scan Line Corrector (SLC) in the ETM+ instrument failed. The SLC consists of a pair of small mirrors that rotate about an axis in tandem with the motion of the main ETM+ scan mirror. The purpose of the SLC is to compensate for the forward motion (along-track) of the spacecraft so that the resulting scans are aligned parallel to each other. Without the effects of the SLC, the instrument images the Earth in a "zig-zag" fashion, resulting in some areas that are imaged twice and others that are not imaged at all. The net effect is that approximately 22% of the data in a Landsat 7 scene is missing when acquired without a functional SLC.
Following the SLC failure, an Anomaly Response Team (ART) was assembled, consisting of representatives from the USGS,
NASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
, and Hughes Santa Barbara Remote Sensing (the manufacturer of the ETM+ instrument). The team assembled a list of possible failure scenarios, most of which pointed at a mechanical problem with the SLC itself. Since there is no backup SLC, a mechanical failure would indicate that the problem was permanent. However, the team was unable to rule out the possibility of an electrical failure, though such a possibility was deemed remote. Nevertheless, on September 3, 2003, USGS director
Charles G. GroatCharles G. "Chip" Groat is an American geologist. He is a distinguished professional in the earth science community with involvement in geological studies, energy and minerals resource assessment, ground-water occurrence and protection, geomorphic processes and landform evolution in desert areas,...
authorized the Landsat project to reconfigure the ETM+ instrument and various other subsystems on board Landsat 7 to use the spacecraft's redundant ("Side-B") electrical harness.
With this authorization, the USGS flight operations team at the NASA
Goddard Space Flight CenterThe Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
uploaded a series of commands to the spacecraft, instructing it to operate using the redundant electrical harness. This operation was successful, and on September 5, 2003, the ETM+ instrument was turned on and acquired data that was sent to the Landsat ground system at
EROSThe Center for Earth Resources Observation and Science is a United States Geological Survey data management, systems development, and research field center. It serves as the national archive of remotely sensed images of the Earth's land surface acquired by civilian satellites and aircraft...
outside of
Sioux Falls, South DakotaSioux Falls is the largest city in the U.S. state of South Dakota. Sioux Falls is the county seat of Minnehaha County, and also extends into Lincoln County to the south...
. It was immediately apparent that the migration to the Side-B electrical harness had not fixed the problem with the SLC. Following this, the instrument was reconfigured to use its primary electrical harness. The subsequent conclusion of the ART was that the SLC problem was mechanical and permanent in nature.
Landsat 7 continues to acquire data in this mode. Data products are available with the missing data optionally filled in using other Landsat 7 data selected by the user. To continue the Landsat legacy, studies are underway to fly an equivalent scientific sensor on a new satellite.
Satellite imagery
In August 1998,
NASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
contracted
EarthSatEarth Satellite Corporation was an early pioneer in the commercial use of Earth observation satellites. Founded in 1969, EarthSat was first headquartered in Washington, D.C. and later moved its offices to Bethesda, Maryland and finally to Rockville, Maryland in the late 1980s. In 2001, EarthSat...
to produce Landsat GeoCover (
Geocover 2000 in
NASA World WindWorld Wind is an open-source virtual globe developed by NASA and the open source community for use on personal computers. Old versions need Microsoft Windows but the more recent Java version, , is cross platform and provides a suite of . The World Wind Java version was awarded in November 2009...
) — a positionally accurate orthorectified Landsat
Thematic MapperOne of the Earth observing sensors introduced in the Landsat program. A Thematic Mapper was first placed aboard Landsat 4 , and one is still operational aboard Landsat 5. TM sensors feature seven bands of image data most of which have 30 metre spatial resolution...
and
Multispectral ScannerThe Multispectral Scanner is one of the Earth observing sensors introduced in the Landsat program. A Multispectral Scanner was placed aboard each of the first five Landsat satellites.-MSS Technical Specifications:...
imagery covering the majority of the Earth's land mass. The contract was part of the NASA Scientific Data Purchase which was administrated through NASA's
John C. Stennis Space CenterThe John C. Stennis Space Center , located in Hancock County, Mississippi, at the Mississippi-Louisiana border, is NASA's largest rocket engine test facility.- History :...
. GeoCover was later enhanced to EarthSat NaturalVue, a simulated
natural color Landsat 7 derived circa year 2000, orthorectified, mosaicked and color balanced digital image dataset. Other commercial simulated true color 15-meter global imagery products built from the NASA Landsat 7 imagery include TerraColor from Earthstar Geographics, TruEarth (found in
Google EarthGoogle Earth is a virtual globe, map and geographical information program that was originally called EarthViewer 3D, and was created by Keyhole, Inc, a Central Intelligence Agency funded company acquired by Google in 2004 . It maps the Earth by the superimposition of images obtained from satellite...
and
Google MapsGoogle Maps is a web mapping service application and technology provided by Google, free , that powers many map-based services, including the Google Maps website, Google Ride Finder, Google Transit, and maps embedded on third-party websites via the Google Maps API...
) from TerraMetrics, BrightEarth from ComputaMaps, simulated natural color from Atlogis and a product of i-cubed used in World Wind.
Largest parts of the earth surface displayed on
web mappingWeb mapping is the process of designing, implementing, generating and delivering maps on the World Wide Web and its product. While web mapping primarily deals with technological issues, web cartography additionally studies theoretic aspects: the use of web maps, the evaluation and optimization of...
services like Google Maps/Google Earth, MSN Maps or Yahoo Maps are based on enhanced and color balanced Landsat 7 imagery.
External links