The Earth-Sensing Legacy
Landsat 7, launched on April 1, 1999, is designed to extend and improve upon the more than 25-year record of images of 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 is providing essential land surface data to a broad, diverse community of national security, civilian, and commercial users.
EO-1 will also demonstrate three advanced land-imaging instruments, each having unique filtering methods for passing light in only specific wavelengths of radiant energy, called "spectral bands." EO-1 spectral bands will allow researchers to best look for specific surface features or land characteristics based on scientific or commercial applications. These advanced imaging instruments will lead to a new generation of lighter weight, higher performance, and lower cost Landsat-type imaging instruments for NASA's Earth Science Enterprise.
The centerpiece of this mission is the Advanced Land Imager (ALI)
instrument. This new instrument will demonstrate remote-sensing
measurements of the Earth that are consistent with data collected by the
Landsat series of satellites. These data are used by farmers, foresters,
geologists, economists, city planners, and others for resource
monitoring and assessment. ALI will lay the technological groundwork for
future land-imaging instruments to be more compact and less costly. A
Landsat-style instrument based on ALI would have a mass of 106
kilograms, consume only 118 watts of power while performing scans,
occupy a volume of .25 cubic meters, and possess finer spectral coverage
over the current Landsat 7 imager, the Enhanced Thematic Mapper Plus
(ETM+). In comparison, the ETM+ has a mass of 425 kilograms, consumes
590 watts of power while performing scans, and occupies a volume of 1.7
|Sensors aboard the Landsat satellites were built in a whisk broom (across track) configuration. In a whisk broom sensor, a mirror scans across the satellites path, reflecting light into a single detector which collects data one pixel at a time. The moving parts make this type of sensor expensive and more prone to wearing out.|
Data from the ALI might help ranchers identify the most suitable lands for livestock grazing, or help farmers improve crop yields by identifying areas that need additional fertilizer or irrigation.
EO-1 will also carry an advanced high-resolution hyperspectral
(capable of resolving a large number of spectral bands per pixel)
imager, called Hyperion. Hyperion will be capable of resolving 220
spectral bands at wavelengths from 0.4 to 2.5 micrometers with a
30-meter resolution (i.e., the smallest object observed will be 30m x
30m). This is a vast improvement over the current Landsat technology,
which supports only eight multispectral bands at a similar resolution.
Because of the large number of spectral bands on Hyperion, complex land
ecosystems can be imaged and more-accurately classified.
|A pushbroom (along track) sensor like ALI consists of a line of sensors arranged perpendicular to the flight direction of the spacecraft. Different areas of the surface are imaged as the spacecraft flies forward. Pushbroom sensors are generally lighter and less expensive than their whisk broom counterparts, and can gather more light because they look at a particular area for a longer time, like a long exposure on a camera. One drawback of pushbroom sensors is the varying sensitivity of the individual detectors. (Animations by Robert Simmon)|
For example, detailed classification of land assets will enable improved remote mineral identification and hazardous waste monitoring. Researchers estimate that there may be more than 20,000 active and abandoned mines in the western U.S. alone. It is a daunting task to use field methods alone to inventory and assess how acidic drainage from mines affects surface water quality and impacts the environment. EO-1 will help land resource managers greatly accelerate this inventory.
The third instrument on EO-1 is the Atmospheric Corrector. Earth
imagery from space is often degraded by the absorption and scattering of
solar radiation due to the aerosol and water vapor content of the
atmosphere (analogous to looking through a dirty window). The
Atmospheric Corrector is a moderate spatial resolution (250 meters)
imaging spectrometer with a 185-kilometer (115 mile) swath, the same as
Landsat 7's ETM+. Using the Atmospheric Corrector, instrument
measurements of actual, rather than modeled, absorption values will
enable more accurate measurement and classification of land resources
and better models for land management in the future. Additionally NASA
will provide its Atmospheric Corrector technologies to U.S. industry with
the explicit purpose of expediting technology transfer to the commercial
|Hyperion, the hyperspectral imager on EO-1, will measure much finer spectral information than the ETM+ or ALI. In nature, spectral information is continuousthe amount of sunlight reflected off a point on the Earths surface varies smoothly with changes in wavelength. Hyperions 220 bands (green line) provide a more accurate depiction than the discrete bands of Landsat (blue dots). (Graph by Robert Simmon)|
||This true-color image of Houston, Texas, (acquired by the Moderate-resolution Imaging Spectroradiometer) was not corrected for the effects of the atmosphere. Note the blue tone of the image, and the overall brightness.|
For each scene, EO-1's three sensors will collect more than 20 gigabits (20 trillion bits) of data that are stored at high rates on the on-board solid state recorder. When the EO-1 spacecraft is in range of a ground station, the spacecraft will automatically transmit its recorded image to the ground station for temporary storage. The ground station will store the raw data on digital tapes which will be forwarded to NASA's Goddard Space Flight Center for processing and sent to the EO-1 science and technology teams for validation and research purposes.
|This image is based on the same data as the image above, but the red, green, and blue channels have been corrected for the scattering that occurs as light passes through the atmosphere. The Atmospheric Corrector aboard EO-1 will allow scientists to improve their data even further, a necessity for the precise measurements made by EOS sensors. (Images courtesy Jacques Descloitres, MODIS Land Team)|