Overview LANDSAT and ASTER Programs
Landsat and ASTER images are the products of programs planned and executed by government agencies and funded by taxpayers. LANDSAT images are products of a United States government program; ASTER images are the products of a joint US-Japanese government program. While there are many technical differences between the Landsat and ASTER instruments, the main difference is that Landsat-7 represents a stage in an operational program extending over 30 years, while ASTER is experimental, supporting other experimental TERRA programs. NASA: Earth Observatory
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The Landsat Program
The Landsat program was a direct spinoff from the Apollo manned space program. Photography acquired by astronauts, especially those from the Gemini missions, was the inspiration for continued earth observation by satellite, (NASA: Remote Sensing Tutorial).
The first satellite, Landsat-1, was launched in 1972 and the last, Landsat-7, in 1999. Apart from Landsat-6, which was lost during launch, the Landsat programs have been successful in acquiring millions of images that have proven useful in fields as diverse as agriculture, forestry, geology, urban and regional planning, and global change research,
(USGS: Landsat History).
The Landsat concept has a future but in a different form. In October 2002 the Earth Observing satellite (EO-1) took up station one minute behind Landat-7. This configuration of two satellites flying in tandem allows the new instruments to be compared to those of Landsat-1 and validated. EO-1 is a very much advanced version of Landsat carrying instruments with higher technology that will provide the basis for continuity and higher technology in the next decade. EO-1 carries the Advanced Land Imager, the Hyperion Hyperspectral Imager, and an Atmospheric Corrector. The spectrum will be covered from the visible to the middle infrared, but there is no instrument sensitive to the thermal infrared, (NASA: EO1).
(The terms visible, middle-infrared, and thermal infrared refer to the the light spectrum.)
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TERRA and ASTER
TERRA, the most important satellite in NASA's Earth Observing System
(EOS), was launched on December 18, 1999. TERRA carries five instruments,
ASTER, CERES, MISR, MODIS, and MOPITT, each dedicated to imaging specific
aspects of the earth's atmosphere. ASTER, the Advanced Spaceborne
Thermal Emission and Reflection Radiometer, as the highest resolution
instrument on board, will support the other instruments.
ASTER may also be seen as part of a continuing Japanese initiative in space. The earlier JERS-1 satellite launched in 1992 continued to provide useful imagery until 1998, far beyond its design life, (World Space Guide:JERS-1). The ASTER program laid the foundation for a much more advanced system, ALOS, to be launched in 2004. Although ALOS has stereo vision (similar to JERS-1 and ASTER) and radar (similar to JERS-1), ALOS has no sensor to record the middle or far infrared, (NASDA: ALOS).
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Landsat and ASTER Compared
Landsat-7 and ASTER differ in both spectral resolution and their spatial resolution. Landsat-7 and ASTER both have bands for green, red, near-, middle-, and thermal-infrared. However, Landsat also has a band for blue and a panchromatic band that covers green, red, and near-infrared in one band. While ASTER lacks the blue and panchromatic bands it nevertheless has 14 bands to Landsat's 8. The extra bands are infrared bands as indicated in the spectral response diagram.
The most remarkable difference between Landsat and ASTER is in the spatial resolution, translated by display software into pixels on the computer screen. Landsat's panchromatic band has a spatial resolution of 15 meters, which means that a pixel on the screen represents an area on the ground of about 15 by 15 meters (50 by 50 feet). However, visible and near-infrared bands have a spatial resolution of 30 meters. By contrast, the visible and near-infrared bands of ASTER have a spatial resolution of 15 meters. This can be seen in a side-by-side comparison of Landsat and ASTER scenes. Side-by-side comparison of Landsat and ASTER images.
Finally, ASTER has a backward facing telescope that records the same spectral range as band 3. To indicate that the two images are related the downward image is labelled band 3N for "nadir", and the rear image is labelled band 3B, for "backwards". A pair of images form a stereoscopic pair that permit 3-dimensional viewing. 3-D view near Muka Head, Penang.
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Pricing of Datasets
Pricing of Landsat and ASTER datasets is not simple. Datasets for both are available with various levels of processing. The Landsat-7 L1g product is regarded as standard because it is radometrically and geometrically corrected. The comparable product for ASTER is the L1B product.
The commercial price for one Landsat-7 standard scene is US$600. However, two contiguous scenes in the same path are sold at a substantial discount. Side-by-side scenes (same row, adjacent paths) are specially priced. Even part-scenes can be purchased. Some older Landsat imagery is sold at a discount. Commercial prices are more or less fixed, but there are so many ways of ordering data that a potential buyer needs to investigate carefully to get the best value for money.
Before purchasing data at the commercial price a potential buyer ought to search for non-commercial sources of data. Some Landsat data is available for less than 10% of the standard price from the Michigan State University's BSRSI (Basic Science and Remote Sensing Initiative). Some Landsat data is available free from governments, such as the Canadian government (Geogratis), and from universities, such as the University of Maryland
(Global Land Cover Facility). The price of Landsat data is high enough to justify the time and effort needed to locate low-cost sources.
Until recently all ASTER data was free. Some ASTER data is still free at the (LP DAAC Datapool). But a user who needs data for Southeast Asia must now either pay or be approved as a registered researcher. The price of an ASTER scene downloaded from the Land Processes DAAC internet site is $60 per granule (scene). Based on price per square kilometer, this is approximately the same as the commercial price for a Landsat-7 scene. See: (ASTER Pricing Policy).
For users who want higher level products ASTER data is still a bargain and here is why: there is still no charge for most higher level products, including digital elevation models (DEMs). Also, users who need data for an area smaller than 60 km x 60 km may find ASTER data considerably cheaper than Landsat data.
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Pricing Policy
The author holds the opinion that pricing of satellite data is driven by bureaucratic and political factors rather than by market forces or the public interest. These opinions are expressed as a footnote.(ASTER Pricing Policy).
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