|
TerraLook
ASTER INSTRUMENT AND DATA
Much of the data in the TerraLook are from an instrument called ASTER. Most people, however, are not familiar with this instrument, and this section provides some basic information. To summarize very briefly: ASTER is quite similar to Landsat, but provides much more detailed images. Landsat, however, has a wider swath width, and a very large historical archive.
Overview
ASTER is a large, space-based, digital camera that started operating in early 2000. It acquires about 600 high-resolution images a day, each one covering an area of 60 x 60 km, with a pixel size of 15 m for bands 1-3. Depending on the version you have and the source of the images, the images on this disk may be "RGB 3-2-1 composite" jpgs, meaning that live vegetation appears red and most human-made areas are blue. For most collections, however, live vegetation is represented as green--obviously closer to real life, though for a variety of reasons this is not as easy as it sounds. Dead vegetation (such as grasslands during the dry season) tend to be purple. The jpg format was used to reduce the size of each image.
The Instrument
ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is basically a large digital camera bolted to a satellite. The satellite, called Terra, is the size of a small bus, was launched in 1999, and has four other instruments. It circles Earth at a distance of 705 km, from pole to pole, about every 100 minutes, crossing the equator at about 10:30 am local time.
ASTER itself takes about 600 pictures ("scenes") a day, each covering an area of 60 x 60 km. Like most satellite sensors, ASTER is much more complex than a hand-held digital camera. First, and most importantly, a separate image is created for each color (or more precisely, each wavelength range, or "band"). Because ASTER has a total of 14 bands, it actually acquires 14 different images for each scene. This is useful because different materials can look very different in different bands--by acquiring images in each of the 14 bands a lot can be learned about the materials being imaged. When an image is "processed", each band can be treated separately, leading to some very powerful (and sometimes very complicated) analysis techniques. To keep things simple, and to save space, the images on this disk are "composite" images derived from bands 1, 2, and 3. A website is planned that will provide the full multi-band images along with some simple tools to help analyze them.
Another difference between ASTER and a typical digital camera is that ASTER has three lenses--(called telescopes because of their size and power)--rather than one. In fact, ASTER is really three separate instruments, each one specializing in a different part of the spectrum. This is because photons in one part of the spectrum behave very differently than in another, so different technologies are used for each part.
One of the most important characteristics of ASTER is its high resolution--a pixel size of 15 m for bands 1-3 (compared to Landsat's 15 to 30 m resolution). High resolution is useful for park managers because it provides more detail and, consequently, greater ability to detect changes and to observe the status of the park.
The ASTER instrument and its operation is a joint project between the US and Japan. Japan designed and built the instrument, the Level 1 processing system, and the operations system, and performs the day-to-day mission planning and the Level 1 data processing. The US designed, built, and operates the Terra spacecraft and the associated ground system.
The Images
All digital images, whether from personal digital cameras or from those in space, are composed of pixels (picture elements). Each ASTER image on this disk has about 16 million pixels (4200 x 4200), and is a "composite" color image derived from bands 1, 2, and 3, which are sensitive to green, red, and near-infrared, respectively. Each pixel in these images corresponds to about a 15 x 15 m patch on the ground. The jpg format was chosen to decrease the size of the images so a sufficient number of them could be placed on a single CD; in most cases, however, the loss in data quality due to compression is minimal.
In some versions of PAA or in some images, live vegetation appears red--the brighter and redder the more healthy the vegetation. Man-made materials like concrete and buildings tend to be a light blue or gray. Bare soil can vary in color and brightness depending on what materials it is made of. Water is very dark.
In such a case people wonder why the scenes are not displayed in their natural colors, and there are several reasons for this. The first is due to historical reasons. Much early remote sensing work used infrared-sensitive film because healthy vegetation strongly reflects those wavelengths (a plant cannot use them for photosynthesis). The human eye can not see infrared, yet some visible color has to be used to represent it if the images are going to be useful. For infrared-sensitive film, that color was red, and so red has been used to represent the infrared ever since, even for digital images that use no film, such as ASTER.
The second reason is that this is the way the "color assignments" have been made. The color red is assigned to band 3 (sensitive to part of the infrared spectrum), green is assigned to band 2 (sensitive to red) and blue is assigned to band 1 (sensitive to green). So, a piece of ground that reflects highly in band 3 will appear bright red in the processed image, one that reflects highly in band 2 will appear bright green, and one that reflects highly in band 1 will appear bright blue. Of course, most things are actually a combination of these, though often one band predominates.
In most versions of PAA and in most images, however, vegetation appears green. However, this requires some special processing because ASTER does not have a "blue" band (this was omitted because blue light tends to be scattered a lot by the atmosphere and so is rather "noisy"). Without blue a "natural color" image is not possible, so the existing bands were combined in a way that leads to a fairly--but not completely--natural looking image.
The Full ASTER Data Archive
All ASTER scenes (currently numbering roughly one million) are archived at a data center in South Dakota, USA (as well as at the equivalent data center in Tokyo). Access to the data in the US archive is by one or both of the following tools (the first provides both search and order capability, the second only search--but a much friendlier search-- and an easy path to ordering):
EOS Data Gateway (EDG)
Global Visualization Viewer
For More Information
For more information on ASTER, the various ASTER data products, how to submit data acquisition requests, the work of the ASTER Science Team, and much more, please visit the US ASTER website:
http://asterweb.jpl.nasa.gov/
Brief Specifications
Launched: December 1999
Expected lifetime: 6+ years
Number of bands: 14
Number of telescopes: 3 (VNIR, SWIR, TIR)
Pixel size:
15 m (VNIR)
30 m (SWIR)
90 m (TIR)
Stereo: yes
Repeat frequency: 1-16 days
|
