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ASTER Level 2 Surface Leaving Radiance TIR Product - AST09
Version 2.7, [March 2002]
This Validated version of the TIR surface leaving radiance product provides an estimate of the total radiance leaving the surface including any reflected sky component and an estimate of the sky irradiance for each of the five ASTER TIR bands. This version is a Validated version intended to provide a data product which we have found to be computationally correct and whose accuracy has been estimated (see below). This product is suitable for use in publications given the user understands the accuracy of the product is dependent on the amount of water vapor in the atmosphere and the accuracy of the source of the atmospheric profiles used in the correction procedure.
This product corrects the measured at sensor radiance (which has been itself validated to be within ±1%) for the effects of atmospheric transmission, path radiance and scattering using atmospheric profile information from NOAA (NCEP) derived assimilation models. The results of the atmospheric correction are only as good as he atmospheric profiles which are used. (Later versions of this product will use atmospheric temperature and water vapor profile information derived from MODIS, a sister instrument to ASTER on the TERRA platform when these atmospheric profile products are validated.)
Table of Contents
1. Where to get detailed information on this product
2. How to report problems
3. Description of this version
* Product Quality
* Problems and limitations
4. Version history
1. Where to get detailed information on this product
There are several sources of information that users will find useful:
* "An Atmospheric Correction Method for ASTER Thermal Radiometry Over
Land" is the title of the theoretical basis document (ATBD) for this
product This document describes the algorithm used to create the product. It is
available in pdf format as document atbd-ast-05 at the EOS Project website:
http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/viewInstrument.php?instrument=ASTER
* ASTER Higher-Level Product User Guide. This document describes each of the
ASTER Higher Level products. It is available in pdf format at the ASTER site:
http://asterweb.jpl.nasa.gov/documents/documents.htm
* US ASTER Website. This site describes the ASTER instrument including provision
of the relative spectral response for each band, how to obtain data, the various
data products that are available, the activities of the US ASTER Science Team,
sample images and other ASTER information The address for this site is:
http://asterweb.jpl.nasa.gov/
* Level 1 product information. Because the TIR Surface Leaving Radiance product is derived from the Level 1B product, understanding the L1 products is useful.
* Level 2 product ordering
site.
In order to order a level 2
ASTER product it is necessary to know the Granule Identification (ID) of the
corresponding Level 1B product from which it will be generated.
You may search for the Level 1 product and its ID at the site: http://edcimswww.cr.usgs.gov/pub/imswelcome/
When you find the Level 1B
Granule IDs for the scenes of interest you need to create a list of the IDs to
enter into your request for the Level 2 products.
The site for ordering Level 2 products is:
http://edcdaac.usgs.gov/asterondemand/
2. How to report problems
To report a problem with the product, please send mail to edc@eos.nasa.gov or call EDC DAAC User Services at (605) 594-6116.
3. Description of this version
ASTER TIR Surface Leaving Radiance AST09 Version 2.7
This is the third release of this product now considered a validated version.
The units after application of a scaling factor for the surface leaving radiance are Watt/m^2/steradian/micrometer and for sky irradiance Watt/m^2/micrometer.
In converting image Data Numbers (DN) to radiance or irradiance there are no offsets, that is radiance is obtained by multiplying the image DN by the appropriate band scale factor (DN*band scale factor). The band scale factors are listed in the metadata of the product in an array called BANDSCALEFACTORS.
The current (March 2001) band scale factors are;
Band 10: 0.006882
Band 11: 0.006780
Band 12: 0.006590
Band 13: 0.005693
Band 14: 0.005225
These are the same band scale factors that are used with the Level 1B TIR image data except that for the Level 1B data there is an offset so the conversion formula becomes ((DN-1)*band scale factor).
Note: If the L1 processing is updated so that the scale factors change, the AST09T Version 2.7 product will use the new factors. The factors are copied from the L1B product.
Changes
Version 2.5 was released in March 2001 as a Beta Release
Further validation upgraded Version 2.5 to a Provisional Release
Version 2.7 Validated Release
Product Quality & Limitations
Extensive validation of the TIR Level 1B (L1B) radiance at sensor product has been conducted by several ASTER groups over the last year and all comparisons indicate performance within ±1% or better.
The Level 2 surface leaving radiance and irradiance products are dependent on the quality not only of the at sensor radiance but the quality of the atmospheric profiles and the topographic data based used. In the case of one of the test sites (Lake Tahoe, CA, elevation ~1.9 km) the validation has indicated agreement at the 1% level for two widely separated dates (March 2000 and November 2000). For another test site (Salton Sea, CA, elevation -70m) there was significant disagreement between the atmospheric profiles obtained from the NOAA assimilation model (used in the products described here) and in-situ radiosonde measurements. This lack of agreement in profiles resulted in a 3-6% disagreement between the Level 2 product surface leaving radiance and that derived from in-situ measurements.
Because there is currently no uniform global high spatial resolution (i.e. 90m postings) topographic data sets available, the radiance and irradiance products may contain artifacts which reflect the topographic interpolations necessary. Users who find artifacts which appear to be topography related are urged to report such problems (See 2. above on reporting problems.)
The surface leaving radiance and sky irradiance products are only valid for cloud-free pixels. The algorithms used do not correct or account for the presence of water or ice clouds and the products are of uncertain value when such clouds are present. Note that the algorithm operates on all pixels in a scene whether or not they contain clouds or are at the edge of clouds. If clouds are of a concern the user should look at the corresponding Level 1B VNIR/SWIR/TIR images for guidance.
This data product has been moved from provisional status to validated status based on evidence that the product is computationally correct and the resulting radiance has reasonable quantified uncertainty. This assessment was made using a limited number of field measurements in which the image based surface leaving radiance and irradiance from the product were compared with the same quantities derived from independent field-based results.
Validation of the ASTER TIR atmospheric correction algorithm on a global basis is not directly possible as the necessary measurements of surface temperature and surface spectral emissivity and atmospheric properties at the time of ASTER data acquisition are not available. However, an indirect approach is possible and has been carried out by Hideyuki Tonooka of the ASTER team (Hideyuki Toonoka and Frank Palluconi, "Verification of the ASTER/TIR atmospheric correction algorithm based on water surface emissivity retrieved", SPIE, San Diego, USA, August 2001). This approach makes use of the fact that the spectral emissivity of water is known. One hundred ASTER scenes distributed in latitude and season were used to extract the spectral emissivity of the water bodies in these scenes (fresh water lakes and the oceans). The difference between the extracted spectral emissivity of water and the know spectral emissivity was used as a measure of the quality of the atmospheric correction. Using this technique the mean error in surface leaving radiance was about 5% (corresponding to an error of 3 C in surface temperature) for a column water abundance of 3 cm with larger errors for higher amounts of water. Users of ASTER surface leaving radiance may use this test on scenes of their interest if they contain bodies of water larger than several pixels in extent. One simple way in which this can be done is to order the ASTER surface temperature/emissivity data product along with the surface leaving radiance product and examine the emissivity values for any water bodies in the scene. For scenes which are high, dry or both (column water amount under 1 cm of water) the radiance error is generally less than 2%.
4. Version History
Version 2.5 was released in March 2001 as a Beta product.
The designation for Version 2.5 was upgraded to Provisional following further validation.
Version 2.7 upgraded the release designation for NCEP-based processing to Validated.
Future releases will produce validated products using MODIS atmospheric profiles.
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