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EDG Data Set Name
ASTER On-Demand L2 Surface Radiance SWIR Crosstalk-Corrected
Granule Shortname
AST_09XT
Data Set Characteristics
Area: ~60 km x 60 km
Image Dimensions: 2100 rows x 2490 columns
File Size: ~75 Megabytes
Spatial Resolution: SWIR: 30 m
Projection: Universal Transverse Mercator (UTM)
| Data Format: | HDF-EOS |
| GeoTIFF |
The ASTER On-Demand L2 Surface Radiance is a multi-file product that contains atmospherically corrected data for both the Visible Near-Infrared (VNIR) and Shortwave Infrared (SWIR) sensors. Each product delivery includes two hdf files: one for the VNIR, and the other for the SWIR. VNIR is always the bigger of the two. The ASTER On-Demand L2 Surface Radiance product description is presented individually for each sensor. This document describes the SWIR crosstalk-corrected radiance product.
SWIR Crosstalk Correction
The ASTER SWIR sensor is affected by a crosstalk signal scattering problem, a phenomenon discovered after the launch of ASTER aboard the Terra platform in December 1999. The SWIR detector contains 2048 Pt-Si (platinum-silicide) arrays for each of its six spectral bands. There are six pairs of staggered linear CCD arrays for each band that are spaced 1.33 µm apart in the band order 7, 8, 9, 4, 5, and 6. In front of each CCD array pair, there are interference filters that spectrally separate the radiation reflected from the Earth.
The source of the crosstalk problem is the ASTER Band 4 detector, whose incident light is reflected by the detector’s aluminum-coated parts (especially from the area between the detector plane and band-pass filter), and is then projected on to the other detectors. The problem is further worsened by the band-to-band parallax effect and the distance between the CCD array pairs. Bands 9 and 5 are most affected because of their closeness to the Band 4 detectors. The spectral range of Band 4 is between 1.6 and 1.7 microns (0.092 µm bandwidth), which is not only the widest bandwidth of the SWIR bands (average of 0.052 µm bandwidth for Bands 5 through 9), but is also the strongest in its reflectivity component. Therefore, Band-4’s incident radiation is about 4 to 5 times stronger than that of the other bands. All the light hitting the detectors is not absorbed. Some of the light that strikes between the detectors is reflected, and some of the reflected light is re-reflected by the interference filters. Evidence of crosstalk along with the photon spread and ghosting effects is visible in images with strong contrast, especially coastlines and islands.
Crosstalk Correction Algorithm
The Japanese Science team developed the original crosstalk correction algorithm that is used to correct an ASTER Level-1B data set. The original model is based on the fundamental understanding that incident radiation to Band 4 that is reflected or leaked to the other bands will follow a certain pattern of line-shifts in the along-track direction. The kernel function used in the convolution (in the original algorithm) is not considered symmetrical in the cross-track direction. Improved kernel functions are used in the updated algorithm. The radiometric sensitivity coefficients are statistically derived to ensure that a calibration consistency is maintained in both pre- and post-crosstalk correction. Using the Japanese crosstalk correction algorithm, the ASTER Project at JPL has implemented a crosstalk-correction process that is applied to ASTER Level-1B data before deriving the reflectance product.
Additional information on the SWIR crosstalk phenomenon is available in the following papers:
Iwasaki, A., Fujisada, H., Akao, H. Shindou, O., and Akagi, S., 2002, Enhancement of Spectral Separation Performance for ASTER/SWIR, SPIE Proceedings, v. 4486, p. 42-50.
Tonooka, H., and Iwasaki, A., 2003, Improvement of ASTER/SWIR Crosstalk Correction, SPIE Proceedings, v. 5234, p. 168-179.
Iwasaki, A., and Tonooka, H., 2005, Validation of a crosstalk correction algorithm for ASTER/SWIR, IEEE Transactions on Geoscience and Remote Sensing, v. 43, Dec. 2005, p. 2747- 2751
Product Description
The ASTER On-Demand L2 Surface Radiance (SWIR Crosstalk-Corrected) is a higher-level product that contains atmospherically corrected shortwave infra-red data. It is generated using the 6 SWIR bands (between 1.60 and 2.43 µm) from an ASTER Level-1B image. Atmospheric correction involves deriving a relationship between the surface radiance/reflectance and the top of the atmosphere radiance from information on the scattering and absorbing characteristics of the atmosphere. Once this relationship is established, it is used to convert ASTER SWIR's original radiance values to atmospherically corrected surface radiance and reflectance values. The atmospheric correction algorithm for SWIR is based on a Look-Up Table (LUT) approach that uses results from a Gauss-Seidel iteration of the Radiative Transfer Code (RTC). This methodology is derived from the reflectance-based, vicarious calibration approach of the Remote Sensing Group at the University of Arizona. The algorithm is based on the relationship between the angular distribution of radiance, scattering and absorption in the atmosphere, and the surface properties. The RTC used to generate the LUT for the atmospheric correction is based on the following parameters: solar zenith angle, satellite view angle, relative azimuth angle between the satellite and sun, molecular scattering optical depth, aerosol scattering optical depth, aerosol scatter albedo, aerosol size distribution parameter, and surface reflectance. The size distributions for aerosol are based on either a Junge size distribution or on the set of aerosol types used in the atmospheric correction of Multi-angle Imaging Spectroradiometer (MISR) data. The initial versions of the algorithm rely on external climatological sources for information on atmospheric absorption and scattering parameters. Eventually, this information is likely to come from other Terra sensors like MISR and the Moderate-Resolution Imaging Spectroradiometer (MODIS). A digital elevation model provides the slope and elevation information for accurate modeling of surface reflectance.
Product Output Information
| Vgroup Data Fields/ Spectral Range (µm) |
Units | Data Type | Valid Range | Band Scale Factor |
|---|---|---|---|---|
SWIR (30 Meters) | ||||
| Band 4 (1.600 - 1.700) | w/m2/sr/µm | 16-bit unsigned integer | 0 - 32767 | 0.217400 |
| Band 5 (2.145 - 2.185) | w/m2/sr/µm | 16-bit unsigned integer | 0 - 32767 | 0.069600 |
| Band 6 (2.185 - 2.225) | w/m2/sr/µm | 16-bit unsigned integer | 0 - 32767 | 0.062500 |
| Band 7 (2.235 - 2.285) | w/m2/sr/µm | 16-bit unsigned integer | 0 - 32767 | 0.059700 |
| Band 8 (2.295 - 2.365) | w/m2/sr/µm | 16-bit unsigned integer | 0 - 32767 | 0.041780 |
| Band 9 (2.360 - 2.430) | w/m2/sr/µm | 16-bit unsigned integer | 0 - 32767 | 0.031800 |
Three groups of ancillary data inputs are used in the atmospheric correction of ASTER radiance and reflectance:
- Ozone data input: The NCEP/TOVS (National Centers for Environmental Prediction/TIROS (Television & Infrared Observation Satellite) Operational Vertical Sounder) data are acquired from a NOAA satellite, and provides the ancillary column ozone data twice daily. This was the default option with the alternative Naval Research Laboratory’s (NRL) Ozone Climatology data set. Starting on April 18, 2005, NOAA’s TOVS developed an irreversible instrument problem; as a result, use of the NCEP/TOVS ancillary data was discontinued for ASTER Level-1 data acquired April 18, 2005. This ancillary data source is available for ASTER data acquired prior to April 18, 2005. Currently, the only alternative in use for column ozone is NRL’s Ozone Climatology data set.
- Aerosol data input: Presently, NRL’s Aerosol Climatology ancillary data are the only available input.
- Temperature, Atmospheric Pressure, and Moisture Profile data inputs: The source of these data is the NCEP-GDAS (Global Data Assimilation System), which are available every 6 hours. The alternative is the NRL Climatology data set. These data sets are based on modeling, simulation, and prediction; they are static, monthly, averaged data sets that are used only as a last resort.
Ordering ASTER On-Demand L2 Surface Radiance
The ASTER On-Demand L2 Surface Radiance product is orderable through the EOS Data Gateway. The ordering process and procedures are described in the following tutorial: http://lpdaac.usgs.gov/tutorial/. As part of that process, it is necessary to first select an ASTER Level-1A granule from the EOS Data Gateway.
Product Information
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