Sample image of L2 Surface Reflectance VNIR.
Sample image of L2 Surface Reflectance SWIR.
Users are advised that ASTER SWIR data acquired from late April 2008 to the present exhibit anomalous saturation of values and anomalous striping. This effect is also present for some prior acquisiton periods. Please refer to the ASTER SWIR User Advisory Document for more details.
The ASTER On-Demand L2 Surface Reflectance 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 data files in Hierarchical Data Format (HDF), one for VNIR, one for SWIR.
VNIR & SWIR Descriptions
The ASTER On-Demand L2 Surface Reflectance (VNIR) is a higher-level product that contains atmospherically corrected visible and near-infrared data. It is generated using the three VNIR bands (between 0.52 and 0.86 µm) from an ASTER Level-1B image.
The ASTER On-Demand L2 Surface Reflectance (SWIR Crosstalk-Corrected) is a higher-level product that contains atmospherically corrected shortwave infra-red data. It is generated using the six 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 VNIR's original radiance values to atmospherically corrected surface radiance and reflectance values. The atmospheric correction algorithm for VNIR 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.
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 m apart in the band order 7, 8, 9, 4, 5, and 6. In front 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 detectors 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-4s 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.
VNIR
Area | ~60 km x 60 km |
Image Dimensions | 4200 rows x 4980 columns |
File Size | ~180 MB |
Spatial Resolution | 15 m |
Projection | Universal Transverse Mercator (UTM) |
Data Format | HDF-EOS or GeoTIFF |
Vgroup Data Fields | 3 |
Crosstalk-Corrected SWIR
Area | ~60 km x 60 km |
Image Dimensions | 2100 rows x 2490 columns |
File Size | ~75 MB |
Spatial Resolution | 30 m |
Projection | Universal Transverse Mercator (UTM) |
Data Format | HDF-EOS or GeoTIFF |
Vgroup Data Fields | 6 |
VNIR (15 Meters)
Vgroup Data Fields/ Spectral Range (µm) |
Units | Data Type | Valid Range | Band Scale Factor |
Band 1 (0.52–0.60) | none | 16-bit signed integer | 0–1000 | 0.001 |
Band 2 (0.63–0.69) | none | 16-bit signed integer | 0–1000 | 0.001 |
Band 3N (0.78–0.86) | none | 16-bit signed integer | 0–1000 | 0.001 |
Crosstalk-Corrected SWIR (30 Meters)
Vgroup Data Fields/ Spectral Range (µm) |
Units | Data Type | Valid Range | Band Scale Factor |
Band 4 (1.600–1.700) | none | 16-bit signed integer | 0–1000 | 0.001 |
Band 5 (2.145–2.185) | none | 16-bit signed integer | 0–1000 | 0.001 |
Band 6 (2.185–2.225) | none | 16-bit signed integer | 0–1000 | 0.001 |
Band 7 (2.235–2.285) | none | 16-bit signed integer | 0–1000 | 0.001 |
Band 8 (2.295–2.365) | none | 16-bit signed integer | 0–1000 | 0.001 |
Band 9 (2.360–2.430) | none | 16-bit signed integer | 0–1000 | 0.001 |
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LP DAAC Acknowledgement and Citations Policy
In the event that data distributed from the Land Processes DAAC are incorporated into your research, please supply the following acknowledgment within your published work: "These data are distributed by the Land Processes Distributed Active Archive Center (LP DAAC), located at the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center (lpdaac.usgs.gov)." If possible, please e-mail or send us reprints/citations of papers or oral presentations based on data obtained from the LP DAAC (see below for mailing address and e-mail address). This will help us to stay informed of how the data are being utilized.
LP DAAC User Services
U.S. Geological Survey (USGS)
Earth Resources Observation and Science (EROS) Center
47914 252nd Street
Sioux Falls, SD 57198-0001
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U.S. Geological Survey (USGS)
Center for Earth Resources Observation and Science (EROS)
47914 252nd Street
Sioux Falls, SD 57198-0001
Phone Number: | 605-594-6116 |
Toll Free: | 866-573-3222 |
866-LPE-DAAC | |
Fax: | 605-594-6963 |
Email: | lpdaac@eos.nasa.gov |
Web: | http://lpdaac.usgs.gov |