On May 24, 2006, the LP DAAC started offering ASTER Level-1 data sets as on-demand products. Two reasons are responsible for this change:
The corrected and updated ASTER Level-1A data are used to generate the Level-1B data set, which in turn are used as the input for generating all the higher-level products except the ASTER DEM, which uses an ASTER Level-1A data set. Users may choose to order the new Level-1A, Level-1B, or any of the higher-level products (subject to the appropriate band availability). All the Level-1 data sets and higher-level products are disseminated via the FTP-Pull process only. The Level-1B data sets are offered with standard projection and resampling options.
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Since the launch of ASTER in 1999, the ASTER Ground Data System (GDS) in Japan has been responsible for Level-1 processing of ASTER data. This included processing virtually all ASTER Level-0 data to Level-1A (L1A) data, as well as processing approximately one-third of the L1A data to Level-1B (L1B) data. Copies of all L1A and L1B data processed by GDS were sent to the LP DAAC for archiving and distribution to users. Users who desired L1B data not available at the DAAC could order those L1B data from ASTER GDS in Japan. The LP DAAC has long been interested in being able to process any L1A data archived at the DAAC to L1B data, so that our users would have routine access to L1B data (and higher-level products) from the entire ASTER data archive.
In late 2005, agreement was reached to change the approach to ASTER Level-1 processing. Under the new approach, ASTER GDS in Japan will continue to process ASTER Level-0 data to L1A data, and send copies of all L1A to the LP DAAC. The LP DAAC and ASTER GDS each will assume responsibility, using software developed by GDS, for processing on-demand L1B data they distribute to their users and/or use in producing higher-level products ordered by their users. ASTER GDS implemented their Level 1 on-demand processing system on April 19, 2006, and the LP DAAC system became operational on May 24, 2006.
The on-demand approach to L1B data processing not only opens the entire ASTER L1A archive to routine access by users, it also allows the LP DAAC and GDS to apply appropriate and current radiometric and geometric correction coefficients. This ensures that the L1B data received by the user and/or used in higher-level product generation are as accurate as current knowledge of the system allows. Likewise, users who order L1A data will receive a product with the most current and accurate correction coefficients appended.
In addition to the routine access to fully corrected L1B data from the complete ASTER data archive, users should bear in mind other important characteristics of ASTER On-Demand Level-1 processing at the LP DAAC:
Data Access. Users can search and order ASTER L1A data, on-demand L1B data or any of the ASTER higher-level products (from the entire L1A archive) through the EOS Data Gateway (EDG). Currently, the EDG is the only method by which users can search and order on-demand ASTER L1B data from the entire L1A archive. Within a month or two, following necessary modifications, ASTER L1B data from the entire L1A archive will also be available on-demand via GloVis. Currently, only data acquired after March 22, 2003 are available through GloVis due to incompatibility of the smaller browse images associated with those data. LP DAAC is scheduled to process all the small browse images to a size consistent with the current GDS-produced browse images. Finally, the LP DAAC will purge the DataPool of ASTER L1B data, and repopulate it over a period of a few months with ASTER L1B data acquired over the U.S. and Territories. The Data Pool ultimately will contain a rolling archive of approximately two-years worth of qualifying ASTER L1B data automatically processed from their L1A source data.
Distribution Media. As an on-demand product, ASTER L1B data sets are available to users via FTP-Pull only. Furthermore, because ASTER L1A data now are routed through the on-demand system to ensure that appropriate and current correction coefficients are appended, they too are available via FTP-Pull only.
Level-1 Browse. The primary browse for ASTER Level-1 data, both L1A and L1B is the L1A browse routinely generated by ASTER GDS. For a few months following implementation of the on-demand system, the EDG will have a "mixed browse" as it will continue to utilize the smaller L1A browse images generated by ASTER GDS prior to March 22, 2003 acquisition date. New L1A browse images generated by the LP DAAC matching the size and quality of the current L1A browse images soon will replace the smaller browse images. GloVis, in due course, will utilize the current (larger) L1A browse images for all the data sets, as well.
Data Cost. The cost of ASTER data and products was not affected by Level-1 on-demand processing, except that there is no media charge, because all data and products are shipped via FTP-Pull only. The cost of ASTER data and products remains $80 per product.
Higher-Level Products. All LP DAAC-produced ASTER Level-2 standard data products utilize fully corrected, on-demand L1B data as input, with no added cost to the user. The ASTER DEM product uses L1A data as input, and the ASTER DEM generation system implemented on May 24, 2006 automatically applies the appropriate and current radiometric and geometric correction coefficients.
Default Parameters Only. Following previous practice, ASTER Level-1B data from the LP DAAC are available with default map projection and resampling parameters only. Users with alternate parameter preferences should contact ASTER GDS.