7.2. Validation and Calibration Activities The Level-1 data processing structure is composed of two parts: processing modules and on-line database files. Each module function is carried out using contents in the Level-0 data and processing parameters accommodated in the on-line database files. The on-line database files accommodate processing parameters that may need revision throughout the operation period to maintain data quality. The radiometric and geometric database files are major parts of the on-line database files from the data quality viewpoint. Preparation and updating of the database files are carried out by the ASTER Science Team by means of calibration/ validation activities that include the on-board calibration and acquired image analysis. The calibration/validation activity process is shown in Figure 7-1. On-board calibration is carried out at a maximum frequency of every 17 days during the normal operation period. The image analysis for vicarious calibration, the band-to-band registration and GCP matching are carried out less frequently. The version information is accommodated in the generic header of the Level-1 data products. Figure 7-1 Database calibration/validation activity flow Figure 7-2 shows the relationship between the major Level-1 processing modules and on-line data base files. The on-line data base files will be updated in line with the validation results. Figure 7-2 Relation between the major Level-1 processing modules and the on-line data base files Each database version is defined as follows： Version 0 ： Prepared from ground measurements during the preflight phase Version 0.4 ： Updated using early initial images during the early initial checkout period for the Level-1 processing system tests. Version 1 ： Updated using various calibration and validation data during the initial checkout period and used as the first version for the Level-1 data product generation during the normal operation period. Version N ： Validated version. All previous older version data will be reprocessed into this version by early 2003.