DIRSIG simulates the image acquisition of scenes including the effects from the atmosphere and the thermal emission of objects in the scene. Therefore, to accurately model these important attributes, DIRSIG needs to be provided information regarding the atmospheric conditions at the image time (visibility, water vapor amounts, aerosol types and amounts, etc.) to correctly model the atmosphere's optical properties and a meteorological history (air temperatures, wind speeds, etc.) to correctly predict surface temperatures. The ENVIRONMENT section of the CFG file is where these items are specified.
ENVIRONMENT {
TAPE5_FILENAME = mls.tp5
ADB_FILENAME = example.adb
WEATHER_FILENAME = mls.wth
}
Table 32-3 describes each of the variables that may appear in the ENVIRONMENT section of the DIRSIG configuration file.
Table 32-3. List of the variables that can be set in the ENVIRONMENT section.
| Variable Name | Usage | Description |
| TAPE5_FILENAME | Required | Name of the template input file utilized by |
| ADB_FILENAME | Required | Name of the Atmospheric Database (ADB) file generated using |
| WEATHER_FILENAME | Required | Name of the weather history file utilized by the thermal model |
The TAPE5_FILENAME variable indicates the name of the MODTRAN input file (which is normally called TAPE5 or tape5) which will be used as a template for a series of MODTRAN runs which will be used to generate the DIRSIG Atmospheric Database (ADB) for the current simulation. The format of this file is extremely complex and is described in the documentation provided by AFRL with the MODTRAN distribution. Although the creation of this file is beyond the scope of this manual, the DIRSIG distribution does come with several standard TAPE5 files which are listed in Table 32-4 and which can be found in DIRSIG_HOME/lib/tape5 directory. In our example file, the user wishes to use MODTRAN's built-in ``Mid-Latitude Summer'' model for their scene (indicated by the mls.tp5 filename).
Table 32-4. List of the available input files for common atmospheres.
| Name | Description |
| trop.tp5 | Tropical Atmosphere |
| mls.tp5 | Mid-Latitude Summer Atmosphere |
| mlw.tp5 | Mid-Latitude Winter Atmosphere |
| sas.tp5 | Sub-Arctic Summer Atmosphere |
| saw.tp5 | Sub-Arctic Winter Atmosphere |
| us_std.tp5 | 1976 U.S. Standard Atmosphere |
The ADB_FILENAME variable indicates the name of the Atmospheric Database (.adb extension) file that will be generated using MODTRAN. This file is usually unique for almost every different simulation scenario since any change in the time of day, day of year, sensor position, meteorological conditions will change the radiative transfer properties of the atmosphere. The name of this file should be, therefore, appropriately named for the scenario.
The WEATHER_FILENAME variable indicates the name of the file describing the meteorological conditions at the scene for at least the previous 24 hours. The format of this file is described in Weather History File. Table 32-5 lists a small set of idealized weather history files that are included with the DIRSIG distribution. The name of these files reflect the geographical breakdown utilized by MODTRAN's internal atmospheric models, but they do not necessarily imply the same atmospheric conditions. These files are useful for testing purpose but actual site measurements are preferred.
DIRSIG relies on MODTRAN (currently Version 4) to model the transmission, scattering and emission of the atmosphere along the various paths of radiation propagation modeled. Rather than interactively call MODTRAN for every atmospheric path, DIRSIG generates a large look-up table (database) from which any path can be interpolated from. The approach used to generate this database is to utilize a user-supplied MODTRAN input file (tape5 ) as a template and change only the MODTRAN variables required to generate the database. Atmospheric Modeling describes this approach in more detail.