The NewAtmosphere Plugin

The max_processes variable defines the maximum number of processes that the backend model can use. For example, multiple simultaneous MODTRAN processes can be used to generate the paths. On a multi-core computer, you can generally run one process per physical core. On a multi-core computer that supports hyper-threading, you can usually run 2 processes per physical core. The default value is set based on the number of virtual cores available on the host computer.

The time_delta variable defines the time resolution of the database. Since the atmosphere is not expected to vary rapidly, it does not make sense to compute new atmospheric values at 10s, 100s or 1000s of Hz if that is what the readout rate is of the sensor. Instead, the toolchain makes the assumption that the atmosphere is optically and geometrically (sun and moon scattering) "frozen" for some period of time. The default period or time resolution is 900 seconds (15 minutes).

The altitude_delta variable defines the altitude resolution of the database. Since the atmosphere is not assumed to be uniform in the vertical dimension, it needs to be sampled as a function of altitude if the scene(s) span significant altitude range. When the builder loads the scene(s), it looks at the altitudes spanned by the scene bounding box(es). Using the altitude sampling delta, it will construct a list of altitudes within each scene and then merge those into a combined list of altitudes for the database.

The default value for the altitude sampling delta is 500m. In most cases, the number of altitudes used in the database is a scalar multiplier for the number of backend model runs that need to be performed.

The hdf_filename variable defines the name of the HDF file to hold the resulting atmospheric database. The convention so far has been to use the compound extension ".adb.hdf".

The MODTRAN tape backend can optionally extract the atmospheric profile from the MODTRAN output "tape6" file. When this profile data is extracted, it is supplied to the radiometry core to drive refraction through the atmosphere. This option is controlled via the extract_profile variable, which is set to either true or false (note, boolean values in JSON are not strings, so do not assign the variable "true").

The first option for specifying the MODTRAN setup is using the MODTRAN "profiles" in the user’s DIRSIG settings (or preferences). In that case the setup includes the modtran_profile variable:

In this example, the profile named "MODTRAN5_2_4" might be mapped to a MODTRAN5 2.4 installation.

Another option is to specify the path to the executable and data directory directly in the backend description:

In this example, the user is pointing at a MODTRAN4 Version 3, Release 1 installation in their own directory. The modtran_exe variable is assigned the full path to the MODTRAN executable, the modtran_data is assigned the full path to the MODTRAN DATA directory and the modtran_min_delta variable is assigned the limiting frequency resolution for the band model database available with that version of MODTRAN (typically 1.0 wavenumbers for MODTRAN4). The name of the "modroot" file for MODTRAN4 was modroot.in.

Another option is to specify the path to the executable and data directory directly in the backend description:

In this example, the user is pointing at a MODTRAN5 2.4 installation in their own directory. The modtran_exe variable is assigned the full path to the MODTRAN executable, the modtran_data is assigned the full path to the MODTRAN DATA directory and the modtran_min_delta variable is assigned the limiting frequency resolution for the band model database available with that version of MODTRAN (0.1 wavenumbers for MODTRAN5 and later). The name of the "modroot" file for MODTRAN5 was mod5root.in.

Although MODTRAN6 introduced a new JSON input/output mechanism, the program is backward compatible with the traditional "tape" input/output mechanism utilized by this backend. This input/output option is accessed via the -modroot command line option, which is described in the manual and shown in the usage message of the console executable:

This backend employs the "modroot" approach for all versions of MODTRAN, where the basename of the input/output files for the MODTRAN run is stored in a separate file. MODTRAN5 and MODTRAN4 automatically looked for the mod5root.in and modroot.in files, respectively. MODTRAN6, however, needs the -modroot option to be specified as the 1st command-line argument for it to look in the mod5root.in file. Additionally, the MODTRAN6 executable needs to be supplied the full path to the MODTRAN6 DATA folder as the 2nd command-line argument. Hence, the complete command-line to launch MODTRAN6 needs to look like:

This backend has an optional configuration variable that allows the user to customize the command-line arguments that are passed to the MODTRAN executable. These arguments are defined in the JSON using the modtran_options variable, which is a JSON array of strings. The example configuration below shows how these two additional arguments are provided to the MODTRAN executable using the options array variable:

The tape5_filename variable defines the name of the input "tape5" file that is used as a template for the MODTRAN runs. In general, the backend is using the description of the atmosphere in this file (which model, aerosols, visibility, etc.) and is only updating the key parameters that are relevant to the specific DIRSIG simulation (the wavelengths, the source position, the path geometry, etc.).

The user also has the option to store the MODTRAN "tape5" file within the input JSON description via the tape5_contents variable. However, because the "tape5" file is multiple lines and you cannot embed newlines inside a JSON variable, the user will need to make the file a single string by replacing the newlines with the \n character:

Below is an example where the plugin description is contained in the JSIM file:

Alternatively, the JSON can be stored in an external JSON file and the name of that file specified in the JSIM file. This approach allows a single copy of a configuration to be stored and shared among multiple simulations. The example below contains the plugin configuration using the same JSON structure but in an external file (named mls_rural_23km_dis4.jatm in this example):

and then this external file is referenced in the JSIM file:

The following command-line options provide additional utility:

In most situations, the source of errors will be related to the external model executed by the backend. When an error occurs with either the running of the backend model process or in reading the output of the backend model, an error will be generated and the related files will be preserved in the reported path for debugging.

The "MODTRAN tape" backend creates a folder for all the MODTRAN executions (named modtran_runs) within which each model execution is stored in a sub-folder using a simple numerical numbering scheme from 0 to N. The backend will purge the contents of the model request folders after the model has been successfully run and the output ingested. However, these folders can be preserved by using the --save_files command-line option to the builder.

The file listing below shows the contents of a preserved model request folder after execution:

The README.txt file contain a "human readable" description of the inputs to MODTRAN. The mod5root.in contains the basename of the input/output files (in the case of this backend, it is always tmp), the modtran_stderr.txt and modtran_stdout.txt files contain the standard error and standard output (console) of the MODTRAN program, respectively. The tmp.tp5 and tmp.7sc files are the respective input and output files of the model execution.