One of the key features of the atmospheric modeling support developed during the DIRSIG5 lifecycle was a unified interface to different, third-party atmospheric physics and radiative transfer models.
These external atmospheric models are referred to "backends", and there are currently two backend interfaces that have been created:
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The "dummy" backend, which doesn’t interface to an external model. Instead, it has analytical models for the atmosphere that are similar to the capabilities of the Simple atmosphere model in DIRSIG4.
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The "MODTRAN Tape" backend, which interfaces with MODTRAN using the classic "tape-based" input/output files. This should be compatible with MODTRAN4, MODTRAN5 and MODTRAN6.
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At a future date, backends will be developed to (1) use MODTRAN6 via JSON input/output method and (2) interface with the 6SV model. |
These supported backend models are used by the DIRSIG5-era atmospheric database builders including atm_builder and fourcurve_builder.
This document outlines the JSON configuration options for the available backends.
The Dummy Backend
The "Dummy" backend is built-in (it does not call an external program) and it implements a set simplified calculations to approximate nominal illumination conditions:
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The sun is modeled with a 5800K blackbody spectral distribution that is magnitude normalized to approximate the Earth reaching irradiance of the sun.
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The moon is modeled using the same solar spectral distribution, but is magnitude normalized to approximate the Earth reaching irradiance of the moon.
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The sky is modeled with an exponentially decaying spectral distribution that approximates clear sky scattering conditions in the visible.
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There is no path radiance or transmission along other paths.
This backed is configured without any additional options:
"backend" : { "name" : "dummy" }
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The "dummy" backend is primarily for testing purposes and rapid use during simulation setup and debugging. It should not be used for data production simulations. |
The MODTRAN Tape Backend
The "MODTRAN Tape" backend is configured with a variety of options that provide details about the external MODTRAN installation and user input files. For the examples included here, the specific paths to the MODTRAN executable, data folder, etc. may differ from your installation.
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Although the examples below show various configurations for older versions of MODTRAN, the user is strongly encouraged to use the latest version of MODTRAN6. Visit the MODTRAN page at the Spectral Sciences, Inc. website for more information. |
The option to extract atmospheric profile data
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"
).
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This option is disabled by default. |
Using a MODTRAN configuration profile from your settings
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:
"backend" : { "name" : "modtran_tape", "modtran_profile" : "MODTRAN5_2_4", "modroot_filename" : "mod5root.in", "tape5_filename" : "./mls.tp5", "extract_profile" : false }
In this example, the profile named "MODTRAN5_2_4" might be mapped to a MODTRAN5 2.4 installation.
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This backend uses the "modroot" approach to naming the input and
output files associated with the MODTRAN run (see the MODTRAN
documentation for more information about this mechanism). The
modroot_filename variable indicates the name of the file containing
the base name of the MODTRAN input and output files. Since the name
of this file has changed depending on the version of MODTRAN, it is
provided as a user configuration variable.
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Supplying the installation paths: MODTRAN4
Another option is to specify the path to the executable and data directory directly in the backend description:
"backend" : { "name" : "modtran_tape", "modtran_exe" : "/Users/dirsig/Mod4v3r1/Mod4v3r1.exe", "modtran_data" : "/Users/dirsig/Mod4v3r1/Mod4v3r1/DATA", "modroot_filename" : "modroot.in", "modtran_min_delta" : 1.0, "tape5_filename" : "./mls.tp5", "extract_profile" : false }
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
.
Supplying the installation paths: MODTRAN5
Another option is to specify the path to the executable and data directory directly in the backend description:
"backend" : { "name" : "modtran_tape", "modtran_exe" : "/Users/dirsig/Modtran5.2.4/mod5_cons.exe", "modtran_data" : "/Users/dirsig/Modtran5.2.4/DATA", "modroot_filename" : "mod5root.in", "modtran_min_delta" : 0.1, "tape5_filename" : "./mls.tp5", "extract_profile" : false }
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
.
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The specific paths to the MODTRAN executable will vary based on your installation. |
Supplying the installation paths: MODTRAN6
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:
$ ./mod6c_cons -h Usage: ./mod6c_cons input_file [data_path] JSON and TAPE5 input formats are supported. [data_path] specifies an optional path to the MODTRAN data directory. optional flags -modroot Process legacy input formats using mod5root.in or tape5 files. ...
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:
$ /some/path/MODTRAN6.0.0/bin/mod6c_cons.exe -modroot /some/path/MODTRAN6.0.0/DATA
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:
"backend" : { "name" : "modtran_tape", "modtran_exe" : "/Users/dirsig/MODTRAN6.0.0/mod6c_cons.exe", "modtran_data" : "/Users/dirsig/MODTRAN6.0.0/DATA", "modroot_filename" : "mod5root.in", "modtran_options" : ["-modroot","/Users/dirsig/MODTRAN6.0.0/DATA"], "modtran_min_delta" : 0.1, "tape5_filename" : "./mls.tp5", "extract_profile" : false }
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The specific paths to the MODTRAN executable will vary based on your installation. |
The input "tape5" filename
Using an External File
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.).
Using an Internal File
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:
"name" : "NewAtmosphere", "inputs" : { "info" : { [deleted for documentation purposed] }, "backend" : { "name" : "modtran_tape", "modtran_profile" : "MODTRAN4", "modtran_exe" : "/Users/dirsig/pkg/Mod4v3r1/Mod4v3r1.exe", "modtran_options" : [""], "modtran_data" : "/Users/dirsig/pkg/Mod4v3r1/DATA", "extract_profile" : false, "tape5_contents" : "M 2 2 2 0 0 0 0 0 0 0 1 1 1 0.000 0.00\n 2 0 0 1 0 0 0.000 0.000 0.000 0.000 0.218\n 20.218 0.218 180.000 0.000 0.000 0.000 0\n 1 2 236 0\n 43.000 77.000 0.000 0.000 12.000 0.000 0.000 0.000\n 450 35050 100 2\n0" }, "hdf_filename" : "mls_rural_23km_dis4_scaled.adb.hdf" }
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The user needs to be very careful when performing this newline replacement operation to otherwise not disrupt the precise format of the MODTRAN "tape5" file. |
Errors and Debugging
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.
Under the hood of the MODTRAN Tape backend
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:
$ ls modtran_runs/0 README.txt modtran_stderr.txt tmp.7sc mod5root.in modtran_stdout.txt tmp.tp5
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.