MODTRAN Model

The MODerate resolution atmospheric TRANsmission (MODTRAN) model is an application that predicts the signal transmission for infrared through visual and into the ultra violet spectral regions (VIS/NVIS regions). MODTRAN is a community standard model within the government, commercial, and educational sectors and has been in use and development for decades.

The molecular makeup of the atmosphere, along with the contaminants, can impair the transmission of a signal in the visible and near visible spectrum (VIS-NVIS). This band of the spectrum has very small wavelengths, and because these wavelengths approach the size of the atmosphere particulates, absorption, and scattering become apparent.

MODTRAN takes into consideration the atmospheric molecular content, aerosols, particulates, and clouds, all of which can have severe effects on VIS-NVIS signals. MODTRAN represents the atmosphere as a series of horizontally homogenous layers and utilizes vertical profiles of the constituents, considering the signal path through each layer. MODTRAN incorporates various radiative transfer models, including narrow band and line-by-line models. There are several different atmospheric models available for selection within MODTRAN as well as various aerosol models and cloud models. Setting up MODTRAN for execution can entail an overwhelming number of input parameters and settings. MODTRAN within the Systems Tool Kit® (STK®) application has a limited number of user inputs, so it is easier for your setup and analysis.

At each time step, the STK software passes your inputs along with certain STK time-dynamic parameters into MODTRAN. This provides MODTRAN with time-dynamic variations such as the positions of mobile platforms and direction of payloads. Upon execution of MODTRAN, the STK application will utilize the transmittance MODTRAN provides to produce link budget and system performance analysis.

Input parameters

The following two tables describe the input parameters you specify for the MODTRAN option.

You need to set these parameters, which are also required for the MODTRAN-derived lookup table option:

Parameter	Description
Aerosol Model	Select one of the aerosol models available from the drop-down menu.
Visibility	To set the visibility, specify the greatest distance a black object located near the ground can be seen and recognized against a bright background.
Relative Humidity	Specify the percentage of relative humidity.
Surface Temperature	Specify the temperature at the surface of the Earth in the units given.

You also need to set these parameters that are specific to the MODTRAN option:

Additional MODTRAN Input parameters

Parameter	Description
Atmosphere Model	Select one of the atmosphere models available from the drop-down menu.
Cloud Model	Select one of the cloud models available from the drop-down menu, which includes a "no clouds" option. If you choose to apply a cloud layer, MODTRAN applies the default thickness and altitude that is typical for the selected cloud type. You can choose to override the default cloud thickness and altitude using the parameters below.
Override Thickness	Select this check box and enter a thickness value to change the default cloud thickness for the selected cloud type in Cloud Model.
Override Altitude	Select this check box and enter an altitude value to change the default cloud altitude for the selected cloud type in Cloud Model.

MODTRAN output file writing parameters

You can specify certain aspects of MODTRAN’s output file generation. MODTRAN can write out several different files, some of which can be very large in size. Since the STK software will execute MODTRAN at every time step, this can generate an enormous amount of output, so you should be careful when using this feature. The files will be placed in the project directory, and the file names will have the following format: t<timestep>_<transmittingObjectPath>-<receivingObjectPath>.<ext>

The following output file writing parameters are available for you to adjust:

MODTRAN output file writing parameters

Parameter	Description
Minimum start time	Enter the time at which you want MODTRAN to start generating output. If you specify a time at which there are no laser communication accesses, then MODTRAN will wait until the first access after this time to start generating output.
Number of time steps	Enter the maximum number of time steps to capture MODTRAN output files.
Include JSON file	Select this check box to tell MODTRAN to include the input file, in JSON format, in the output.