SOSM_atmosphere_model

atmosphere_model SOSM_atmosphere_model

This page describes how to create an SOSM atmosphere definition using the table model.

Creating the atmosphere definition consists of three step:

A pair of Perl scripts are provided with the WSF release which facilitate the use of MODTRAN in preparing the atmosphere tables needed by the model. These scripts are located in sosm/modtran and operate as follows:

Executing MODTRAN

A Perl script ‘modtran_execute.pl’ is included with the standard WSF distribution in the directory sosm/modtran. This script facilitates the creation of the raw spectral atmospheric data using MODTRAN over the desired altitude, elevation and range values. Execute the script as follows:

*<path-to-wsf>*/sosm/modtran/modtran_execute.pl *<basename>*

The file ‘<basename>.def’ which indicates the observer altitudes, the line of sight elevation angle and range values and the MODTRAN input cards. The preparation of a ‘.def’ file requires some knowledge of MODTRAN. Several examples of ‘.def’ files are present in the sosm/modtran directory. The output from this script are the files:

*<basename>*_bgr.plt - Background radiance
*<basename>*_bgt.plt - Background transmittance *(Not currently used)*
*<basename>*_fgr.plt - Foreground radiance
*<basename>*_fgt.plt - Foreground transmittance

Note that it can take several hours to perform this process.

The following observations should be noted when creating the MODTRAN input:

  • Card 4, FLAGS(1:1) should be blank or ‘W’ (wavenumber input)

  • Card 4, XFLAG must be set to ‘W’ (wavenumber output).

  • Card 4, DV should be set to 20 (cm-1) unless you are also prepared to update your sensor and target models.

  • Card 4, V1 and V2 should be a multiple of DV.

  • The size of the output files will be directly proportional to the number of spectral samples, i.e.: ((V2 - V1) / DV) + 1.

Converting MODTRAN Output

Once MODTRAN has been run to create the necessary files, the files must be converted into tho format that can be used by SOSM. This can be accomplished by using the Perl script modtran_convert.pl that is supplied with the standard WSF distribution in the directory sosm/modtran. Execute the script as follows:

modtran_convert.pl *<basename>*

The script takes the files produced by the previous step and produces the tables needed by SOSM:

*<basename>*.bgr
*<basename>*.bgt *(Not currently used)*
*<basename>*.fgr
*<basename>*.fgt

Creating the Atmosphere Definition File

After running the previous steps, the file ‘<basename>.txt’ should be created which contains the following:

atmosphere_model table
   background_radiance      *<basename>*.bgr
   foreground_radiance      *<basename>*.fgr
   foreground_transmittance *<basename>*.fgt
end_atmosphere

Example

Assume you wish to create a new atmosphere type that is based on the US Standard Day atmosphere that is provided with the standard WSF distribution. Copy ‘sosm/modtran/us_std_day.def’ to ‘us_std_day_mod.def’ and edit as desired. Then execute MODTRAN:

*<path-to-wsf>*/sosm/modtran/modtran_execute.pl us_std_day_mod

Convert the files:

*<path-to-wsf>*/sosm/modtran/modtran_convert.pl us_std_day_mod

Create the file ‘us_std_day_mod.txt’ with the following contents:

atmosphere_model table
   background_radiance      us_std_day_mod.bgr
   foreground_radiance      us_std_day_mod.fgr
   foreground_transmittance us_std_day_mod.fgt
end_atmosphere

To use the new model in your simulation, add the following commands to your simulation input stream after all other sosm_interface commands:

sosm_interface
   sosm_interface.load_atmosphere     US_STD_DAY_MOD from us_std_day_mod.txt
   sosm_interface.default_atmosphere  US_STD_DAY_MOD
end_sosm_interface