The DIRSIG model relies on other models to handle specific elements of the modeling chain. In these cases, existing models are utilized that are accepted by the modeling community to be the state-of-the-art tools for those applications.
For temperature prediction, DIRSIG utilizes a passive thermodynamic model called THERM that was developed by DCS Corporation for the Air Force in the late 80's. The model utilizes a meteorological history that includes air temperature, wind speed, direct (solar) insolation, diffuse (sky) insolation, humidity, etc. The meteorological history is used to drive a passive slab model that incorporates conventional thermodynamic parameters such as thermal conductivity, density, heat capacity, etc. to produce surface temperatures.
The use of this model is discussed extensively in the Thermal Modeling. The THERM model is built into the DIRSIG model and does not need to be acquired separately.
The DIRSIG model uses both MODTRAN and FASCODE to create the atmospheric databases used to simulate the scenes. These programs are used by the DIRSIG atmospheric database building utility program called make_adb. The use of MODTRAN and FASCODE by the make_adb program is discussed extensively in The make_adb User Manual.
The MODTRAN and FASCODE programs must be acquired by the user from the Air Force Research Laboratory. Instructions for obtaining access to these models can be found at the following websites: http://www.vs.afrl.af.mil/ProductLines/IR-Clutter/modtran4.aspx and http://www.vs.afrl.af.mil/ProductLines/IR-Clutter/fascode.aspx.