This manual was originally designed and written for DIRSIG Version 3 (or what is commonly referred to as DIRSIG3). In 2005, DIRSIG Version 4 (or what is commonly referred to as DIRSIG4) was released to the user community. The DIRSIG4 code bas was a complete rewrite of the DIRSIG model from the ground up. The new model's modular design was a result of years of experience upgrading the DIRSIG3 code base and from experience with true, object-oriented programming. The DIRSIG4 code base is exceptionally flexible and has allowed the developers to introduce new features with relative ease. This code base also features an improved radiometry core that handles representations that would have been very difficult to add to the DIRSIG3 code base.
The DIRSIG4 model features backward compatibility with DIRSIG3 inputs and will produce the same results as DIRSIG3 when given such inputs. However, the true power of the model is when the user utilizes the new features unique to the DIRSIG4 model. These improved features include:
An improved radiation core that allows the model to propagate polarized light, which allows DIRSIG4 to model polarization sensitive imaging systems.
An improved material subsystem that allows the user to use a growing set of surface and bulk optical properties. These include various BRDF functions, scattering functions, etc.
A flexible radiometry solver subsystem that gives the user increased control over how radiometric calculations are performed for radiation elements in the scene.
DIRSIG3 could model instruments with misregistered focal planes only if you made a separate Input Configuration File for each focal plane (each simulation computed only 1 of the focal planes.) DIRSIG4 allowed the user to configure instruments with multiple misregistered focal planes and will generate data for all of them in a single simulation.
The addition of laser radar instruments which allows the user to model both topographic and atmospheric LADAR/LIDAR systems.
An improved platform model that gives the user more flexibility and control over platform location and orientation.
The addition of instrument mount models that allow the user to point the instrument (relative to the platform) as a function of time.
The chapters in this section of the manual describe and explain features that are unique to the DIRSIG4 model. At this time, the DIRSIG4 computation engine is operational, however, many of the file and user interfaces have not be created and documentation is in the process of being written. That does not mean that the DIRSIG4 model cannot be used, but it does mean that the user will need to manually configure many of the advanced features without the aid of a graphical user interface. During this transition period from DIRSIG3 to DIRSIG4, the user needs to be aware of several limitations:
All of the DIRSIG4 input files will eventually be formatted as Extensible Markup Language (XML ) files. This is a robust file format (a parent to the HyperText Markup Language, or HTML) that can be easily created with software tools and validated for correct syntax. In addition, XML is a richer format than the custom DIRSIG3 "tag" format, which will eventually allow for improved graphical user interfaces.
The DIRSIG4 specific input simulation (the analogous file to the DIRSIG3 Input Configuration File) has not been finalized at this time. Eventually, a simulation will be described by an XML file that contains much of the information in the DIRSIG3 Input Configuration File. But, during this transition many features unique to DIRSIG4 will be accessed through new variables and sections in the DIRSIG3 Input Configuration File. This results in simulation configuration files that are specific to DIRSIG4 and will not work with DIRSIG3.
In most cases, there are not graphical user interfaces (GUIs) for the DIRSIG4 specific features, which means that they must be configured by hand editing the files. In many cases, these files will be XML files which will require careful formatting to create a valid file.
The DIRSIG4 model uses a specific version of the make_adb program. This version of make_adb understands how to correctly work with the new platform and instrument models and to create a polarizing atmosphere. The user must use the DIRSIG4 version of make_adb when using these DIRSIG4 specific features. A DIRSIG3 Atmospheric Database File file can be used with DIRSIG4 in many cases, but a DIRSIG4 Atmospheric Database File cannot be used with DIRSIG3.
DIRSIG4 does not have the "probe pixel" run-time option that was available in DIRSIG3. This capability will be handled at a later time by a separate program.
DIRSIG4 is currently not available for parallel processing machines. A parallel version of DIRSIG4 will be created at a future date.
DIRSIG4 does not currently have plume modeling enabled.