The BasicAtmosphere Plugin

The BasicAtmosphere plugin and make_adb tool date back to the late releases of DIRSIG3 (1996) and were the primary mechanism to include a MODTRAN-driven atmosphere in DIRSIG4.

In DIRSIG4 the atmosphere was described via the .atm file, which supported a three different radiative transfer (RT) modeling modes for the atmosphere. In DIRSIG5, the BasicAtmosphere plugin provides the backwards compatibility for these modeling modes.

Table 1. Atmospheric modeling modes supported by the `BasicAtmosphere` plugin.
Mode	Summary	Suggested Usage
`SimpleAtm`	Spectrally varying direct visible and diffuse thermal illumination, unity path transmission, zero path radiance.	Preview simulations, testing
`UniformAtm`	Spectrally constant direct + Diffuse illumination, unity path transmission, zero path radiance.	Preview simulations, testing
`ClassicAtm`	MODTRAN-driven direct and diffuse illumination, path transmission and path radiance.	Production simulations

Simple Atmosphere

Summary

The "simple" atmospheric RT model (sometimes referred to simply as "SimpleAtm") is a parametric model that has the following characteristics:

The solar irradiance is modeled as a 5,800K blackbody but has been scaled to a magnitude generally found on a clear, equatorial day.
The lunar irradiance is modeled as the same 5,800K blackbody but has been scaled to a magnitude generally found on a clear, equatorial day.
In the visible, the sky irradiance is 0. In the thermal, the sky irradiance is driven by the user-supplied apparent sky temperature.
There is no scattering or absorption (extinction) along any paths within the atmosphere.

Usage

Because this model is fully parametric, this mode can be used without the burden of any pre-simulation computations that can impede rapid configuration iterations. This makes this model useful for preview simulations while a complex simulation is being assembled and/or debugged.

Due to the simple nature this model it is also used extensively in DIRSIG’s automated testing because it is possible to write closed-form solutions to radiometric scenarios.

Due to the unrealistic illumination and path propagation employed by this mode, the SimpleAtm model is not recommended for production data generation.

Uniform Atmosphere

Summary

The "uniform" atmospheric RT model (sometimes referred to simply as "UniformAtm") is a parametric model that has the following characteristics:

The total irradiance from the hemisphere above the scene is driven by a user-supplied, spectrally constant irradiance.
This total irradiance can be partitioned into direct (from the location of the Sun) and diffuse (from the rest of the sky) using a user-supplied scalar fraction (referred to as the "sky fraction").
There is no scattering or absorption (extinction) along any paths within the atmosphere.

Usage

Due to the simple nature this model it is also used extensively in DIRSIG’s automated testing because it is possible to write closed-form solutions to radiometric scenarios.

Due to the unrealistic illumination and path propagation employed by this mode, the UniformAtm model is not recommended for production data generation.

Classic Atmosphere

Summary

The most commonly used mode is the "ClassicAtm" mode, which used a pre-generated database of spectral atmospheric components that were computed by MODTRAN using the make_adb tool. The user must provide make_adb with a MODTRAN input file (referred to as the MODTRAN "template" file) that describes the type of atmosphere to be modeled (e.g., clearly, hazy, humid, etc.). The make_adb tool would then update this template with simulation specific values (geolocation, time of day, day of year, solar and lunar state, wavelengths, etc.) to generate the spectral components required. The results of the MODTRAN runs were then stored in an atmospheric database (referred to as the ADB).

The database contains a sub-set of all the atmospheric paths that might be traversed in a given simulation. This is because running MODTRAN for every path would be computationally impractical. Instead, the database is contains a set of look-up tables (LUTs) that are angularly interpolated on-the-fly.

This database is specific to the simulation (the scene location, date, time, sensor, etc.) and generally cannot be shared across simulations. The atmospheric database provided this mode with the following characteristics:

The ground reaching solar irradiance was computed using the location of the scene, date and time. The position of the Sun is driven by the ephemeris model employed in the simulation and not by MODTRAN’s internal ephemeris model.
The ground reaching lunar irradiance was computed using the location of the scene, date and time. The position of the Moon is driven by the ephemeris model employed in the simulation.
The paths between the sensor and the scene are computed as a function of angle across the field-of-view of the sensors in the simulation. This allows for the path transmission and path radiance (scattering and self-emitted) to vary across the FOV.
The sky irradiance is computed by sampling the sky dome on a fixed zenith and azimuth sampling scheme. This allows the scattered and self-emitted flux gradients across the sky to be captured.

Usage

Unlike the "SimpleAtm" and "UniformAtm" modes, the "ClassicAtm" mode requires the user to pre-generate the ADB file before running the simulation. This is accomplished by supplying the make_adb tool with the name of the DIRSIG4 era .sim file:

$ make_adb foo.sim

The details of how the make_adb tool and MODTRAN interact as well as the various options available for the tool are described in greater detail in the make_adb manual.

For DIRSIG4 simulations, "ClassicAtm" is considered the best choice since it employs specific, physics-driven solutions from the MODTRAN model for the given DIRSIG simulation.