Refraction1

This demo shows how to use the built-in atmospheric profiles in DIRSIG5 to enable refraction with any classic atmosphere setup. It also demonstrates the use of the "--split_channels" option to enable dispersion.

DIRSIG5 contains an internal, layer-based model of the atmosphere that can be used to model the effects of refraction. While the refractive index of the atmosphere is always close to 1 (only about 1.0003 at maximum) the cumulative effect of a varying refractive index from a high altitude, expecially when traveling a slant path, can be significant.

Practically speaking, refraction manifests itself in two ways. First is a difference between the direct line of sight of the sensor and what is actually seen as light travels to the sensor. Assuming that the sensor is oriented so the sky is "up" this effect manifests itself as an apparent vertical offset in the resulting image (more refraction means that the image moves farther up) and a corresponding horizontal offset in the locations on the ground that are being imaged. The second manifestation is the dispersion of energy of different wavelengths as it passes through the atmosphere. Even at visible wavelengths this effect can be significant, especially at edges.

Since both behaviors are dependent on the spectral refractive index at any point in the atmosphere, it is important to have a comprehensive and efficient model of the properties of the atmosphere in order to calculate the values during propagation and "bend" the rays of the ray tracer appropriately. We take the approach of using a layer model that models the atmosphere as concentric spheres (really concentric ellipsoids since they are based on the eccentricity of the Earth). The ray is refracted at every layer boundary and the properties are assumed to be constant within each layer. The properties being provided are the temperature, pressure and dew point, which are then used to calculate the refractive index for the current spectral state (dispersion will only be seen using multiple spectral states, as explained below).

All classic D4 atmosphere setups can enable refraction by using one of the built-in atmosphere profiles as an attribute to the radiative transfer entry in the ".atm" file. For example,

which enables the mid-latitude summer profiles (corresponding to the profiles of the same name in MODTRAN). There are 6 built-in profile models available that match the basic MODTRAN profiles:

as well as:

The resulting profiles are shown below:

The scene contains a resolution chart positioned so that it is (more or less) centered in a 512x512 framing array looking through a mid-latitude summer atmosphere at a slant angle of 30 degrees from nadir and about 20km distance. It is a purely constructed example and is not meant to represent a realistic system.

If run with the default command-line arguments, there are no differences in refraction between the bands, this is because a single spectral state is being used to represent the propagation of rays throughout the scene.

Since this is the default behavior of DIRSIG5 (dispersion does not commonly impact the sensor), a command-line option has to be set to tell DIRSIG to use a different spectral state for each sensor channel (in this case, one for each of the Red, Green and Blue channels). This is done by running "dirsig5 — split_channels=true …​". This triggers a different state for each channel with the center wavelength being used as the representative or "reference" value when choices have to be made about where a path goes. The result using the MLS atmosphere demonstrates how the light for each channel travels a slightly different path:

Note that if the atmosphere profiles are turned off (either by removing the "use_profile" attribute or setting it to "none") the image will no longer contain the resolution target. This is due to the direct line of sight looking above and past the top of the target. Even using a different atmosphere will result in a visible shift in the target location, just due to differences in the paths taken. The sub-arctic winter simulation is shown below:

As additional atmosphere model plugins are developed, they will have the opportunity to provide their own profiles directly and are not limited to the bulit-in profiles.