This software simulates Sun images observed on the ground and from space with an easy-to-use analytical approach and with high accuracy. It is a simple, analytical approach with the accuracy of a rigorous, multilayer model. The existing analytical approach cannot predict when the image of the Sun is blocked by the surface of the Earth, but the current approach can.
When imaging a setting or rising Sun on the ground or from space, the shape and the intensity distribution of a captured image are affected by the following three factors: limb-darkening of the solar-disk, atmospheric extinction, and atmospheric refraction. A numerical tool was developed for the simulation of Sun images that takes into account all of the three factors.
The tool uses the following innovations.
- Cutoff-angle: The elevation-dependent apparent altitude angle at which the rays from the Sun are blocked by the surface of the Earth, and thus are not detected by a sensor;
- Elevation or observation-height-dependent relative air-mass; and
- Extension of optical depth from visible spectral range to infrared in the atmospheric extinction calculation.
When dealing with atmospheric refraction, the relationship between the true altitude angle of geometric Sun and the apparent altitude angle of a model or photo Sun is described by a transfer curve.
This work was done by Erkin Sidick of Caltech for NASA’s Jet Propulsion Laboratory.