Polarized Skylight Navigation Using Full Sky Imager

Abstract

Navigation is a skill used by all life. Although many plants and animals have developed natural navigation methods, humans typically employ a compass. A magnetic compass relies on the Earth's magnetic field. In the presence of other magnetic fields, such as in many electrical systems, its efficacy is degraded. For this reason, a new form of compass not reliant on the orientation of a magnetic field is investigated.
 The proposed method uses the atmospheric polarization of sunlight to determine heading. As light rays hit the atmosphere, they are scattered by gas molecules. During a scattering event, the E-vector of a ray is uniformly oriented with respect to the sun position, as described by the Rayleigh model. E-vector angles (AoLP) are consistently orthogonal to the solar meridian. This feature of the Rayleigh model can be used to determine the solar meridian from a polarized sky image.
 Sky images were taken using a polarizing camera and fed to a computer for classification. The classifier was based on a method published by Lu et al. It has three steps:
 
 Extract the AoLP from intensity data.
 Generate a binary image by selecting pixels representing an orthogonal AoLP.
 Apply a Hough transform to the binary image to extract heading.
 
 During testing, the compass was aligned with north. Then, an image was taken and classified. The solar azimuth angle in the camera reference frame was read as 125°. The recognized solar meridian with respect to north was 131°. Since the camera was aligned with north, these angles are comparable. The compass error was estimated at ±6°.
 The help of my colleagues has been integral, and I’d like to thank them. Truly, the work done on this project was made possible by ongoing guidance from Dr. Muhammad Alam. I am grateful for his support.