For a number of applications, the pointing direction towards the Sun must be known or measured. Con ventional Sun sensors determine the pointing direction towards the Sun while the Sun is in the field of view of the sensor. The disadvantage of a conventional Sun sensor is that it only operates when there is an unobstructed line of sight to the Sun. At some locations/seasons, it is virtually impossible to use a conventional Sun sensor.
The sensor measures Rayleigh scattering in the sky. The idea of using Rayleigh scattering for finding the Sun is not novel. It is believed that the Vikings utilized Rayleigh scattering in the sky for navigation. Also, some insects utilize this principle. The described polarization Sun sensor is a fully automated digital implementation of a well-known phenomenon.
The sensor operates by imaging the sky with a motorized linear polarizing filter. Each pixel on the sky is polarized orthogonally to the orientation of the Sun. A great circle is projected on a sphere from each pixel towards the Sun. All the great circles converge in the same point where the Sun is located.
The proposed Sun sensor method estimates the Sun vector by observing the polarization pattern in the sky. No Sun sensor technology previously has used this method as way of estimating the Sun vector. Furthermore, unlike traditional Sun sensors, the Sun vector can be determined even when the Sun is below the horizon (i.e., in twilight conditions).
An experimental setup used to acquire actual images of the sky consisted of a monochrome CMOS (complementary metal oxide semiconductor) camera, in front of which is a laminated film polarizer mounted on a motorized rotation stage. Both the camera and stage were controlled by a computer (see figure). At every observation time, six images were taken, each with a different polarizer orientation. Results of field testing with the Sun sensor indicate an accuracy of approximately 1°.