This solar tracker integrates a camera with a field of view (FOV) large enough to easily image the entire solar disc — co-boresited with a narrow-band spectrometer that measures the frequency of a Doppler shifted line — with an algorithm designed to drive zero Doppler shift rotation axis of the Sun.
The tracker camera is pointed towards the Sun so that the Sun is within its field of view. Although a search pattern could be used, this first pointing step can also be accomplished using known orbital data. For example, for ISS (International Space Station) applications, knowledge of the solar ephemeris and ISS orbital characteristics allow for this initial pointing.
The tracker gimbal (or other pointing mechanism) is then commanded to center the Sun in the camera FOV. An edge detection algorithm is used to determine the upper limb of the Sun (farthest from the horizon and therefore least distorted by atmospheric refraction). The upper limb is fitted to an ellipse form that can locate the geometric center of the Sun, and either the gimbal is commanded to center upon it, or the tracker provides an error function to enable corrective pointing of an instrument with known offset from the center of the tracker FOV. This algorithm provides accuracy of 10 arcsec, adequate for many applications, but not when the Sun nears the horizon.
Once the image of the Sun enters the Earth’s atmosphere (as seen from ISS, for example) or nears the horizon (for terrestrial instruments), the centroiding algorithm begins to lose accuracy, but the loss is greater in the longitudinal (i.e., orthogonal to the horizon), as opposed to the transverse direction (unless environmental conditions create a particularly asymmetrical solar image). The narrow-band spectrometer is then scanned along the longitudinal axis to locate where the Doppler direction shifts at the solar rotation axis, thus greatly improving the accuracy of the longitudinal coordinate. This approach of integrating ellipse matching and the location of the solar rotation axis can also be used during sunrise to accurately track the center of the Sun.