Locating the North Celestial Pole From Skylight Polarization Patterns and Solar Declination

Global navigation satellite systems (GNSS) provide robust and accurate navigation information. However, GNSS can be spoofed, potentially leading to dramatic errors in localization estimation. Inspired by nature, research has been undertaken into skylight polarization as a reliable alternative to the existing navigation systems. Recent studies have used skylight polarization to estimate the observer’s position. Nevertheless, such methods generally require a precise knowledge of time, date, and solar ephemeris. Alternatively, time invariances of the skylight polarization pattern, obtained from at least three polarization snapshots, were used to estimate the position of the north celestial pole (NCP) in the sky, without additional information, providing the true north bearing and the latitude of the observer. Here, we explain how these skylight polarization time invariances are related to the solar declination, and how this relation can be used to find the position of the NCP. Based on this property, we introduce a method to find the position of the NCP requiring only two skylight polarization observations over time and a prior knowledge about the solar declination. We tested our method on real skylight polarization images acquired by a polarimetric camera during a 1 month observation. A comparison between estimated and ground truth coordinates validates the theoretical model. Finally, our method can also be used in reverse to find the solar declination from the skylight polarization time invariances and the position of the NCP, demonstrating how skylight polarization can be used as a celestial calendar.