Polarized skylight

Geolocation without GNSS

Navigating in Global Navigation Satellite System-denied or unmapped environments will, over the coming decade, become one of the 10 biggest challenges in robotics. Currently, autonomous robots rely on Global Navigation Satellite System (GNSS), Inertial Navigation Systems (INS) and ground-based antennas to triangulate or correct GNSS signals (5G networks or Real-Time Kinematic (RTK) networks), astronomical navigation, gyrocompass navigation, and vision-based or lidar-based SLAM. However, as summarized in (Serres et al., 2024), many animals are able to navigate or migrate over extremely long distances without using localization techniques developed by humans. For instance, desert ants use a powerful navigational tool termed optical path integration to locate their nest. When returning, desert ants follow the shortest possible route, a straight line, even in featureless or unfamiliar terrains. By integrating a directional compass and distance information from their vision, desert ants calculate a vector from their visual inputs, and this leads them home (see R. Wehner’s book ).

The aim of this project is precisely to study new methods based on the processing of the polarized skylight to determine in real time the heading to the true North but also the location on earth (latitude and longitude) by means of only polarimetric information. We developped a fully open access and open source simulator, named OpenSky (Moutenet et al., 2024) providing realistic images from a simulated polarimetric camera oriented toward the sky. We recently developed a method named SkyPole (Kronland-Martinet et al., 2023) able to locate the celestial pole (pole point) from the pattern of degree of polarization of the sky measured by a polarimetric camera.

(Left) Global view Credit: Léo Poughon) and mechanical layout of the measurement device used to collect polarimetric images during two months (Poughon et al., 2023). A fully anotated database composed of several thousand of images is available here including Python code to make it user friendly. (Middle) OpenSky simulate the measurement of sky polarization properties, i.e., the light intensity, the angle of polarization and the degree of linear polarization of light seen through a polarimetric camera. OpenSky is an open source simulator available here. (Right) The green circle is the radial invariance circle, the red circle is the plane invariance circle computed from analytical calculations (see CITE). The colored half sphere is the simulated absolute difference between two DoLP patterns associated with the Sun’s positions S1 and S2 at two different times. The red dot is the NCP, and the maximum value of the absolute difference of DoLP is in yellow. The NCP can be found at the intersection of the two invariances when the solar declination is null or by estimating the axis of rotation of the invariance circles resulting from the rotation of the sun.

References

2024

  1. Passive Polarized Vision for Autonomous Vehicles: A Review
    Julien R Serres, Pierre-Jean Lapray, Stéphane Viollet, and 4 more authors
    Sensors (Basel, Switzerland), 2024
  2. OpenSky: a modular and open-source simulator of sky polarization measurements
    Antoine Moutenet, Léo Poughon, Bruno Toulon, and 2 more authors
    IEEE Transactions on Instrumentation and Measurement, 2024

2023

  1. SkyPole—A method for locating the north celestial pole from skylight polarization patterns
    Thomas Kronland-Martinet, Léo Poughon, Marcel Pasquinelli, and 3 more authors
    Proceedings of the National Academy of Sciences, 2023
  2. A stand-alone polarimetric acquisition system for producing a long-term skylight dataset
    Léo Poughon, Vincent Aubry, Jocelyn Monnoyer, and 2 more authors
    In 2023 IEEE SENSORS, 2023