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The direction of True North, being strong and free, is also better aligned.


Image of a blue sphere with lights near the top and bottom

Throughout history, people have created various techniques to identify true north, but each has its own drawbacks. For instance, using the stars as a guide only works when the sky is not illuminated. Magnetic compasses can be disrupted by a variety of factors, from iron-containing rocks to magnets in modern gadgets. Even satellite navigation systems like GPS are prone to issues like space weather.

Certain creatures use the sky as a means of navigation. The polarization of skylight, undetectable to humans but detectable to animals like desert ants, is caused by Rayleigh scattering. This refers to the process in which sunlight is dispersed by tiny particles in the Earth’s atmosphere.

In a recent study, scientists were inspired by nature and devised a way to determine orientation using polarized skylight. By analyzing the polarized sunlight, they were able to accurately determine the true north pole of Earth and the observer’s latitude on the ground.

SkyPole

The polarization of the light from the sky is affected by the Sun’s position in relation to the observer on Earth. In many locations, the polarization of the light fluctuates throughout the day. When the Sun is directly overhead, the sky will appear horizontally polarized along the entire horizon. During sunrise and sunset, the sky will be most strongly polarized in a vertical direction at the horizon.

According to Thomas Kronland-Martinet, the primary researcher of a recent study and a PhD candidate in biorobotics at Aix-Marseille Université in France, the scattering angle remains consistent at the celestial poles. These are specific points in the sky that are directly above observers located at the North and South Pole of Earth, and the rotation of polarization patterns is centered around them.

Kronland-Martinet and his team examined the level of linear polarization (DoLP), which is calculated by dividing the intensity of polarized light by the total intensity of light. By measuring the DoLP during at least three time periods, they were able to accurately determine true north with an average error of 2.6°. This is slightly higher compared to the precision of current methods that use polarization for geolocation, which have an average error of 0.1° in latitude, longitude, and north bearing. However, these methods also incorporate additional information such as time, date, ephemeris, position estimation, and inertial navigation systems, making them more efficient in data processing. Once the position of the north celestial pole (true north) is known, an observer can determine their latitude.

The scientists conducted an experiment known as SkyPole, which involved using a polarimetric camera with a fish-eye lens attached, placed on top of a structure in Marseille, France. They published their findings in the journal Proceedings of the National Academy of Sciences of the United States of America.

Proof of Concept

The research indicates a remarkable, though not entirely novel, approach.

The research indicates a remarkable method, although it is not completely novel. There have been previous, less successful efforts to utilize the polarization patterns of daylight to accurately determine true north. Wisam Eldin Mahammed, an expert in geographic information systems (GIS) at Imam Abdulrahman Bin Faisal University in Saudi Arabia, who was not involved in the study, made this statement.

According to Mahammed, the proposed tool is not appropriate for immediate global positioning. This means that it may not be useful for autopilot and drone systems that require real-time location data.

Kronland-Martinet recognized that the SkyPole approach is primarily a demonstration of a concept rather than a immediately functional tool. However, Kronland-Martinet stated that with further refinement, the time required to calculate position could potentially be reduced to mere minutes or even seconds.

According to Dmitry Kishkinev, a professor at Keele University in the UK who specializes in animal behavior and behavioral neuroscience, the results of this study appear to be accurate and intriguing as a demonstration of the concept. Although Kishkinev was not directly involved in the research, he conducts research on animal navigation. He stated that the paper does not offer entirely new insights in the field, as this method was already known, but it does provide a practical example, confirms its validity, and estimates its margin of error.

Kishkinev stated that it is fascinating to observe how a basic camera and fish-eye lens can be used to determine latitude and north. However, it is not unexpected that this method has a significant margin of error.

According to Kronland-Martinet, as SkyPole continues to advance, it could potentially enhance compass calibration and marine navigation by allowing for the creation of automated polarimetric sextants.

—Mohammed El-Said (@MOHAMMED2SAID), Science Writer

Reference: El-Said, M. (2023), The Importance of Being Properly Oriented: True North, Strong and Free, Eos, 104, https://doi.org/10.1029/2023EO230345. Published on September 14th, 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0

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