Design and Calibration of a High-Accuracy Polarized Compass Based on Vortex Retarder

We have constructed a compact, low-cost, high-accuracy polarized compass based on spatial modulations of vortex retarders that enable rapid and precise angle of polarization (AOP) measurement of skylight. A traditional optical imaging system incorporated with a vortex retarder and an analyzer in front of it is employed to calculate the skylight AOP through analyzing the modulated light intensity distributions. A periodic error function with 90° period is observed in the wide-range AOP measurement and an effective calibration method is implemented to reduce such periodic error. Outdoor orientation experiments confirm that, after calibration, the max/standard deviation of orientation error over a 180° range was less than 0.03°/0.01° respectively, obtaining the highest accuracy in outdoor measurements to our knowledge. Besides high accuracy, our compass also features a simple and compact structure with strong optical path compatibility, making it easier to be miniaturized for broad applications such as autonomous navigation.