Currently, in situations where Global Position System (GPS location information is not available, a user typically relies on dead reckoning localization (e.g., using inertial measurements from an inertial measurement unit (IMU)). Such localization analysis, however, is subject to drift and error accumulation. Yaw, for example, is typically calculated using a compass, which can be unreliable due to variations of the earth's magnetic field direction on the surface of the earth, meaning yaw measurements using a compass can be inaccurate by many degrees. Other methods for calculating yaw include measuring celestial features such as the Sun, moon and star positions. These methods can be accurate (less than 1 degree of error), but are subject to reduced availability due to cloud cover, the Sun being out of the field of view, stars not being visible during the daytime, and the like.
According to theory, the observed polarization at any position in the sky depends on the Sun and the sensor platform positions, as well as the sensor pointing direction, where “sensor pointing direction” is the center point of the field of view of the sensor, also known as the target point. The target point, sensor platform position, and sun position together define a plane. Given the Sun's position, which is a function of the time of day, and polarization measurements at one or more unique pointing directions, the sensor absolute position and orientation may be derived. As used herein, “orientation” generally refers to roll, pitch and yaw. “Position” generally refers to latitude and longitude.
A method according to the present disclosure calculates orientation and position parameters using a sky polarimeter that takes polarized images of multiple simultaneous target points in the sky. The orientation and position parameters can be useful to a navigating vehicle (especially if GPS is denied, spoofed, or unavailable), and can work in all types of vehicles (including ground, air and naval vehicles). The orientation and position parameters can also be useful to target locating systems such as far target locators and surveying equipment. The method can provide 0.1 degree yaw accuracy. Further, while the method is typically applied during daylight hours, it is conceivable that the method could be executed at night with some accuracy using the moon instead of the sun.
A system according to an exemplary embodiment of the present disclosure comprises an imaging sensor, polarization state analyzer, optics, mechanical housing, memory and logic circuitry, IMU, GPS, clock, and embedded software that determine the orientation and position parameters. A method according to an exemplary embodiment of the present disclosure comprising using polarization images and prior position/orientation/time data from the GPS, IMU and clock, respectively, to determine expected Sun azimuth and elevation, comparing this expected Sun position to the measured sky polarization pattern, and then filtering to calculate a better orientation and position estimate of the desired object. This localization estimate can be provided in any number of interfaces to a navigation system, a user, a display, or a target locator.