Ground Based Attitude Determination Using a SWIR Star Tracker

Sammanfattning: This work investigates the possibility of obtaining attitude estimates by capturing images of stars using a SWIR camera. Today, many autonomous systems rely on the measurements from a GPS to obtain accurate position and attitude estimates. However, the GPS signals are vulnerable to both jamming and spoofing, making any system reliant on only GPS signals insecure. To make the navigation systems more robust, other sensors can be added to acquire a multisensor system. One of these sensors might be a ground based SWIR star camera that is able to provide accurate attitude estimates. To investigate if this is possible, an experimental setup with a SWIR camera was placed at the office of FOI Linköping, where the camera in a rigid position has captured images of the sky. The SWIR camera possesses several advantages over a camera operating in the visual spectrum. For example, the background radiation is weaker and the transmission through the atmosphere is higher in certain wavelength bands.  The images captured by the SWIR camera was provided to a star tracker software that has been developed. The star tracker software contains algorithms to detect stars, position them in the image at subpixel accuracy, match the stars to a star database and finally output an attitude based on the stars from the image and the identified stars in the database. To further improve the attitude estimates, an MEKF was applied. The results show that attitude estimates could be obtained consistently from late evenings to early mornings, when the sky was dark. However, this required that the weather conditions were good, i.e., a limited amount of clouds. When more clouds were present, no attitude estimates could be provided for a majority of the night. The SWIR camera was also compared to a camera operating in the visual spectrum when clouds were present, to see if the results were any different. With the camera settings applied in this work, the two cameras seemed to perform equally. The accuracy of the estimated attitudes is hard to validate, since no true attitude is available. However, the variance of the estimates was low, and the major differences in the attitude estimates over a night's measurements seemed to be a drift present in all angles. The maximum estimated error in declination during a night's measurements varied from about 40 to 60 arc seconds, depending on the data set. The maximum estimated error in right ascension varied between 200 and 2000 arc seconds, and the same metric in the roll estimate were about 100 to 2500 arc seconds. The reason for the drifts is assumed to be atmospheric effects not being accounted for, and astronomical effects moving the direction of the rotation axis of the earth, creating errors in the star positions given in the database.