Attitude Determination using Infrared Earth Horizon Sensors

Infrared Earth horizon sensors (EHS) are capable of providing attitude knowledge for satellites in low-Earth orbit, even during periods of eclipse. Attitude information is acquired by detecting Earth’s infrared electromagnetic radiation and, subsequently, determining the region obscured by Earth in the sensors’ fields of view to compute a nadir vector estimation in the satellite’s body frame. Due to the limited computational resources and source code modification ability of most small satellites on orbit, a compact and robust EHS solution is required to efficiently achieve high-accuracy attitude knowledge. This paper presents the analytic form and simulated model of an attitude estimation method to compute a nadir vector using inputs from infrared EHS with Gaussian response characteristics. The proposed method can be applied when two sensors, each with known and distinct pointing directions, detect the horizon, which is defined as having their fields of view partially obscured by Earth. The accuracy of the estimation was quantified through simulations to be approximately 0.2 for a satellite in low-Earth orbit under a maximum attitude disturbance level of 4. The sensitivity of the estimation accuracy relative to mounting uncertainty was also analyzed, yielding an additional error of 0.7 on nadir vector estimation for every 0.25 of boresight offset.