A High Accuracy Horizon Sensor for Small Satellites

The conceptual design for a novel, high accuracy horizon-sensor is presented to be used on-board Low Earth Orbit small satellites. The concept consists of a multihead infrared sensor capturing images of the Earth limb. By fitting an ellipse to the imaged limb arcs, and exploiting some analytical results available from projective geometry, a closed form solution for computing the attitude matrix is developed. To quantify the expected attitude estimation performance of the proposed method, a simulator is developed which generates Earth images as gathered by a low Earth orbit satellite including also the presence of the atmosphere. The algorithm is developed in a dimensionless framework, requiring the knowledge of the shape of the planetary target, but not of its size. As a result, the solution is less sensitive to the limb shift caused by the atmospheric own infrared radiance.Results show that our sensor concept returns rms errors of few hundredths of a degree or less in determining the local nadir direction. Furthermore, coarse information on the orientation about nadir is also obtained, with accuracy in the order of some degrees for most operating conditions.