Optical Navigation Using Planet's Centroid and Apparent Diameter in Image

Recent interest in sending humans to the moon, Mars, or other destinations beyond low Earth orbit has created an increased need for autonomous spacecraft navigation. Because traditional navigation approaches rely on ground-based tracking, new onboard techniques are necessary for the crew to safely return to Earth in the event of a communication systems failure. This paper investigates how images of a planet or moon may be used to perform autonomous navigation through the centroid and apparent diameter measurement type. If a planet is modeled as a triaxial ellipsoid, then it will project to an ellipse in an image. A new approach, which allows estimation of the spacecraft position relative to the observed planet using the full set of parameters describing an ellipse that is fit to points along the planet's lit horizon, is presented. The performances of a number of different ellipse-fitting algorithms are also evaluated. Finally, the covariance of the solution is derived, and simulation results are presented.