Histogram Filter for Attitude Determination of Small Asteroid Lander

Abstract

The Mobile Asteroid Surface Scout (MASCOT) is a small landing package build by the German Aerospace Center (DLR) jointly with the French Space Agency (CNES). MASCOT will fly onboard the Japanese space probe Hayabusa-II, built by the Japan Aerospace Exploration Agency (JAXA) and is scheduled to be launched in late 2014 on a 5-year sample return mission to the Near-Earth Asteroid 1999 JU3. The lander is equipped with four science instruments which will take detailed close-up pictures and make a variety of in-situ measurements. MASCOT has a cubic shape roughly 29 cm x 27,5 cm x 20 cm and weighs about 10 kg. An internal mechanism with an eccentric tappet can be used to reorient MASCOT if needed and relocate to a different site to increase its investigation area. After the arrival and a characterization of the asteroid, MASCOT will be dropped from the Hayabusa-II mothership and will land passively on the asteroid surface. Before starting science measurements MASCOT must lie on the correct side. The attitude determination system consists of 2 types of sensors: solar cell based sun sensors on each side of MASCOT as well as 5 optical proximity sensors (OPS). The latter consists of an LED and a photodiode which can detect the reflected LED light when MASCOT is in proximity to the surface. A histogram filter is used for attitude determination and multi sensor data fusion. It is a bayesian filter used to estimate states which can be divided in a finite number of possible values. This is useful for MASCOT since all that is needed is an estimation of the side on which it is lying. This paper focuses on the attitude determination system of MASCOT. It describes the attitude determination sensors and the details of the filter algorithm. It also presents simulation results for system verification, as well as planned Monte-Carlo simulation and Hardware-in-the-Loop tests.