Spacecraft Attitude Determination Using a Combination of GPS Attitude Sensor and Star Sensor Measurements

Abstract

The applications of GPS in space, such as orbit and attitude determination, offer serveral advantages, e.g. complete navigations information on board, low weight, volume and power consumption. Besides this, GPS also influences the operations of satellites. Ground stations and operations can be reduced, which means a reduction of the total operation cost. GPS based attitude determination has many important advantages like the fast and reliable intial acquisition, the resistance against spin rates and the fact, that at least two satellites for deterministic attitude determination are always visible during the mission. GPS could also be used as a safe mode sensor. Therefore GPS based attitude determination seems very promising. But due to the achievable accuracy, implied by the baseline length and the signal noise, a GPS attitude sensor cannot be used for missions were high accurate attitude measurements are needed and the baseline lengths are resticted. For these missions, a star sensor will be the adequate attitude sensor. Star sensors are high accurate, obust and have well established algorithms. The disadvantages using star sensors are e.g. resticted field of view, operation only at low angular rates, sensor performance degradation and sometimes problems with the star indentification. Taking all thes individual advantages and disadvantages into account, the combination of a GPS attitude sensor and a star sensor seems extremely promising, because such a sensor combination can be applied to a broad area of missions and their reqiriements of a high accurate attitude. This paper deals with the development of new algorithms for the combination of a GPS attitude sonsor and a star sensor and the investigation of the various possibilities to support each other and therefore improve the attitude solution and the range of applicability. The results of this paper are obtained from an experiment on a tur table with a four antenna GPS attitude sensor and a star sensor on top of it during a ground night experiment. Data were collected for several inclined static and spinning cases. New attitude algorithms for both sensor typers were tested and verified.