Development of an Alignment Process to Be Used on Flight Tests Campaign of a GPS Attitude Determination System

N. P. Oliveira Leite, F. Walter

Abstract:	The development of a GPS attitude determination system requires a flight test campaign to validate the attitude determination algorithm, in which the measured aircraft attitude angles, expressed in terms of the Euler Angles, should be correlated to a reference, that in this case will be measured from a Flight Test Instrumentation (FTI) System. In the latter system, all measurements are observed from a set of transducers, (e.g. rate gyros, accelerometers and a attitude gyro) which are mounted on the aircraft’s instrumentation bay. Although it is possible to have a very accurate alignment process of all sensors on their hardware mounting, as well relative to the others sensors mounted on the same hardware, it’s very difficult to find the relationship between the instrumentation and the aircraft coordinate frames. Also, from the GPS antennas reference frame to the aircraft where are taken the GPS attitude angles. Taking into account that the flight test transducers are lower accurate than those used in the aircraft’s Inertial Navigation System (INS) (e.g. a Strap down INS), the well-known alignment process applied on the latter sensors, capable of sensing the earth rotation, could not be used with the former one. The development of an algorithm to compute the angular relationship, expressed by the misalignment angles, between both the FTI and GPS reference frames to the aircraft reference system is described. The proposed algorithm estimates the initial transformation matrices from FTI and GPS to North-East-Down (NED) coordinate frame, and then estimates the residual misalignment refined via Kalman Filter. The filter design is based on an easy model. The filter validation process uses real instrumentation data taken from the FTI, a XC-95B Brasilia turbo propeller aircraft, and data from a GPS antenna array alignment device, which was developed for this application. Also to guarantee that the results are satisfactory, a complete residual analysis is employed to show that the filter could extract all available information from the FTI and GPS alignment transducers. Now considering that all coordinate frames are properly aligned, it is possible to perform an accurate correlation between the attitude angles measurements provided by the reference system and those taken from the GPS-based system in order to validate the attitude determination algorithm.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	152 - 162
Cite this article:	Leite, N. P. Oliveira, Walter, F., "Development of an Alignment Process to Be Used on Flight Tests Campaign of a GPS Attitude Determination System," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 152-162.
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