The Development and Simulation Results of a GPS Attitude Determination Algorithm

Nelson Paiva Oliveira Leite and Fernando Walter

Abstract:	Attitude determination with GPS interferometry is achieved by the adjustment of the body-to-reference transformation matrix LRB, expressed in terms of the Euler Angles [è, ö, ø]. For short baselines, the usage of the carrier phase double differences method minimizes most of the systematic errors that affects the GPS measurement. Several efficient algorithms for attitude determination are based on a least square fit of the attitude data to vector measurements. The latter being derived from the phase measurements expressed in the aircraft’s body reference system. To accomplish that, it is required to form a reference system composed of 04 (four) GPS antennas, to define the x; y, and z reference axes. Due to the aircraft geometry, however, it is not always possible to install the antennas array on the optimum location; this problem is mostly related to the installation of the z axis antenna. Therefore, many algorithms propose the computation of the z-axis data, the positive root of the measure from the x and y components. This computation assumes that the antenna receives only the signals of those GPS satellites, which are above its plane. This also reduces the number of antennas down to 03 (three). During some field tests, it was verified that sometimes it was possible to receive GPS signals from satellites, which are below the antenna plane, which results in a bad attitude solution. Also, during simulations, it was verified that the number of interactions required for the convergence of the algorithm was highly dependent on the initial condition of the LRB matrix (initial values of the Euler Angles). During the landing maneuvers, the aircraft is more prone to lose the GPS signals, due to the rate of change of its attitude and, in this case, the attitude solution algorithm should be re-initialized upon the reacquisition of the GPS signals, Therefore, speeding up the convergence process would improve the flight safety and process confidence. This paper will present a modified attitude determination algorithm, which estimates the right attitude solution even when the GPS satellites are below the antenna plane. This algorithm works in a two-step mode: firstly, it uses GPS single differences code measurements to initialize the LRB matrix (guess Euler Angles initial values), and then for the convergence process it uses the GPS double difference phase measurements, to determine the best attitude solution. There will also be presented simulation results that show the adequate performance of this algorithm in terms of the accuracy and processing time (0.131 ± 0.64 s @ 1ó down to 0.024 ± 0.013 s @1ó per simulation) showing a better performance than the REQUEST algorithm.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	1100 - 1107
Cite this article:	Leite, Nelson Paiva Oliveira, Walter, Fernando, "The Development and Simulation Results of a GPS Attitude Determination Algorithm," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 1100-1107.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In