Improving the GPS/INS Integrated System Performance by Increasing the Distance Between GPS Antennas and Inertial Sensors

Jörg F. Wagner and Günther Kasties

Abstract:	Today integrated navigation systems based on a low-cost inertial measurement unit (IMU) and a single antenna GPS receiver are a powerful navigation means. However, during certain phases of operation, such a configuration shows significant performance degradations and stability problems. This behavior results from an incomplete observability of the system state describing the vehicle motion and additional inertial sensor errors. The rigorous observability analysis of Hong et al. (2000) led to the mathematical conditions for such critical circumstances: These situations appear if the vehicle dynamics is too low; additionally, one insight was that a vanishing distance between IMU and GPS antenna should be avoided at all. Considering besides numerical aspects leads to two consequences: - The probability of entering the state of too low dynamics decreases if the antenna lever-arm increases. - A 3-antenna-array, which theoretically ensures a full observability, requires distinct antenna distances. Thus, the intention of the paper is to illustrate the effect of extending the distances between IMU and GPS antenna(s) on the system performance. This is done using simulated measurements to avoid, in a first step, disturbances caused by different satellite signal errors and by vehicle distortions. Yet, the employed Kalman filter was previously tested and its parameters were tuned by means of real flight test data. In addition, the filter is based on a total state formulation and uses an earth fixed Cartesian navigation frame, as this approach shows a better estimation quality compared with the classical error state description (Wagner and Wieneke, 2001). It turns out that increasing the lever-arm of one single antenna effects slightly better estimates for the inertial sensor errors. Employing instead a 3-antenna-array with usually short lever-arms and GPS range measurements of classical DGPS accuracy generates similar results. Combining both measures, however, leads to clear improvements of the attitude accuracy and filter stability. Assuming additionally the precision of carrier phase measurements amplifies those enhancements. A generalization of the GDOP that includes also the antenna configuration makes this plausible. Therefore, using GPS antenna arrays with long leverarms can greatly improve the accuracy and stability of GPS/INS integrated systems. On the other hand, as such an approach imposes questions on structural flexibility, the paper concludes with outlining a concept of integrated systems for nonrigid vehicles.
Published in:	Proceedings of the 2002 National Technical Meeting of The Institute of Navigation January 28 - 30, 2002 The Catamaran Resort Hotel San Diego, CA
Pages:	103 - 115
Cite this article:	Wagner, Jörg F., Kasties, Günther, "Improving the GPS/INS Integrated System Performance by Increasing the Distance Between GPS Antennas and Inertial Sensors," Proceedings of the 2002 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2002, pp. 103-115.
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