Optimising the Integration of Terrain Referenced Navigation with INS and GPS

Paul D. Groves, Robin J. Handley and Andrew R. Runnalls

Abstract:	Accuracy and robustness are vital to present and future air navigation, for both military and civil applications. The strengths of INS and GPS and hence those of integrated INS/GPS systems are well known. However, the knowledge required to jam GPS is becoming public, and the hardware to achieve this is basic. When GPS data are unavailable, and a low grade INS is used, navigation accuracy quickly degrades to an unacceptable level. Terrain referenced navigation (TRN) techniques offer a solution. The most established TRN technique, terrain contour navigation (TCN), uses measurements from a radio altimeter (radalt) and requires undulating terrain. Performance may be improved by using a laser rangefinder as the sensor. A second, and complementary, technique is scene/line feature matching, which uses a dedicated imaging sensor. The current state-of-the-art in line feature matching systems is represented by the Continuous Visual Navigation system (CVN). TRN techniques are described and compared. The addition of one or more TRN systems to an integrated INS/GPS navigation system enables the INS to be calibrated during GPS outages, increasing the robustness of the overall navigation solution. Integration architectures are reviewed. For the initial studies of INS/GPS/TRN integration, radalt based TCN has been selected because a relatively simple simulation can be used and the research is readily applicable to current systems. A batch processing TCN algorithm is used in conjunction with a centralised integration filter. To determine how best to use these TCN fixes to calibrate the INS whilst obtaining the optimum balance of accuracy, robustness and processing efficiency, four different approaches are compared. These are a best fix method, a weighted fix method using a probabilistic data association filter (PDAF), and single and multi-hypothesis versions of the Iterative Gaussian Mixture Approximation of the Posterior (IGMAP) method. Simulation results are presented showing that the single hypothesis IGMAP technique offers the best balance between performance and processor load. It is shown that, where TCN is combined with INS and GPS, optimum performance is obtained by weighting the TCN and GPS measurements equally in the integration filter. Where only a limited number of GPS signals can be tracked or the GPS measurements are noisy due to jamming, the TCN measurements improve the overall navigation solution.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1048 - 1059
Cite this article:	Groves, Paul D., Handley, Robin J., Runnalls, Andrew R., "Optimising the Integration of Terrain Referenced Navigation with INS and GPS," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1048-1059.
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