OEM4 Inertial: A Tightly Integrated Decentralised Inertial/GPS Navigation System

Tom Ford, Janet Neumann, Pat Fenton, Mike Bobye and Jason Hamilton

Abstract:	The synergy between GPS and inertial navigation has been well known in the industry since the inception of GPS. In theory, the continuity of the inertial system can both fill in positioning gaps left by GPS satellite outages and reduce the effect of high frequency GPS errors, while the unbiased nature of the GPS signals can limit the size of the low frequency errors in the inertial system. NovAtel Inc. embarked on a development process more than 2 years ago in order to build a prototype GPS/INS integrated system designed to take advantage of the complementary nature of the two systems. The components of the prototype system were a Honeywell HG1700 IMU and a NovAtel Inc. OEM4 GPS receiver. The objective of the development was to provide a tightly integrated system at reasonable cost which could give positioning continuously at a 10 cm level provided the GPS signal outages were of short duration. The approach taken during the system development was to take advantage of the existing GPS navigation algorithms and supplement these with a set of inertial algorithms and to use these in a decentralised process that could run on the target processor on the OEM4 board. The result of this development is a modular system which fulfils the accuracy requirements noted and resides entirely on the NovAtel Inc. OEM4 receiver. A decentralised filter has both advantages and disadvantages compared to a centralised filter. The advantages of simplicity, modularity, component size and independence, are offset to a certain extent by the requirement that at least 4 satellite observations are needed to generate a GPS solution that can be used to control the inertial errors. Another drawback of the decentralised approach is related to eliminating the use of data with poor integrity. In addition, a decentralised filter does not incorporate integer ambiguity states with the states related specifically to the inertial system, so another method of aiding the ambiguity resolution process needed to be devised. Solutions to these problems are described. In an inertial system, the measurements of the system are used to provide the coefficients of the differential equation set which describes the dynamics of the system errors. Noise on these measurements tricks the Kalman filter into a condition of 'false observability', in which theoretically unobservable states experience incorrect reductions in standard deviation. This condition was investigated and a solution to this problem was implemented. In this paper, the authors propose to describe the system architecture, the particulars of the system development noted above and to provide test results which demonstrates the system performance in various environments.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	3153 - 3163
Cite this article:	Ford, Tom, Neumann, Janet, Fenton, Pat, Bobye, Mike, Hamilton, Jason, "OEM4 Inertial: A Tightly Integrated Decentralised Inertial/GPS Navigation System," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 3153-3163.
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