Temporary DGPS position loss, in circumstances such as an overhead bridge, can be alleviated by an inertial navigation system (INS) that uses onboard sensors to determine vehicle position. This work introduces a postprocessing DGPS/INS integration method based on using the INS solution during DGPS outages or periods of low accuracy DGPS position solutions. The INS solution is initialized using the DGPS solution before position solution loss, and measurements from the Inertial Measurement Unit (IMU). The final INS solution is a weighted average of the INS forward and backward solutions. The initialization methods for different degree of freedom vehicle positioning models and the developed weighting model, are fully explained in this work. Experimental results are included. ION GPS '99, 14-17 September 1999, Nashville, TN 1931 1.0 INTRODUCTION Vehicle navigation is an emerging product in the U.S. vehicle market where navigation system deployment follows widespread use in Japan and to a lesser extent in Europe [1]. Navigation systems are essentially selfcontained systems comprising a GPS receiver, a speed sensor, a yaw rate sensor for positioning, and a map database upon which navigation algorithms plan the route and track the route taken. High accuracy DGPS makes it possible to determine the position of a moving vehicle within a nominal vehicle operation regime to under 10cm [2]. DGPS, however, operates only when unobstructed view of at least four satellites is maintained. Satellite visibility is generally not an issue on the open highway, but it does become an issue when the GPS equipped vehicle passes under a highway overpass. Satellite signal lock is lost, and no position measurement is possible from GPS. Experiments performed in [3] show that in urban conditions, GPS can be available less than 50% of total time. The gaps in DGPS position data can be filled with an INS that uses onboard sensors such as accelerometers and rate sensors to provide back up data. The data are used with a model of vehicle motion to calculate vehicle position. In this work, DGPS is the primary navigator, and the INS is used as a back up navigator during DGPS outages or low accuracy DGPS position solution. 2.0 ANALYSIS 2.1 Proposed Method Fig. 1 illustrates the proposed method, which can be summarized by the following approach, 1. Initialize the INS forward and backward solutions at both ends of the gap or of low accuracy DGPS solution period. 2. Run the INS forward solution. The forward solution is the solution that is updated forward in time. 3. Run the INS backward solution. The backward solution is the solution that is updated backward in time. 4. Calculate the INS forward/backward combined solution with appropriate weighting.
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