TABLE OF CONTENTS Navigation employing GPS and inertial sensors is a synergistic relationship. The integration of these two types of sensors not only overcomes performance issues found in each individual sensor, but also produces a system whose performance exceeds that of the individual sensors. GPS provides bounded accuracy, while inertial system accuracy degrades with time. Not only does the GPS sensor bound the navigation errors, but the GPS sensor calibrates the inertial sensor. In navigation systems, GPS receiver performance issues include susceptibility to interference from external sources, time to first fix (i.e., first position solution), interruption of the satellite signal due to blockage, integrity, and signal reacquisition capability. The issues related to inertial sensors are their poor long-term accuracy without calibration and cost.
This section first discusses in more detail the relative weaknesses of GPS (Section 9.2.1) and inertial sensors (Section 9.2.2) as outlined previously. Next, an introduction to Kalman filtering is provided (Section 9.2.3), followed by a description of a variety of practical GPS/inertial integrations and their performance features (Sections 9.2.4 9.2.6).
One primary concern with using GPS as a stand-alone source for navigation is signal interruption. Signal interruption can be caused by shading of the GPS antenna by terrain or manmade structures (e.g., buildings, vehicle structure, and tunnels) or by interference from an external source. An example of signal interruption is shown in Figure 9.1. Each vertical line in this figure indicates a period of shading while driving in an urban environment.
