On the Observability of Loosely Coupled Global Positioning System/Inertial Navigation System Integrations With Five Degree of Freedom and Four Degree of Freedom Inertial Measurement Units

This article examines the observability of a modified loosely coupled global positioning system/inertial navigation system (GPS/INS) filter and analyzes the sideslip and attitude estimation capability of the filter. The modified filter is a loosely coupled integration which does not include a pitch rate gyro and which uses GPS course information as a measurement of heading when the vehicle is driving straight. Experimental tests are conducted which show that the modified filter has the same observability characteristics as a standard loosely coupled filter during turning events. The observability of a loosely coupled integration using only a four degree of freedom (DOF) inertial measurement unit (IMU) is also discussed and examined by experiment, as well as the sideslip and roll angle estimation performance. Finally, the error characteristics of the modified loosely coupled integration with the five DOF IMU when the filter is unobservable are studied. Monte Carlo simulations of long periods of straight driving with various sensor qualities are presented to show the worst case attitude errors when the filter is unobservable.