f²IMU-R: Pedestrian Navigation by Low-Cost Foot-Mounted Dual IMUs and Interfoot Ranging

Foot-mounted inertial sensors become popular in many indoor or GPS-denied applications, including but not limited to medical monitoring, gait analysis, soldier, and first responder positioning. However, the foot-mounted inertial navigation relies largely on the aid of zero velocity update (ZUPT) and has encountered inherent problems, such as heading drift. This article implements a pedestrian navigation system based on dual foot-mounted low-cost inertial measurement units (IMUs) and interfoot ultrasonic ranging. The observability analysis of the system is performed to investigate the roles of the ZUPT measurement and the foot-to-foot ranging measurement in improving the state estimability. A Kalman-based estimation algorithm is mechanized in the Earth frame, rather than in the common navigation frame, which is found to be effective in depressing the linearization error in Kalman filtering. An ellipsoid constraint in the Earth frame is also proposed to further restrict the height drift. Simulation and real field experiments show that the proposed method has better robustness and positioning accuracy (about 0.1%–0.2% traveled distance) than the traditional pedestrian navigation schemes do.