An Indoor and Outdoor Seamless Positioning System for Low-Cost UGV Using PPP/INS/UWB Tightly Coupled Integration

Seamless positioning is critical for mobile robots to achieve ubiquitous availability in all scenarios. The precise point positioning (PPP) technique provides a centimeter-level solution without a reference station and is often integrated with an inertial navigation system (INS) for outdoor navigation of unmanned ground vehicles (UGVs). However, in challenging environments such as indoors, the performance of PPP/INS systems is severely degraded, while ultrawideband (UWB) is widely used due to its high accuracy over short distances. This article proposes a tightly coupled (TC) PPP/INS/UWB integrated system, in which satellite-difference ionosphere-free (SD-IF) pseudorange and carrier-phase measurements from a single receiver, microelectromechanical system (MEMS) inertial measurements, and the UWB rangings are tightly fused to accomplish continuous and precise positioning throughout indoor and outdoor coverage. To mitigate the effects of nonline-of-sight (NLOS), multipath, and faulty signals, a two-step weighting approach is proposed to improve UWB positioning performance, including segmentation weighting based on the signal strength, and weight adjustment based on area identification. Besides, motion constraints are imposed on velocity, yaw, and height to suppress drifts during signal-harsh periods. Real-world experiment results show that the proposed system can achieve continuous positioning in the overall indoor and outdoor environments, with the mean absolute errors (MAEs) of (0.214, 0.186, and 0.243 m) and the root mean square errors (RMSEs) of (0.339, 0.291, and 0.414 m) in the east, north, and up directions. In transitional areas with severe signal interference and interruption, a 0.5-m positioning accuracy and smooth switching are still attainable.