John M. Elwell

Indoor geolocation in the absence of direct path

Severe multipath in indoor areas causes undetected direct path (UDP) conditions, which pose a serious challenge to the design of robust precision indoor geolocation systems. Based on a scenario on the third floor of the Atwater Kent Laboratory at the Worcester Polytechnic Institute, we explain the reason for frequent absence of direct path, and introduce and analyze the effectiveness of two novel approaches to mitigating the large ranging errors caused by UDP conditions. The first technique exploits nondirect paths for ranging, while the second approach relies on cooperative localization for wireless sensor and ad hoc networks

A Nonlinear Code Tracking Filter for GPS-Based Navigation

Current Global Positioning System (GPS)-based navigation systems are highly susceptible to unintentional and intentional jamming due to relatively low signal power at the receiver antenna and, in part, due to suboptimal code tracking loop designs that do not account for measurement nonlinearities near loss-of-lock. A nonlinear code tracking filter is developed whose architecture is based on a rigorous minimum-variance solution of the navigation problem, rather than using prespecified tracking loop architectures. The filter implementation can be viewed in terms of the classical notions of error detector functions, which depend on signal-to-noise ratio (SNR) and root mean square (rms) code tracking error. Detector functions are defined for both code tracking error and code tracking error variance. The filter responds more rapidly than current designs to rapidly varying jammer power due to a measurement-dependent term in the covariance calculations. Extended-range tracking is utilized, yielding linear state vector error detector functions (i.e., the filter is essentially optimum) out to the maximum allowed by the correlator range, and reducing the need for reacquisition. Significant antijam improvements relative to current designs are predicted from high-fidelity simulation and hardware demonstrations. Computational requirements are comparable to extended Kalman filter/vector tracking loop techniques.