Ultra-Wideband Aiding of GPS for Quick Deployment of Anchors in a GPS-denied Ad-hoc Sensor Tracking and Communication System

Global Navigation Satellite Systems (GNSS), even when used outdoors, have limitations in localization accuracy due to multipath reflections from nearby buildings, trees, and other obstructions. The result is localization performance is significantly degraded or inconsistent, requiring expensive and/or time consuming techniques to minimize these effects. The system under development requires quick on-site adhoc setup combined with precision geodetic location of outdoor landmarks in support of dynamic tracking of mobile sensors for mapping throughout buildings and other GPS-compromised areas. Because relative geometric errors in the landmark nodes can cause large localization errors for mobile sensors, the relative accuracy of the anchor locations is more important than their absolute accuracy. Ultra-Wideband (UWB) peer-to-peer two-way time-offlight ranging between anchors has the potential to significantly mitigate these problems. The UWB transceivers provide range errors of a few centimeters even in high multipath environments. In addition, the communications capability of these UWB transceivers support a time division data network, thus coordinating enhanced accuracy of GPS/UWB automatic survey of landmark nodes as well as greatly improved accuracy of ad-hoc setup. In this study the accuracy of landmark nodes is compared using GPS alone, GPS aided with UWB ranging, and user-assisted fiducial marking of landmark locations using satellite imagery. In practice GPS alone provided the least accurate and most inconsistent relative localization. GPS with UWB range enhancement provided better results. Our best results were attained using GPS/UWB localization followed by user manipulation of at least two landmark locations visible on a satellite image of the area readily available from the Internet. Accurate landmark registration of any two coordinate system anchors defines position and orientation of the others anchors. Extraction of the geodetic coordinates of those landmarks from map imagery was straightforward and produced the best anchor network locations. Unlike previous GPS/UWB systems our approach uses a network of distributed extended Kalman filters (EKFs) integrated in each node, rather than centralizing the solution into one massive filter burdened with all states and measurements of the composite system. While a fully centralized solution could theoretically provide a more accurate solution, practical wireless data communication and processing limitations led us to distributing individual EKF solvers in each node. As each node sequentially blends its own GPS estimates with UWB range inputs to neighboring nodes the entire system settles quickly to optimized coordinates. The localization accuracy was monitored over time as the distributed solution converged. The results indicate a large improvement of relative localization accuracy using UWB augmentation in the presence of multipath. After ad-hoc placement of coordinate system anchor nodes, the mobile nodes utilize Two-Way Time-of-Flight (TW TOF) UWB ranging and communication to transmit sensor data. The motion model of the each mobile navigator supports sparse deployment of anchors. The solution is queried at any time and, unlike a trilateration solver, a single distance measurement can be folded into a mobile node’s navigation solution.