ION GNSS 2011
Session F6: Urban Navigation Technology and Location Based Services (Commercial focus)

Ultra-wideband Aiding of GPS for Quick Deployment of Anchors in a GPS-denied Ad-hoc Sensor Tracking and Communication System
J. Johnson, B. Dewberry, Time Domain

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 that localization performance is significantly degraded or inconsistent, requiring expensive and time consuming techniques to minimize these effects.
Our system under development requires quick on-site ad-hoc setup combined with precision geolocation of outdoor anchors in support of dynamic tracking of mobile sensors for mapping throughout buildings and other GPS-compromised areas. Because relative geometric errors in the anchoring/registration nodes can cause large localization errors for mobile sensors, the relative accuracy of the anchor locations is more important than their absolute accuracy.

Ultrawideband (UWB) peer-to-peer two-way time-of-flight ranging between anchors has the potential to significantly mitigate these problems. The UWB tranceivers under study showed, after a short packet-based conversation, consistent range errors of a few centimeters in high multipath environments. In addition, the communications capability of these UWB transcivers supported a time division data network, thus coordinating range measurements between mobile sensor units and static anchor units. The anchor units were localized both using GPS alone, GPS aided with UWB ranging, and surveyed locations. The results are compared.

In practice GPS/WAAS 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 manual manipulation of two anchor locations onto landmarks visible on a satellite image of the area readily available from the internet. Accurate landmark registration of any two 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 (EKF) in each node, rather than centralizing the solution into one massive filter including all states and measurements in the system. While a fully centralized solution could theoretially provide a more optimized 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 anchors the mobile nodes wirelessly coordinate ranging conversations to the anchors and while simultaneously transmitting their sensor data using a the UWB network. The motion model of the each mobile navigator supports sparse deployment of anchors. The solution can be queried at any time and, unlike a trilateration solver, a single distance measurement can be folded into the solution.

As these devices move into and through the building the GPS is de-valued while UWB preferred. On the other hand, as these devices move away from the localized anchors, the GPS solution is preferred. The result is a combination with much greater performance than the individual technologies by themselves.

Our next step is incorporating dynamic UWB and GPS quality metrics into an advanced EKF solver with dynamic selection of optimal anchor and/or peer-to-peer ranging to nearby mobile devices wth higher position confidence. In addition tighter coupling of UWB with GPS pseudo-ranges willl be developed. These improvements will be reported at the conference.

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