Virtual Differential GPS & Road Reduction Filtering by Map Matching

A novel method of map matching using the Global Positioning System (GPS) has been developed for civilian use, which uses digital mapping data to infer the <100 meters systematic position errors which result largely from “selective availability” (S/A) imposed by the U.S. military. A method of rapidly detecting inappropriate road centrelines from the set of all possible road centre-lines that a vehicle may be travelling on has been developed. This is called the Road Reduction Filter. The S/A error vector is estimated in a formal least squares procedure as the vehicle is moving. This estimate can be thought of as a position correction from a “virtual” differential GPS (DGPS) base station, thus providing an autonomous alternative to DGPS for in-car navigation and fleet management. We derive a formula for “Mapped Dilution of Precision” (MDOP), defined as the theoretical ratio of position precision using virtual DGPS corrections to that using perfect DGPS corrections. This is shown to be purely a function of route geometry, and is computed for examples of basic road shapes. MDOP is favorable unless the route has less than a few degrees curvature for several kilometers. MDOP can thus provides an objective estimate of positioning precision to a vehicle driver. Precision estimates using MDOP are shown to agree well with “true” positioning errors determined using high precision (cm) GPS carrier phase techniques.