Improving positioning accuracy in WAW location-based services

Location-based services rely on the accuracy of the user\´s position estimates. In WAW ("Where Are We?") location- based services, users want to be aware, not only, of their own position but also the position of the other elements on their group. If this information is available in real-time, this kind of service can be used to support coordinated positioning operations. This paper presents an algorithm for the estimation of a user\´s position, based on measurements from a group of mobile Global Positioning System (GPS) receivers. Receivers are assumed to have some common satellites in view, but measurements from all the satellites are used in the estimation process. Instead of estimating the user\´s position on an autonomous way, based only on the measurements of his own receiver, the proposed approach takes advantage from all the GPS measurements available in this kind of location-based service. None of the GPS receivers, used in the estimation process, is considered to be fixed or known. The approach leads to an improvement in the user\´s position accuracy and can particularly benefit those with a more restricted view of the satellite constellation. As such, the methodology, here presented, can be applied with advantage in the deployment of WAW location-based services in urban environments, more prone to the canyon effect. The paper describes in detail the positioning model and estimator used to achieve the accuracy improvements. Simulation results are presented based on real GPS satellite ephemeris, collected at the university campus.