Omni-directional WiFi Localization Center for Embedded Networked Sensing Omni-directional WiFi Localization Rohun Bansal, Jessica Chen, Kevin Chen, Nathan Schloss, Deborah Estrin, Cameron Ketcham, Rakhee Patel, Martin Lukac OWL - http://owl.t4so.com/ Introduction: Determining location through use of sensors Sensors on Mobile Phones OWL: Localization System Modern smart phones (such as the Android G1) have a variety of sensors GPS sensors can generally acquire fairly accurate location data. WiFi devices can scan for access points as well as create ad-hoc networks. Mobile phones are becoming increasingly more widespread As the number of phones in use increases, applications involving networks of phones become progressively more practical to implement. An Android application for the G1 which takes advantage of both GPS and WiFi sensors in a network of phones to estimate location The system is generally functional in many different situations. The application works to minimize battery usage and time to location fix while trying to improve accuracy. Relies more heavily on the phone itself for location fix Problem Description: Current localization methods have disadvantages GPS: Global Positioning System WPS: WiFi Positioning System Requires line-of-sight view to satellites in order to function Although GPS is very accurate outdoors under an unobstructed sky, it works poorly or not at all in environments lacking such a view (notably indoors and near large buildings). Long time to first fix Because each GPS satellite only broadcasts ephemeris data every 30 seconds, the time for a receiver to get a fix is the time it takes to acquire each GPS satellite’s signal plus up to 30 seconds for ephemeris information. Requires an internet connection A list of visible access points must be sent in an HTTP request to a central server; a location estimate will be sent in the server's response. Accuracy is largely inconsistent Because the system uses a pre-compiled database of access points, the accuracy of location estimates depends completely on the coverage of these access points in the area, resulting in rather rough estimates. Proposed Solution: A networked localization system that uses both GPS and WiFi OWL: Localization System ` Devices that can acquire an accurate GPS fix become anchors An anchor sets up its own ad-hoc networks, with its IMEI number (used as a unique identifier) and GPS location encoded into the network SSID. Devices that cannot acquire an accurate GPS fix become receivers A receiver periodically scans for available wireless networks set up by anchors. Distance to each anchor network is estimated using a simplified logarithmic correlation to signal strength (in dBm). Once there are at least 3 visible anchor networks, a receiver can estimate its position using multilateration. X 80 m 30 m Z Y 50 m Inside Building Screenshots of the OWL application: initial check of GPS accuracy (left) receiver mode (right) A A can determine its location through multilateration if it knows the positions of X, Y, and Z, and its respective distances to them. Comparison with GPS and WPS Results Further Work Average accuracy of receiver mode is almost that of GPS 29.16% of receiver calculations were 11m or better. 21.09% of GPS locations were 11m or better. Location fix time was lower than both GPS and WPS Battery usage was less than WPS and similar to GPS Fix issues with ad-hoc networks There were numerous problems with the anchor networks, including limited range, inconsistent signal strengths, and slow update rates. This seems to be a mainly hardware-specific issue. More testing The system needs testing in more controlled environments. UCLA – UCR – Caltech – USC – UC Merced