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GPS (Global Positioning System) is a world wide positioning service using satellites. Its accuracy is the best of known systems but it needs a special GPS-receiver and straight view to at least three satellites. GPS can't be used indoors or between high buildings. GPS is provided by US Department of Defence and they are able to disturb the signal so that the signal is useless to civilians. Normally the accuracy of the GPS is under ten meter and it's the only of these systems that could be used to locate a person in emergency situation. Mobile phone can be positioned with several different methods but most of them requires changes in mobile phones or in the network. Some methods to mention: Cell of Origin (COO), Time of Arrival (TOA), Round Trip Time (RTT), Timing Advance (TA), Observed Time Difference (OTD), Angle of Arrival (AOA), Received Signal Levels and combinations of these. Easiest way to locate mobile phone is to use COO but its accuracy depends on density of the cells in network. GSM cell size varies from 200 m to 35 km. In the future COO may provide us an easy way to indoor positioning when mobile phone is in micro and pico cells.
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