Mobile phone tracking is a process for identifying the location of a mobile phone, whether stationary or moving. Localization may be effected by a number of technologies, such as the multilateration of radio signals between (several) cell towers of the network and the phone or by simply using GNSS. To locate a mobile phone using multilateration of mobile radio signals, the phone must emit at least the idle signal to contact nearby antenna towers and does not require an active call. The Global System for Mobile Communications (GSM) is based on the phone's signal strength to nearby antenna masts. Mobile positioning may be used for location-based services that disclose the actual coordinates of a mobile phone. Telecommunication companies use this to approximate the location of a mobile phone, and thereby also its user. Technology The location of a mobile phone can be determined in a number of ways. Network-based The location of a mobile phone can be determined using the service provider's network infrastructure. The advantage of network-based techniques, from a service provider's point of view, is that they can be implemented non-intrusively without affecting handsets. Network-based techniques were developed many years prior to the widespread availability of GPS on handsets. (See for one of the first works relating to this.) The technology of locating is based on measuring power levels and antenna patterns and uses the concept that a powered mobile phone always communicates wirelessly with one of the closest base stations, so knowledge of the location of the base station implies the cell phone is nearby. Advanced systems determine the sector in which the mobile phone is located and roughly estimate also the distance to the base station. Further approximation can be done by interpolating signals between adjacent antenna towers. Qualified services may achieve a precision of down to 50 meters in urban areas where mobile traffic and density of antenna towers (base stations) is sufficiently high. Rural and desolate areas may see miles between base stations and therefore determine locations less precisely. GSM localization uses multilateration to determine the location of GSM mobile phones, or dedicated trackers, usually with the intent to locate the user. The accuracy of network-based techniques varies, with cell identification being the least accurate (due to differential signals transposing between towers, otherwise known as "bouncing signals") and triangulation as moderately accurate, and newer "advanced forward link trilateration" timing methods as the most accurate.