MOZER mobile phone tracking system works between (several) radio towers of the network and the phone or simply works via GPS. Referring to the ascertaining of the position of a mobile phone by GPS, Mobile phone tracking can be defined as attaining the current position of a mobile phone whether stationary or moving. Tracking is possible in a restricted area either by multilateration of radio signals between radio towers and the mobile phone or via GPS. While multilateration requires emission of roaming signal to contact the next closest antenna tower without an active call, GSM is based on signal strength of the closest antenna towers. Measurement of power levels and antenna patterns linked with the concept that mobile phones can always be wirelessly used for communications with one of the nearest base stations, and forms the foundation for using technology in mobile phones tracking. Use of multilateration for mobile phone tracking is referred to as GSM localization.