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 GPS tracking. 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.^[1]

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.^[2]

The location of a mobile phone can be determined using several complementary technologies. Modern phones typically use a hybrid positioning system that combines these methods to provide the fastest and most accurate location possible.^[3]

In order to route calls to a phone, cell towers listen for a signal sent from the phone and negotiate which tower is best able to communicate with the phone. As the phone changes location, the antenna towers monitor the signal, and the phone is "roamed" to an adjacent tower as appropriate. By comparing the relative signal strength from multiple antenna towers, a general location of a phone can be roughly determined. Other means make use of the antenna pattern, which supports angular determination and phase discrimination.

Newer phones may also allow the tracking of the phone even when turned on but not active in a telephone call. This results from the roaming procedures that perform hand-over of the phone from one base station to another.^[12]

A phone's location can be shared with friends and family, posted to a public website, recorded locally, or shared with other users of a smartphone app. The inclusion of GPS receivers on smartphones has made geographical apps nearly ubiquitous on these devices. Specific applications include:

• Geo-fence specific locations of interest such as No Fly Zones
• GPS navigation and maps
• Locator apps like Find My Friends
• Dating apps like Grindr
• Recording a journey, for example to show a hiking accomplishment
• For quantified self purposes such as fitness tracking
• GPS drawing

In January 2019, the location of her iPhone as determined by her sister helped Boston police find kidnapping victim Olivia Ambrose.^[13]

Locating or positioning touches upon delicate privacy issues, since it enables someone to check where a person is without the person's consent.^[14] Strict ethics and security measures are strongly recommended for services that employ positioning.

In 2012 Malte Spitz held a TED talk^[15] on the issue of mobile phone privacy in which he showcased his own stored data that he received from Deutsche Telekom after suing the company. He described the data, which consists of 35,830 lines of data collected during the span of Germany's data retention at the time, saying, "This is six months of my life [...] You can see where I am, when I sleep at night, what I'm doing." He partnered up with ZEIT Online and made his information publicly available in an interactive map which allows users to watch his entire movements during that time in fast-forward. Spitz concluded that technology consumers are the key to challenging privacy norms in today's society who "have to fight for self determination in the digital age."^[16][17]

• Varshavsky, Alex; Chen, Mikey; Froehlich, Jon; Haehnel, Dirk; Hightower, Jeffrey; Lamarca, Anthony; Potter, Fred; Sohn, Timothy; Tang, Karen; Smith, Ian. "Are GSM phones THE solution for localization?"(PDF). In 7th IEEE Workshop on Mobile Computing Systems and Applications (HotMobile). IEEE Computer Society: 20–28.
• Dan Goodin (17 February 2012). "Location tracking of GSM cellphones: now easier (and cheaper) than ever". Arstechnica. Conde Nast.