From a Guardian article
Mobile phone companies can provide police with real-time location information about the whereabouts of suspects or missing people at 15-minute intervals. More commonly, police request a "cell site dump", which gives a complete historical record of the whereabouts of person's mobile phone.
There are two ways the data is obtained. When a phone is used for a call or SMS message, details of its location are logged. Alternatively so-called "pinging" can be used when a phone is not in use, by sending the device signals and triangulating the results from cellphone masts. The level of accuracy ranges from a few hundred metres to around two kilometres, depending on proximity to the masts.
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