There are two ways a cell phone can be detected (or tracked): through a built-in GPS (global positioning system) device or by being GPS-enabled. The latter involves a process called triangulation, where signals between cell phones and cell towers are calculated to determine the location of the cell phone. Although it's possible to track most cell phones, it's also possible to prevent and minimize detection.
Turn off your cell phone. It's as easy as that. If your cell phone is turned off, it's not transmitting signals that can be picked up by cell towers, and if it has a built-in GPS device, it can't be tracked. Keep in mind that this is an immediate means to preventing detection--there is still a track-able history of your past calls and locations for periods of time when your cell phone was on.
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Disable the location feature. Some cell phone models have a GPS-location feature that you can turn off, which effectively disables tracking unless you make a 911 call. This feature might be called "Location" or listed under your "Security" settings. If you're unsure if your cell phone has this feature, look up the security features for your phone in its documentation or contact customer support for the phone manufacturer (this phone number is on the manufacturer's main website or call directory assistance for the manufacturer's number).
Refuse tracking invitations.There are various free, online GPS-locator services, such as Google Latitude, that let you register your own cell phone for tracking, as well as track others' cell phones with their permission. Such services have options for viewing cell phone locations on computers, as links in text messages, on websites and more. If you want to minimize others detecting your cell phone, refuse their tracking invitations.
Decline cell phone locator services. Many cell phone providers offer GPS-locator services for cell phones under an account--for example, Sprint offers the Sprint Family Locator plan that enables tracking for up to four phones under a single account. Similar to tracking invitations (see Step 3), each cell phone in the plan must accept the invitation to be tracked. There is typically a $5 to $10 monthly fee for this service.
Protect your cell phone from spyware. If malicious spyware is illegally downloaded into your cell phone, your locations can be tracked, your stored information retrieved, and any conversations you have in the vicinity of the cell phone--even if it's turned off--can be heard. To prevent such spyware from being downloaded, set a password to lock access to the phone, never leave your phone unattended, refuse connections from devices you don't know or trust, and never loan it to strangers.