Cell phones are powerful tools for communication and instant Internet access, but they can also be used to track your location, contacts, and daily activities. You can generally presume you are not being monitored--there are millions of cell phone subscribers, after all--but if you are concerned about your privacy, these are the potential methods by which you can be tracked.
GPS and Network Triangulation
There are two methods by which your cell phone location (and presumably, you) can be tracked, global positioning systems and network triangulation.
Global positioning systems (GPS) were included in only certain phones as of 2009, but they are becoming increasingly common due to the popular mapping features these services offer users. GPS systems can generally track your location within a radius of several feet, and have global reach.
Network triangulation can occur when your cell phone is within range of at least two cellular towers connecting you to the network. This creates a triangle with three points: you and the two towers. In these cases, the strength of your signal can be measured by both towers, and can mathematically determine your location. This is usually less precise than GPS, but can still be highly accurate, especially in dense cellular networks.
You can minimize network triangulation by turning off "location services" in your phone. This will turn off your phone's registration with ongoing location monitoring by your network. This is not a complete fix, however: by law, the network must be able to determine your location if you make a 911 call, and you cannot turn this off. Ultimately, the best way to defeat both triangulation and GPS is to turn off your phone when you are not actively using it.
Conversation Traces and Taps
An alternate method of surveillance is to monitor any conversations held on it. Methods range from duplicating the serial number the cell phone uses to identify itself with the network, to installing tapping mechanisms at the trunk location where the network patches your call into the overall telephone network. When the conversation itself is made available to other parties, this is called a "tap." When information about whom you are calling, and when you called, is available to others, this is called a "trace" or "pen trace."
Unfortunately, there is no way of knowing when this might occur. Taps and traces can be installed at both your network provider, as well as the networks of anyone you call. However, both taps and traces can be made more difficult (and in some cases, impossible) by using Voice over Internet Protocol (VoIP) on encrypted networks to connect your call. The Skype network provides encryption for US calls, and has software available for many cellular handsets.
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