The common wisdom about surveillance is that your phone can only be tracked through its cell signal and through GPS. Relatively few people are aware that the WiFi and Bluetooth capabilities provide additional ways to track phones, and even fewer are aware that it’s possible to use malware to track phones that seem to have been turned off. Even among those who are aware of those possibilities, it’s generally accepted that a faraday bag can protect your phone from being tracked. Turning off the GPS and turning on “Airplane Mode” is supposed to produce similar results – but neither are actually enough to prevent your phone from being tracked (though they can prevent real-time tracking as well as the most common methods of electronic cellphone surveillance).
Every modern smartphone is equipped with three sets of sensors that can’t be disabled and can be accessed by applications without the user needing to grant access: magnetometers, accelerometers and gyroscopes. These sensors can be used to track the cellphone, and of the three only one can be blocked by using a faraday bag. Unfortunately, that one also happens to be the least useful for tracking purposes – the magnetometer. The gyroscope and the accelerometer are more than enough to track the movements of a cellphone that has an active power source through what’s called inertial navigation or inertial tracking.