Mobile Phones: Phone Components and Sensors

Your phone is filled with all sorts of antennas and sensors that collect a wealth of information. Here’s a rundown of some of them.

GPS anchor link

The Global Positioning System (GPS) lets devices anywhere in the world figure out their own locations quickly and accurately. GPS works based on analyzing signals from satellites that are operated by the U.S. government as a public service for everyone. It's a common misconception that these satellites know where the GPS users are, but the GPS satellites only transmit signals. They don't receive or observe anything from your phone, and the satellites and GPS system operators do not know where any particular user or device is located, or even how many people are using the system.

This is possible because the individual GPS receivers (like those inside smartphones) calculate their own positions by determining how long it took the radio signals from different satellites to arrive.

So, if GPS satellites aren’t tracking users, how is “GPS tracking” done? Usually, this tracking is done by apps running on a user’s smartphone. These apps ask the phone's operating system for its location (determined via GPS). In turn, the phone asks if you’d like to share your location with the app. If you grant permission, then the app can transmit this information to someone else over the internet. Multiband support, which increases GPS reliability, is now available in modern phones for other government sponsored positioning systems such as GLONASS (Russia), BDS (China), and GALILEO (European Union).

Law enforcement can obtain GPS data on a particular individual with a warrant to app providers. Law enforcement also compels providers to perform geofence searches of GPS data to identify every device at a particular location at a particular time. EFF has had some success in arguing these geofence warrants are categorically unconstitutional. 

NFC (Near Field Communication) anchor link

This wireless, radio-frequency based, identification system exists in modern phones. It is mainly used for contactless mobile payment technologies (like using your phone to pay for something) or contactless identification systems (like using your phone as a key at a hotel). The range of NFC normally maxes out at four centimeters and can store around a few hundred bytes to a few kilobytes of data at a time. 

NFC technology can also passively read RFID (radio-frequency identification) tags embedded in posters and other objects. Even though this isn’t a technology that accurately knows where you are in the world, it can be logged when you use contactless payment or enter a building, placing you at a specific time and place. NFC technology can also be used to control “smart home devices.” 

If you’re concerned about this capability , turn the NFC sensor in your phone off. 

On Android, follow these directions to disable NFC. 

In iOS, NFC is limited to applications like Apple Pay and the company’s AirDrop services (which enable you to share contact information, photos, or audio with a nearby phone). That limitation means there’s not a lot of information that might get leaked out, but still, if you prefer to turn it off, you can avoid using Apple Pay entirely, and disable AirDrop’s proximity sharing services. To turn off AirDrop’s device sharing, follow these directions.

Biometric Sensors anchor link

These consist of fingerprint sensors or a facial recognition system to help you log into your phone. The facial recognition system may also use an infrared sensor, which uses infrared light to map your face in 3D (this is how features like the iPhone’s FaceID work in the dark). These biometric unlocking features are considered more secure from a phone safety aspect, but are not necessarily considered safer in various situations with law enforcement.

Motion Sensors anchor link

There are other phone sensors that don’t necessarily provide as much information as cell towers, GPS, or Wi-Fi. The following are listed to build on the knowledge of what exists inside your smartphone, as applications often read sensor information that can potentially identify your device, even though the potential to be detected with these methods is minimal.

Accelerometer: A motion sensor, often used in fitness applications to log what type of activity the user is doing. For example, if you keep your phone in your pants pocket, this is the main sensor your phone uses to track the number of “steps” you take in a day. This is also used in many navigation applications that measure the rate of speed you are traveling. This sensor has been flagged in security research as being able to detect the vibrations of and identify different keystrokes on a nearby computer with 80% accuracy. This is hard to detect by normal means, but putting your phone in your bag or pocket can mitigate this concern.

Gyroscope: A motion sensor that detects orientation and angular velocity. This sensor is constantly sensing new information since we are always moving our phone’s position. This sensor has been noted in previous research as acting as a potential crude microphone, due to their capability to pick up sound waves. There are other sensors that are environmental, motion, and position based that can measure room temperature, humidity, amount of light, Earth’s magnetic field, air pressure, etc. These sensors are normally very low powered and not as useful as the methods used by Location Tracking services to locate someone. The usage model for this sensor varies from iOS to Android (more strict on iOS because developers have to define the reason they are using this sensor). Generally, applications don’t need to ask for permission to use these sensors. However, it takes an active amount of targeting and resources to use these sensors in a way that can compromise a user.

Other sensors to be aware of:

• Magnetometer: This works much like a compass, and combined with the gyroscope and accelerometer helps your phone figure out where it is. Its most obvious use is the compass app.
• Barometer: This is used to measure air pressure, which can be helpful to improve GPS elevation results.
• Proximity sensor: This sensor measures proximity by emitting a beam of invisible light. The most common usage is to turn your phone’s screen off when you hold it to your face during a call.
• Ambient light sensor: As the name suggests, this sensor detects lighting conditions in a room and is most commonly used for the “auto brightness” feature.
• LiDAR: This is used to measure distance using technology similar to radar, but with light instead of radio waves. It’s used for everything from taking better pictures in the dark, to the measuring app that allows you to measure things with your phone’s camera.
• U1 and U2 chip (antenna proprietary to iPhone): The U1 and U2 chips are used to improve communication between other devices that have this chip. This improves how iPhone communicate with each other, and how an iPhone communicates with other Apple devices, including laptops, the Apple Watch, and AirTag devices.