Your smartphone is a remarkable feat of engineering. It’s half a dozen or more gadgets packed into a single slab, and many of its coolest feats are accomplished with a wide range of sensors—but what are they and what do they all actually do?
How does your phone count your steps and replace your fitness tracker? Does GPS use up your data? Which sensors should you make sure are in your next handset?
Here’s all you need to know.
Accelerometers handle axis-based motion sensing and can be found in fitness trackers as well as phones—they’re the reason why your smartphone can track your steps even if you haven’t bought a separate wearable.
They also tell the phone’s software which way the handset is pointing, something that’s becoming increasingly important with the arrival of augmented reality apps.
As the name kind of gives away, accelerometers measure acceleration. That means the map inside Snapchat can put a cute toy car around your bitmoji when you’re driving, and a host of other actually useful applications can be enabled too.
The sensor is itself made up of other sensors, including microscopic crystal structures that become stressed due to accelerative forces. The accelerometer then interprets the voltage coming from the crystals to figure out how fast your phone is moving and which direction it’s pointing in.
From switching apps from portrait to landscape, to showing your current speed in a driving app, the accelerometer is one of your phone’s most important sensors.
The gyroscope helps the accelerometer out with understanding which way your phone is orientated— it adds another level of precision so those 360-degree photospheres really look as impressive as possible.
Whenever you play a racing game on your phone and tilt the screen to steer, the gyroscope rather than the accelerometer is sensing what you’re doing, because you’re only applying small turns to the phone and not actually moving through space.
Gyroscopes aren’t exclusive to phones: They’re used in altimeters inside aircraft to determine altitude and position, for example, and to keep cameras steady on the move. Better ones are in the pipeline, though they won’t immediately be cheap and practical enough for consumer mobile devices.
The gyroscopes inside smartphones don’t use wheels and gimbals like the traditional mechanical ones you might find in an old plane—instead, they’re MEMS (Micro-Electro-Mechanical Systems) gyroscopes.
The first time MEMS gyroscopes really hit it big was with the iPhone 4 in 2010. Back then, it was incredibly novel to have a phone that could detect orientation with such accuracy—nowadays, we take it for granted.
Completing the triumvirate of sensors responsible for working out where a phone is in physical space is the magnetometer. Again the name gives it away—it measures magnetic fields and can thus tell you which way is north by varying its voltage output to the phone.
Magnetometers are found in metal detectors as well, as they can detect magnetic metals, which is why you can get metal detector apps for your smartphone.
However, the sensor doesn’t work alone for its primary purpose, which is inside mapping apps—it operates in tandem with the data coming from the phone’s accelerometer and GPS unit to figure out whereabouts you are in the world, and which way you’re pointing (very handy for those detailed navigation routes).
Ah, GPS—Global Positioning System technology—where would we be without you? Probably in a remote, muddy field, cursing the day we ditched our paper maps for the electronic equivalents.
GPS units inside phones get a ping from a satellite up in space to figure out which part of the planet you’re standing on (or driving through). They don’t actually use any of your phone’s data, which is why you can still see your location when your phone has lost signal, even if the map tiles themselves are a blurry, low-res mess.
In fact, it connects with multiple satellites then calculates where you are based on the angles of intersection. If no satellites can be found—you’re indoors or the cloud cover is heavy—then you won’t be able to get a lock.
And while GPS doesn’t use up data, all this communicating and calculating can be a drain on your battery, which is why most battery-saving guides recommend switching GPS off. Smaller gadgets like some smartwatches don’t include it for the same reason.
GPS isn’t the only way your phone can work out where it is—distance to cell towers can also be used as a rough approximation, as Serial taught us—but if you’ve got some serious navigating to do then it’s essential. Modern-day GPS units inside smartphones actually combine GPS signals with other data, like cell signal strength, to get more accurate location readings.
Just about every phone on the market will come with either a fingerprint sensor or a facial recognition system to help you log into your phone. These biometric sensors can be tricked in certain ways, but they’re generally more secure—and a lot more convenient—than using a PIN code or a pattern alone.
Fingerprint sensors have made the move from hardware buttons to in-screen circuitry. There are three main types: Optical (scanning with light), capacitive (scanning with electronic capacitors), and ultrasonic (scanning with sound waves). For the best results, you want ultrasonic, though the other two options are sometimes used on phones towards the cheaper end of the market.
These sensors aren’t working alone, with manufacturers deploying a variety of different software tricks and algorithms to make the fingerprint recognition as accurate as possible. The best premium Android phones now have in-display fingerprint sensors that are almost as good as the ones using physical hardware buttons.
You won’t find a fingerprint sensor on the top-tier iPhones or the Pixel 4, of course: These phones, and others like them, use facial recognition. Again, a variety of technologies are deployed here, with cheaper handsets simply using a normal camera lens and trying to verify your identity with a high-resolution photo.
At the high-end, an infrared sensor maps your face in three dimensions using dots, which are then interpreted by software on the phone: The smarter the software, the quicker the unlock. When face unlock works well, it can feel like magic, but there’s a lot of work going on behind the scenes.
You’ve got plenty more sensors in your handset, though they’re perhaps not all as important as the ones we’ve already mentioned. The Pixel 4 and Pixel 4 XL are unique in having a Soli sensor, which is essentially a radar module: It can detect movement near the phone and just above it, so alarms get quieter as you move to mute them, and face unlock can kick into action as soon as you pick up your phone.
On the Apple side, we’ve seen LiDAR added to the iPad Pros, and it may well come to the iPhone soon. In short, it’s a laser light scanning technology that can judge depth and map out a room very accurately, and it’s going to be most useful for augmented reality apps in the years to come.
Then there’s the U1 chip in the newest Apple phones. This is more of a communications antenna than a sensor, but it can help determine location and the direction you’re pointing your phone in. Many phones, including the iPhone, also have a barometer that measures air pressure: it’s useful for everything from detecting weather changes to calculating the altitude you’re at.
The proximity sensor usually sits up near the top speaker and combines an infrared LED and light detector to work out when you have the phone up to your ear, so that screen can be switched off. The sensor emits a beam of light that gets bounced back, though it’s invisible to the human eye.
Meanwhile, the ambient light sensor does exactly what you would expect, taking a measuring of the light in the room and adjusting your screen’s brightness accordingly (if indeed it’s set to auto-adjust).
Like the rest of the tech packed inside your handset, these sensors are getting smaller, smarter, and less power-hungry all the time, so just because phones five years apart both have GPS doesn’t mean they’re both going to be as accurate. Add in software tweaks and optimizations too and it’s more reason to upgrade your handset on a regular basis, even if you’ll almost never see these sensors listed on a specs sheet.
This guide was originally posted on 7/23/17 and was updated on 6/29/20 to highlight additional sensors found in modern smartphones.