A phone that only listens
A common misconception is that your phone sends a signal up to satellites and they track it. In fact the satellites only broadcast. Your phone’s receiver listens, much as a radio listens to a station, and does the arithmetic itself. The satellites do not know that your phone exists. This is why a GPS receiver can work for millions of people at once.
GPS stands for Global Positioning System. It is the system operated by the United States. The general term for such systems is global navigation satellite system, and there are others, such as Europe’s Galileo, Russia’s GLONASS and China’s BeiDou. Many phones can use several of them together. People often say “GPS” to mean any of them.
The key idea: distance from time
Each navigation satellite carries extremely accurate atomic clocks and continually broadcasts a radio message. The message includes the satellite’s identity, the exact time the message was sent, and data describing where the satellite is in its orbit.
Radio waves travel at the speed of light, roughly 300,000 kilometers per second. Your phone notes when the message arrived. The difference between the send time and the arrival time is the travel time. Multiply the travel time by the speed of light and you have the distance to the satellite.
As a rough example, the satellites orbit at an altitude of about 20,000 kilometers. A signal from a satellite directly overhead would take about 20,000 divided by 300,000, or around 0.07 seconds, which is about 67 milliseconds. Because light moves so fast, a timing error of just one microsecond (a millionth of a second) corresponds to about 300 meters of distance. This is why the clocks must be so precise.
From distances to a position
One distance alone tells you only that you are somewhere on a sphere centered on that satellite. A second distance narrows your position to a circle where two spheres meet. A third narrows it to two points, and one of these is usually far out in space and can be ignored. So three satellites could, in principle, fix a position.
There is a catch. Your phone’s own clock is an ordinary, cheap one, not an atomic clock, so its time is slightly off. Because a small error means a large distance error, the phone treats its own clock error as an unknown to be solved too. With three position unknowns (for example latitude, longitude and altitude) and one clock unknown, it needs signals from at least four satellites. The receiver finds the single position and clock correction that fit all the distances at once. A side effect is that the phone ends up with very accurate time, which is why timekeeping is among the precise timing services these systems provide to other technologies.
In practice, a phone usually sees many satellites at once, and using more than the minimum improves the result.
Corrections the system makes
The calculation has several refinements. Relativity affects the satellite clocks slightly because of their speed and their distance from Earth’s gravity, and the system is designed to compensate for this. The signal slows as it passes through the atmosphere, especially the charged upper layers, and receivers apply models to correct for that. Space weather, such as solar storms, can disturb the upper atmosphere and degrade accuracy at times.
Why your phone is faster than an old GPS unit
Getting a first fix from scratch can be slow, because a receiver must find the satellites and download their orbit data. Phones speed this up with assisted methods. They may get approximate satellite positions through a mobile data connection, and they can use a rough location from nearby cell towers or Wi-Fi networks to narrow the search. Once your phone has recently had a fix, it typically locks on quickly.
Phones also combine GPS with other sensors. The accelerometer and gyroscope sense movement and help smooth the position when the satellite signal is poor, such as briefly in a tunnel. Wi-Fi and cell tower information help indoors, where satellite signals are weak because they are faint by the time they reach the ground and walls block them.
Why accuracy varies
Under open sky, a consumer phone is commonly accurate to within a few meters, though this varies with the device and conditions. Several things can make it worse:
- Tall buildings. Signals bounce off walls and arrive late, which the receiver can mistake for a longer distance. This is called multipath error.
- Poor sky view. Dense tree cover, canyons and being indoors reduce the number of satellites available or weaken their signals.
- Satellite geometry. If the satellites you can see are all in one part of the sky, the position estimate is less certain than when they are spread out.
- Interference. Radio noise can drown out the faint signals. Deliberate interference with signals is known as jamming, and sending false signals is spoofing; both are areas of concern discussed under signal security and spoofing.
The “accuracy circle” shown on some map apps expresses this uncertainty.
What GPS does not do
GPS gives your position. It does not by itself provide maps, directions or traffic. Those come from map data and a data connection, usually using the position as input. Many phones can still show your location without mobile data if maps are stored offline.
Using location also has privacy implications, since apps can record where you go. Phone settings usually let you choose which apps can use location and whether they get a precise or approximate one. Satellite positioning is also used for emergency response positioning, where a phone can share its location with emergency services in some places, though how this works depends on the region and the phone.
Key points
Satellites broadcast the time and their own position. A phone measures how long each signal took to arrive, converts that to distance, and solves for its position and the error in its own clock using at least four satellites. Buildings, weather and weak signals can reduce accuracy, which is why phones also lean on Wi-Fi, cell towers and motion sensors.