How Does GPS Work? Satellites, Timing and Your Phone
GPS finds your distance to each satellite from how long its radio signal takes to arrive. The satellites keep broadcasting the exact time and their own position, and your phone only listens. From the distances to four or more satellites, it works out your position and its own clock error at the same time.
What you'll learn
- How GPS differs from GNSS, with Japan's QZSS (Michibiki) as an example of a regional system
- How signal travel time gives distance, and why four satellites are needed
- The satellites' atomic clocks, and how they are corrected for relativity
- Why phones find your position quickly, and why it sometimes drifts
Prerequisites: none. The basics of radio waves and frequency are covered in "How Does Wi-Fi Work?"
In everyday life
Open a map app, and a blue dot shows where you are. This dot is worked out mainly from the radio signals of satellites circling the Earth.
Among tall buildings, though, the blue dot sometimes jumps to the other side of the street. The pale circle around the dot can also grow much larger. Let's look, step by step, at how satellite signals reveal your position, and why it sometimes drifts.
GPS and GNSS: names for satellite positioning
Systems that find positions from satellite radio signals are called satellite navigation systems. They are also known by the initials GNSS (global navigation satellite system). GPS is the name of one of them: the system run by the United States.
The US built GPS for its military, and today anyone in the world can use it for free. The satellites fly about 20,000 km up and circle the Earth about twice a day. The US says it keeps at least 24 satellites available. As of 2026, a little over 30 satellites are in operation.
Other countries and regions have their own systems.
| Name | Country or region | Notes |
|---|---|---|
| GPS | United States | The main example in this article |
| GLONASS | Russia | Works worldwide |
| Galileo | EU | Initial service began in December 2016 |
| BeiDou | China | Global service began in July 2020 |
| QZSS (Michibiki) | Japan | Focused on the skies over Japan; service began in November 2018 |
Most phones today use several of these together. For example, the specs of the iPhone 16 Pro (2024) list the five above plus NavIC (India). The more satellites a phone can use, the easier it is to get a position where little sky is visible. Much of what we call "GPS" in daily life is really GNSS as a whole.
Position comes from the time a signal takes to arrive
Radio waves are a kind of light. In a vacuum, they travel about 300,000 km per second. If you know the speed, the travel time gives you the distance, because distance is speed × time.
Each GPS satellite keeps broadcasting a signal that carries two things: the time it was sent, and where the satellite was at that moment. Your phone compares the sending time with the arrival time to get the travel time. Multiply that by the speed of light, and you have the distance to that satellite.
When a satellite is straight overhead, about 20,200 km away, the signal arrives in about 0.067 seconds (67 milliseconds). Satellites lower in the sky are farther away, so it can take more than 80 milliseconds.
A clock off by a millionth of a second means 300 m of error
This method is very sensitive to timing mistakes. Radio waves travel about 300 m in one microsecond (a millionth of a second). So a clock that is off by 1 microsecond puts the distance off by about 300 m. Even 1 nanosecond (a billionth of a second) means about 30 cm.
That is why GPS satellites carry extremely accurate clocks. These clocks rely on the fact that atoms respond only to radio waves of one exact frequency. A clock like this is called an atomic clock. The latest GPS III satellites each carry three atomic clocks that use the element rubidium.
On the ground, control stations keep watch on the satellites' clocks and orbits. They measure tiny errors and send correction data up to the satellites. The satellites then broadcast that data in their signals too.
The clock in your phone, on the other hand, is an ordinary clock driven by a vibrating quartz crystal. It is far less accurate than an atomic clock, and it soon drifts by microseconds. Even so, your phone does not need an atomic clock. The trick explained next makes up for it.
Why four satellites are needed
A position needs three numbers: east–west, north–south and height. Knowing your distance to one satellite tells you that you are somewhere on a sphere around it, with that distance as the radius. With distances to three satellites, your position is where the three spheres meet. They meet at two points, but one is far out in space, so it can be ruled out.
But that only works if the phone's clock is correct. If the clock is off, the distances to all the satellites come out too long (or too short) by the same amount. Then the spheres no longer meet at one point.
So the clock error is added as one more unknown. That makes four unknowns: east–west, north–south, height and clock error. Finding four unknowns takes four clues, that is, the distances to four satellites.
The receiver adjusts the clock error as it calculates, until the distances to four or more satellites line up at exactly one point. This gives the phone's clock error along with the position. That is why a GPS receiver is also a tool for getting precise time. The more satellites in view, such as five or six, the more reliable the result.
Picking out a signal weaker than the noise
Satellite signals travel more than 20,000 km, so they are very weak when they reach the ground. The GPS specification sets the minimum received strength on the ground at −128.5 dBm (about −130 dBm). Here, dBm is a unit of signal power. 0 dBm is 1 milliwatt, and each step of −10 dBm is ten times smaller. So −130 dBm is a ten-trillionth of a milliwatt, or 10−16 watts.
Inside a receiver's circuits, electrons jiggle randomly because of heat, and this always creates a little noise. The GPS signal is even weaker than this noise. By a rough estimate, the noise is several tens of times stronger.
The signal can still be picked out, because each satellite keeps repeating its own fixed pattern of 0s and 1s. This pattern is called a code. In the civilian GPS signal, a code 1,023 digits long repeats every thousandth of a second.
The receiver holds a copy of the same code. It slides the copy in time, a little at a time, and lines it up with the incoming signal. Where the two match exactly, a strong response appears. Noise is random, so it does not build up when lined up this way. The time shift that gives the exact match tells when the signal arrived.
Relativity: satellite clocks run about 38 microseconds a day fast
Satellite atomic clocks have one more surprising error. According to Einstein's theory of relativity, how fast a clock ticks depends on the strength of gravity around it and on how fast it moves. The difference goes unnoticed in daily life, but GPS cannot ignore it.
The satellites are about 20,000 km above the ground, where Earth's gravity is weaker. Weaker gravity makes a clock run faster, by about 46 microseconds a day. Meanwhile, the satellites move at about 3.9 km per second, and a fast-moving clock runs slow. That effect is about 7 microseconds a day.
Taken together, a satellite clock runs about 38 microseconds a day faster than a clock on the ground. In distance, that equals about 11 km, far too much for a system that measures distance by time. So the satellite clocks are set before launch to run very slightly slow. Their reference of 10.23 MHz (10.23 million cycles a second) is set to 10.22999999543 MHz instead.
This is a simplified picture. Satellite orbits are slightly oval, so the size of the effect changes a little along the orbit. The receiver calculates and corrects this changing part.
Why phones find your position so fast: A-GPS
To calculate a position, the receiver also needs precise data on each satellite's orbit. This data is in the signal too, but it is sent very slowly, at 50 bits per second. A bit is a single digit, either 0 or 1. Reading the part that is needed can take up to about 36 seconds.
So your phone gets the orbit data and the approximate time in advance, over the mobile network or Wi-Fi. The cell towers nearby also give a rough idea of where you are. Knowing ahead where each satellite is in the sky makes the search for signals quicker. This system is called A-GPS (assisted GPS).
Thanks to A-GPS, your position usually appears soon after you open a map app. It also helps in places where signals are too weak to read the orbit data easily.
Why your position drifts
According to the US government's official GPS website, a phone is usually accurate to within a 4.9 m radius under open sky. Accuracy gets worse near buildings, bridges and trees. Let's look at three main causes.
1. Signals reflected off buildings
Among tall buildings, the direct signal from a satellite can be blocked, so only a signal reflected off a wall gets through. The reflected signal takes a detour, so it arrives late. The receiver thinks the satellite is farther away than it really is, and the position shifts. The direct and reflected signals can also arrive together and overlap. A signal that arrives by several paths like this is called multipath.
2. Delays in the layers of the atmosphere
Radio signals slow down slightly as they pass through the ionosphere, high above the ground. The ionosphere is a layer where part of the air has become electrically charged, mainly by sunlight. Lower layers of air also slow the signal slightly, because of water vapor and other factors. The delay changes with the time of day, the weather and the Sun's activity. So correcting it with a prediction formula cannot remove it completely.
The delay in the ionosphere depends on the signal's frequency. So comparing signals on two frequencies lets the receiver estimate the delay. In 2017, Broadcom announced a phone chip that receives two frequencies (L1 and L5). L5 signals have another advantage: reflected signals are easier to tell apart.
3. How the satellites are spread across the sky
If the satellites in view are bunched in one part of the sky, small distance errors turn into large position errors. The spheres meet at shallow angles, so the meeting point blurs. The more spread out the satellites are, the more accurate the position. Among tall buildings, the visible sky is a narrow strip, so the satellites in view also tend to be lopsided.
A Japan example: QZSS (Michibiki) and centimeter-level corrections
QZSS (Quasi-Zenith Satellite System), nicknamed Michibiki, is a satellite system run by Japan's Cabinet Office. Seen from the ground, its main satellites trace a figure-eight path. This keeps them close to straight above Japan for long periods. The point straight overhead is called the zenith, which is where the name "quasi-zenith" comes from. Signals from directly above are less likely to be blocked by buildings.
Michibiki also sends signals in the same format as GPS, so it can be used alongside the GPS satellites. Service began in November 2018 with four satellites, and there is a plan to grow to seven. In December 2025, one satellite was lost in a launch failure, and another was launched in August 2026. Once all seven are in place, the plan is for Michibiki alone to give positions in and around Japan.
Its other role is to send correction data that improves accuracy. Japan's Geospatial Information Authority runs a nationwide network of observation points (GNSS reference stations). Their data is used to work out error information, which the satellites then broadcast. This is called the Centimeter Level Augmentation Service (CLAS). The Cabinet Office's specification for a stationary receiver is a horizontal error within 6 cm, 95% of the time.
However, using CLAS requires a special receiver that can take in this correction data. It is mainly used for surveying and for work that needs precise positions, such as farm and construction machinery. It does not make the blue dot on an ordinary phone accurate to the centimeter.
The receiver circuit inside your phone
In a phone, satellite signals are picked up by part of the radio circuitry. First, an amplifier designed to add very little noise boosts the faint signal from the antenna. Next, the signal is shifted to a lower frequency that is easier to handle. Then it is turned into digital 0s and 1s.
After that, the code-matching calculation described earlier runs for many satellites at the same time. Some phones build this circuitry into the main processor chip (SoC), while others use a small dedicated chip. How it fits with the other chips in a phone is covered in "What Chips Are Inside a Smartphone?"
Indoors: Wi-Fi and cellular signals as clues
Indoors, walls and roofs weaken satellite signals even more, and often almost nothing gets through. So phones combine other clues as well.
According to Apple, iPhones use Bluetooth in addition to GPS. They also use location data for nearby Wi-Fi access points and cell towers. The phone checks which access points it can hear against data on where those access points are. How Wi-Fi signals work is covered in "How Does Wi-Fi Work?", and cell towers in "What Is 5G?"
Common misconceptions
Myth: GPS satellites keep track of where your phone is
In fact, GPS satellites only send signals, and they receive nothing from phones. Your phone works out its own position from the signals it receives. Because phones only listen, it makes no difference to the satellites how many receivers use them.
Your location reaches other people or companies only when an app sends it over the mobile network or Wi-Fi. You can choose in your phone's settings which apps may use your location. People easily confuse "satellites let you find your position" with "satellites know your position."
Myth: GPS doesn't work without mobile data
In fact, receiving satellite signals and calculating a position needs no data connection. Even in mountains with no cell coverage, you get a position if the sky is open. Without help from A-GPS, though, the first position can take a while to appear. Also, many apps download the map images over the network, so the map won't show unless you saved it in advance.
Myth: The blue dot on the map always shows the right place
In fact, the blue dot is the most likely location. On Android, the position comes with an accuracy figure. It means there is a 68% chance that your true position is inside a circle of that radius. Many map apps show this kind of estimate as a circle around the dot. Among tall buildings or indoors, the circle grows, and the dot may jump.
By the numbers
| Item | Figure | Compared with |
|---|---|---|
| Clock error and distance | About 300 m per microsecond | Even 1 nanosecond is about 30 cm |
| Travel time from a satellite overhead | About 0.067 seconds | A distance of about 20,000 km |
| Clock gain from relativity | About 38 microseconds a day | Equal to about 11 km in distance |
| Signal strength on the ground | −128.5 dBm (minimum in the specification) | About 10−16 watts |
| Phone accuracy (open sky) | A radius of about 4.9 m | CLAS specification: 6 cm horizontal (stationary receiver) |
Learn more
- Chips that handle radio in a phone: "What Chips Are Inside a Smartphone?"
- How data rides on radio waves: "How Does Wi-Fi Work?" and "What Is 5G?"
- Try the math: the dBm converter (check how many watts −130 dBm is)
- Compare in a table: Wi-Fi vs Bluetooth vs LPWA (radio that gives clues to your position indoors)
- Terms for the next step: fading (big swings in strength when reflected signals overlap), free-space path loss (how much signals weaken with distance), noise figure (the noise a circuit adds)