How does GPS know where you are?

GPS satellites have no idea where you are. Each one keeps broadcasting two things, where it is and what time its atomic clock says. Your phone listens, works out how long each signal took to arrive, and turns those delays into distances. With signals from at least four satellites, it can solve for its own position and for the error in its own cheap clock.

The satellites are very precise clocks in known places

The United States commits to keeping at least 24 GPS satellites working 95% of the time, and since 2011 the system has effectively run as a 27-slot constellation. GPS.gov counted 31 operational satellites as of July 2023. They sit in medium Earth orbit about 20,200 km up, circle the planet twice a day, and are spread across six orbital planes so that you can see at least four of them from virtually anywhere on Earth.

Each satellite transmits one-way signals giving its current position and the time. Ground stations run the upkeep. They adjust the satellite clocks, upload fresh navigation data and nudge satellites back into their slots. NASA notes that even the atomic clocks on GPS satellites drift, so they get corrections twice a day from larger, more stable atomic clocks on the ground.

Drag sideways or press Turn to see the tilted planes from another side. The markers illustrate spatial coverage, not which satellites a receiver can currently see.

How do you turn time into distance?

You already do a rough version of this during a storm. Count the seconds between the lightning and the thunder and you know roughly how far away the strike was. GPS works the same way, except the "flash" is a timestamp inside the signal and the "thunder" is the moment your phone hears it.

The catch is speed. Radio travels at the speed of light, roughly 30 centimeters every nanosecond, so tiny timing errors become large distance errors. Astronomer Richard Pogge of Ohio State puts the requirement at knowing clock ticks to within 20 to 30 nanoseconds. Each satellite's atomic clock has a nominal accuracy of about 1 nanosecond.

Why does GPS need a fourth satellite?

One distance tells you that you are somewhere on a giant sphere centered on that satellite. Two spheres intersect in a circle. A third narrows it to two points, and one of those is usually nonsense, far out in space.

So why four? Your phone's clock is the weak link. It isn't atomic, which means every distance it calculates is off by the same unknown amount. Engineers call these measurements pseudoranges for that reason. The European Space Agency's Navipedia lays out the result. There are four unknowns, three coordinates plus the receiver's clock offset, so the receiver needs measurements from at least four satellites. With more than four, the system is over-determined, and the receiver uses least squares to find the best fit. It iterates from a rough guess, and the method converges even if the starting guess is the center of the Earth.

Drag the receiver, then move the clock slider. Removing the bias here reveals the known correction; real GPS must solve for it alongside three position coordinates, which needs a fourth satellite.

Solving for the clock error has a second payoff. Once your phone has solved for its clock error, it knows the time very well. GPS.gov's 2024 performance figures put time transfer error at 30 nanoseconds or less, 95% of the time.

Why your phone sends nothing back

GPS works like a lighthouse. The lighthouse doesn't know which ships are watching, and it doesn't need to. The signals flow one way, from satellites to receivers, so any number of receivers can use the system at once without the satellites noticing.

That matters for privacy. The GPS receiver in your phone only listens. If your location ends up on a server, an app or a network put it there afterward.

How big is the relativity correction?

Relativity shifts the satellite clocks by enough to matter, and the system corrects for it. Pogge, drawing on calculations by physicist Neil Ashby, gives the numbers.

• The satellites move at about 14,000 km per hour, and special relativity says moving clocks tick slower. That costs them about 7 microseconds per day.
• They also sit higher in Earth's gravity well, and general relativity says clocks higher up tick faster. That gains them about 45 microseconds per day.
• The net effect is that satellite clocks run fast by about 38 microseconds per day.

Thirty-eight microseconds sounds like nothing. At light speed it is kilometers. Pogge estimates that without the correction, position errors would grow by about 10 kilometers each day.

How accurate is GPS, and what makes it worse?

GPS.gov says GPS-enabled smartphones are typically accurate to within a 4.9 meter (16 foot) radius under open sky. The signal from space is better than that. The government commits to a signal-in-space user range error of 2.0 meters or less with 95% probability, and the measured global average in April 2021 was 0.643 meters or less. Most of the error, in other words, gets added on the way down and inside your phone.

The common culprits, per GPS.gov, are signals blocked by buildings, bridges and trees, use indoors or underground, and signals that bounce off walls before arriving, which engineers call multipath. A city street lined with towers, sometimes called an urban canyon, does both. It hides much of the sky and bounces what's left, which is why your blue dot can wander to the wrong side of the road. GPS.gov also lists satellite geometry, atmospheric conditions and the design and quality of the receiver itself.

Then there's the ionosphere. NOAA's Space Weather Prediction Center explains that its charged plasma bends GPS signals. Receivers compensate using a model of an average, quiet ionosphere. In calm conditions, single-frequency receivers can manage a meter or less, but during a severe geomagnetic storm the model stops matching reality and errors can grow to tens of meters. Dual-frequency receivers can measure the ionosphere's effect directly, because it treats the two frequencies differently, and get to within a few centimeters. Near the magnetic equator, bubbles in the ionosphere can make signals scintillate, a radio cousin of the way the atmosphere makes stars twinkle, and receivers can lose lock entirely.

GPS is one of several satellite systems

People say "GPS" the way they say "Google it." Strictly, GPS is one system, the one the US Space Force operates. The general term is GNSS, meaning any satellite constellation that provides positioning, navigation and timing. GPS.gov lists BeiDou (China), Galileo (European Union) and GLONASS (Russia) as other global systems, plus two regional ones, QZSS from Japan, covering East Asia and Oceania, and NavIC from India. The basic idea, timed signals from satellites in known places, is the same in each. More constellations simply mean more satellites to listen to.

From clocks in orbit to sailors with sextants

Everything here hangs on clocks, so a good next question is how an atomic clock on a satellite keeps time to a nanosecond. Or go back two centuries and ask how sailors found their position before any of this existed. Learnfold turns a question like that into a short planned course and keeps each follow-up as a linked page, which is how a curiosity rabbit hole gets a map. It is an AI tutor and can be wrong, so check its explanations against sources like the GPS.gov pages below.