31 satellites on six rings

PHYSICS · 5 MIN

How does your phone find you to within 5 metres from 20,000 km away?

Your phone never sends a thing to space. It just listens to faint whispers from satellites, times them to a billionth of a second, and even corrects for Einstein. Let's climb from a car on a highway up to the clock inside a satellite.

8 steps · 8 quick challenges

Step 1 of 8 · On the highway

A blue dot that follows you

You're on a highway, phone on the dashboard, and a blue dot slides along the map with you. No one told it where you are. So how does it know?

Your car finds its place on Earth by quietly listening to radio signals from satellites far overhead, and it never sends anything back.

Blue dot. That moving blue dot comes from GPS, the Global Positioning System. Under open sky it places a phone within about 5 metres, roughly the length of this car, and updates many times a second. It works the same in a desert, at sea or on this highway, with no Wi-Fi and no phone signal needed. Like knowing exactly which parking space you're in, from anywhere on the planet.
GPS is run by the US Space Force and is free for anyone to use. Europe (Galileo), Russia (GLONASS), China (BeiDou), Japan and India run similar systems, and most phones listen to several at once, which is why your dot is usually steady even between tall buildings.
"About 5 metres" is the official figure for a phone under open sky: 95% of the time it is closer than 4.9 m. Cars that also use wheel-speed sensors and a gyroscope can keep the dot on the right lane even in a tunnel, by guessing until the satellites come back.
When the sky is blocked, accuracy drops. Signals bounce off glass towers and arrive late, which can throw the dot across the street. That's why your map sometimes thinks you are in the river next to the road.
Only listening. Here's the surprise: your car never talks to the satellites. It doesn't send a request or a signal; it only listens to what they broadcast. All the clever maths happens inside the car, using the signals that arrive at that small fin on the roof. The satellites have no idea you exist. Like a radio station: you can listen in, but it can't tell who's listening.
Because receivers only listen, GPS can't track you by itself. Apps that share your location do it separately, over the mobile network or Wi-Fi, after your phone has worked out its own position.
It also means the system never gets crowded. A billion phones can use the same satellites at the same time, just as a billion radios could all play the same station without slowing it down.
What if a satellite stops working? Receivers just use the others. There are spares in orbit, and ground control can flag a faulty satellite as 'unhealthy' in its broadcast, so receivers ignore it within seconds.
Everyone at once. That car ahead hears exactly the same signals, and so do the trucks on the other side. Each one works out its own position from them. Because the signals are the same for everyone, each receiver's answer depends only on where it is, which is exactly what we want. Like a lighthouse: one light helps every ship, and each ship uses it to find itself.
The signals spread out from about 20,000 km up, so by the time they reach the ground they cover nearly half the planet. Two cars 100 m apart get almost identical signals, just arriving a tiny fraction of a second apart.
That tiny difference is the whole trick. The car ahead is a bit closer to some satellites and a bit further from others, so its signals arrive at slightly different moments from yours, and that is what gives each of you a different dot.

Phone accuracy: ≈5 m
Cost to you: Free

Did you know? GPS was built for the US military. It was opened to everyone in the 1980s, and in 2000 the deliberate 'fuzz' added to civilian signals was switched off, making GPS about 10 times more accurate overnight.

GPS stands for Global Positioning System. It has three parts: satellites in space that broadcast signals, a ground-control network that keeps those satellites accurate, and the receivers (phones, cars, watches, ships, planes) that use the signals.

The core idea is surprisingly simple. If you know exactly when a signal left a satellite and exactly when it reached you, you know how long it travelled. Radio waves travel at the speed of light, so travel time tells you the distance. Measure the distance to enough satellites, and only one spot on Earth fits all of them.

The hard part is the timing. Light covers 30 cm in a billionth of a second. To find you to within a few metres, the whole system has to agree on the time to a few billionths of a second, which is why every GPS satellite carries atomic clocks.

In this story we climb up, one layer at a time: the car, its antenna and the signal, then the satellites in the sky, then one satellite, and finally the atomic clock inside it.

GPS time signals also keep mobile networks, power grids and bank transfers in sync. Even when nobody needs a location, GPS is used as the world's clock.

Step 2 of 8 · On the highway

The ear on the roof

So the car only listens. Then it needs a pair of ears. Let's zoom in on that little fin on the roof and look inside.

The shark fin hides a flat antenna that catches satellite signals and a chip that pulls them out of the noise.

Shark fin. That fin on the roof is a plastic cover for antennas. Plastic lets radio waves through, while the car's metal body would block them. It sits on top, the highest point of the car, so it can see as much of the sky as possible and isn't shaded by the car itself. Like a satellite dish on a house roof, but shrunk and made sleek.
A fin often holds several antennas: GPS, the car's mobile connection, and sometimes radio. The streamlined shape keeps wind noise down and stops it snapping off in a car wash.
The metal roof underneath is useful too. It acts as a 'ground plane', a flat mirror for radio waves that helps the antenna listen upwards and ignore signals bouncing up from the road.
A whisper. Inside sits a patch antenna: a small square of ceramic with a metal plate on top, tuned to 1575.42 MHz, the main GPS radio frequency. By the time a satellite's signal reaches it, it is unbelievably faint, far weaker than the background radio hiss all around us. It's like hearing a whisper in a stadium. Like trying to hear one friend whisper from across a packed football stadium.
Each satellite transmits with about 50 watts, roughly a bright light bulb, from 20,000 km away. Spread over half the planet, only around 0.0000000000000001 watt (10⁻¹⁶ W) reaches your antenna. That is below the random radio noise made by heat in the antenna itself.
The patch is sized to the 19 cm GPS wave; the ceramic shrinks the wave inside it, so the patch can be just 2–3 cm across. Its shape makes it most sensitive to waves twisting in one direction, the same twist GPS satellites use, so it ignores many signals that bounced off buildings, because a bounce flips the twist.
Right behind the antenna is a tiny amplifier that boosts the signal before it travels down a cable to the receiver. Without it, the cable would lose more signal than there is.
Receiver chip. Next to it, the receiver chip finds the whisper using a trick: each satellite repeats its own known pattern of 1,023 on/off 'chips', every thousandth of a second. The chip slides a copy of that pattern along the noise until it lines up. When it clicks, the faint signal stands out, and its timing is measured. Like spotting a friend in a crowd because you already know their face.
This is called correlation. The receiver multiplies the incoming noise by its own copy of a satellite's code. Random noise averages out to nothing, but the matching code adds up a thousand times over, lifting the signal clear of the noise.
Every satellite has a different code, so they can all share one frequency without jamming each other. A modern chip tracks dozens of satellites from GPS, Galileo and BeiDou at the same time, on many 'channels' running in parallel.
A cold start, with no idea which satellites are overhead, can take 30 seconds or more. Phones skip that wait using 'assisted GPS': the mobile network tells them roughly where they are and which satellites to look for, so a fix comes in a second or two.

Frequency: 1575.42 MHz
Signal at ground: ≈10⁻¹⁶ W

Did you know? The GPS signal arriving at your phone is so weak that, collecting all of it would take far longer than the age of the universe to warm a cup of tea by a single degree.

Radio waves are the same stuff as light, just with much longer waves. GPS uses waves 19 cm long. They pass through clouds, rain and plastic, but not through metal or thick walls, which is why GPS struggles indoors.

An antenna is a piece of metal shaped so that passing radio waves push electrons in it back and forth. That tiny jiggle of electrons is an electric signal the receiver can measure.

The receiver's job has three parts. First, find each satellite's code in the noise (acquisition). Then keep locked on as the car and satellite move (tracking). Finally, read the slow stream of data hidden in the signal and measure exactly when each part of the code arrived.

That data stream is very slow, only 50 bits per second, about one short text message every few seconds. It tells the receiver where the satellite is, how its clock is doing and the time it sent each message.

If you could see GPS radio waves, the sky would be lit up all the time, yet the light reaching each square metre would still be far too faint for your eyes.

Step 3 of 8 · On the highway

A message from the sky

The chip has found the whisper. But what's actually in it? Follow the beams back up the sky.

Each signal says 'this is satellite X, and I sent this at exactly this time'. How late it arrives tells you how far away the satellite is.

Time stamp. Every signal carries a time stamp: the exact moment the satellite sent it, read off an atomic clock on board, plus where the satellite was right then. So when the message reaches your car, it says: 'I'm here, and I sent this at exactly 10:15:03.000000000.' Those billionths matter. Like a postcard that says where it was posted and the exact second it was sent.
The 'where' part is called the ephemeris: a precise description of the satellite's orbit, good for a few hours, uploaded by ground control. Using it, the receiver works out the satellite's position to within about a metre at the moment of sending.
The time stamp isn't written as a number in every wave. The receiver works it out from where it is in the repeating code, plus a time marker sent every 6 seconds in the slow data stream. Together they pin the sending moment to within a few billionths of a second.
Travel time. Your receiver notes when the signal arrived and subtracts the time it was sent. From a satellite straight overhead, the trip takes about 0.07 seconds. Radio waves travel at the speed of light, 300,000 km per second, so travel time × speed gives the distance to the satellite: about 20,200 km. Like counting seconds between lightning and thunder to tell how far away the storm is.
Light travels 300 metres in one millionth of a second, and 30 cm in one billionth. So if the receiver's timing is off by just one millionth of a second, its distance is off by 300 m. That's why everything in GPS is about precise clocks.
A satellite near the horizon is further away, about 25,000 km, so its signal takes around 0.085 s. The receiver doesn't care: it just turns each travel time into a distance.
The receiver's clock isn't perfect, so these first distances are a bit wrong, by the same amount for every satellite. GPS people call them 'pseudoranges' (pretend distances). We'll see how that error gets fixed when we climb up into the sky.
Tiny errors. On the way down, the signal slows slightly as it passes through the ionosphere, a layer of charged gas high above us, and the damp air lower down. It can also bounce off buildings before reaching the antenna. Each delay makes a satellite seem a few metres further away than it really is. Like a runner slowed by mud on part of the track: same route, later arrival.
The ionosphere is the biggest error. Sunlight knocks electrons off air molecules 50–1,000 km up, and those loose electrons slow radio waves. The slow-down changes with the time of day and solar storms, and can add several metres of error.
Newer satellites broadcast on two frequencies (L1 and L5). The ionosphere slows lower frequencies more, so a receiver that hears both can compare them and cancel most of the delay. Many new phones do this.
Bounced signals (called multipath) are harder. They arrive late and can make a receiver think it's on the other side of the street. Antenna design, the twist of the waves, and software that spots odd results all help to reject them.

Trip time: ≈0.07 s
1 µs of error: 300 m

Did you know? A satellite straight overhead is about 20,200 km away, so its signal takes about 0.07 seconds to reach you. That's quicker than a blink.

Everything in GPS comes down to one equation you already know: distance = speed × time. The speed is the speed of light (about 300,000 km/s). The time is how long the signal took. Multiply them and you get how far away that satellite is.

Because light is so fast, the times are tiny and must be measured incredibly precisely. One billionth of a second (a nanosecond) of timing error is 30 cm of distance error. To get your position right to a few metres, the timing has to be good to about 10 nanoseconds.

The signal is made of a repeating code of 1,023 'chips' sent every millisecond. Each chip lasts about a microsecond, so it's about 300 m 'long' as it flies. The receiver lines up its copy of the code with the incoming one to a small fraction of a chip, which is how it gets timing down to a few nanoseconds.

Mixed into the code is a slow data message: the time, the satellite's orbit (ephemeris), how its clock is drifting, and a rough almanac of where all the other satellites are. A full almanac takes 12.5 minutes to download at 50 bits per second, one reason phones fetch it over the internet instead.

If GPS used sound instead of radio, a signal from the satellite would take about 16 hours to reach you, if sound could travel through space at all.

Step 4 of 8 · Talking to the sky

Four distances, one spot

So each signal gives a distance to one satellite. Let's pull way back, above the whole planet, to see how a few distances become one spot.

Each distance puts you somewhere on a giant sphere around a satellite. Where the spheres meet is where you are.

One distance. Knowing you're 20,200 km from one satellite doesn't tell you where you are. You could be anywhere on a giant imaginary sphere around it, all points that are exactly that far away. Only a small patch of that sphere touches the Earth, but that patch is still thousands of kilometres wide. Like knowing you're 5 km from home: you're somewhere on a circle around it.
Mathematicians call finding a position from distances 'trilateration'. It's different from triangulation, which uses angles. GPS never measures angles, only distances.
The sphere is drawn here as a glowing shell. Where it cuts through the Earth's surface it makes a big circle, and your car sits somewhere on that circle.
Three spheres. Add a second satellite and its sphere crosses the first in a circle. A third sphere cuts that circle at just two points. One of them is out in space or deep inside the Earth, so the receiver throws it away. The other is you. Three distances, one answer, at least in theory. Like finding a friend who says they're 2 km from school, 3 km from the park and 1 km from the shop.
You can try it on a map with three circles drawn with a compass. Two circles cross at two points; the third circle passes through only one of them.
The further apart the satellites are in the sky, the more cleanly the spheres cross. If they're all bunched in one direction, the spheres meet at a shallow angle and a small error in distance becomes a big error in position. Receivers pick a well-spread set when they can.
Fourth satellite. There's a catch: your phone's clock is a cheap quartz one, not an atomic clock, so every distance is off by the same amount. The fourth satellite fixes this. The receiver solves for four unknowns, east, north, height and its own clock error, and only one answer makes all four spheres meet. Like a watch that's 2 minutes off: one more clue and you can work out exactly how wrong it is.
If the phone's clock is a millionth of a second fast, every distance is 300 m too short. The spheres then don't meet at a single point. The receiver nudges its clock guess until they do, and that nudge is exactly its clock error.
A nice side effect: once it's solved, your phone knows the time to within a millionth of a second or better, straight from the atomic clocks in orbit. That's why phone clocks are always right.
With more than four satellites in view (usually 8–12 from GPS alone), the receiver uses them all and averages out errors. If only three are visible, it can assume you're on the ground and solve for a 2D position instead.

Unknowns: x, y, z, time
Satellites needed: 4

Did you know? Your phone doesn't just get a location from GPS: it also gets the time, accurate to around a millionth of a second, beamed down from atomic clocks in space.

A sphere is the 3D version of a circle: every point that is the same distance from a centre. If a satellite is 20,200 km away, you're somewhere on a sphere of radius 20,200 km around it.

In flat 2D maps, two circles meet in at most two points and a third picks one. In 3D, two spheres meet in a circle, a third sphere cuts that circle at two points, and Earth itself rules out one of them.

In reality the receiver's own clock is wrong, which shifts every distance by the same amount. So there are four unknowns: three for position, one for the clock. Four unknowns need four measurements, hence four satellites.

The receiver solves it by guessing a position and a clock error, checking how well they fit the measured distances, then improving the guess, a few times over, in a fraction of a millisecond. It repeats that many times a second as you drive.

With many satellites from several systems, phones can now spot and reject a single bad signal on the fly, by checking which one disagrees with the rest.

Step 5 of 8 · The satellites

Four satellites at once, anywhere on Earth, all the time. How many satellites does that take, and who keeps them in line?

About 31 GPS satellites circle the Earth twice a day on six tilted rings, arranged so at least four are always above any spot on Earth.

Six rings. The satellites fly on six circular paths, called orbits, each tilted 55° to the equator and turned evenly around the planet. They're about 20,200 km up, nearly 50 times higher than the Space Station, and each one circles the Earth in just under 12 hours, at nearly 14,000 km/h. Like six hula hoops spun around a ball at different angles, with beads sliding along each.
An orbit is a never-ending fall. The satellite moves sideways so fast that, as gravity pulls it down, the Earth's curved surface falls away beneath it at the same rate. At this height, the speed that matches is about 3.9 km per second.
The height was chosen so each orbit takes half a day (11 h 58 min). Every satellite then passes over the same spots at almost the same times every day, which made the system easier to plan and check.
There are 24 'slots' needed for full coverage, and the extra satellites are spread among them as spares. When one fails, the others already fill the gap while a replacement is launched.
Always 4 in view. The rings are spread so that from almost anywhere on Earth, day or night, at least four satellites are above the horizon, and usually eight or more. That's what the earlier step needed. Because they're so high, each satellite can see nearly half the Earth at once, so a few dozen are enough for everyone. Like streetlights spaced so there's never a dark patch on the road.
Lower satellites would be cheaper to launch, but each would see a much smaller patch of Earth and race overhead in minutes. You'd need hundreds of them. High orbits trade a weaker signal for a lot fewer satellites.
The satellites rise and set like stars, so the four you use slowly changes as you drive. Receivers switch smoothly, always picking a set spread around the sky for the best answer.
Ground control. On the ground, a network of stations listens to every satellite all day. A control centre in Colorado works out how each orbit and clock has drifted, then big dishes send corrections back up. The satellites broadcast those updates to you, so their positions stay right to about a metre. Like a coach on the sidelines, shouting small corrections to players who can't see the whole field.
The Master Control Station sits at Schriever Space Force Base in Colorado, with a backup in California. Monitor stations around the world track each satellite's signal, and ground antennas send it new orbit and clock data, usually at least once a day.
Orbits drift because the Earth isn't a perfect ball, the Sun and Moon tug on the satellites, and even sunlight pushes on their solar wings. Ground control models all of that, and occasionally fires a satellite's small thrusters to nudge it back into its slot.
If a satellite misbehaves, ground control can mark it unhealthy within minutes, so receivers stop using it before it can send anyone the wrong way.

Height: 20,200 km
One orbit: ≈12 hours

Did you know? Each GPS satellite goes around the Earth twice a day, so it rises about 4 minutes earlier every day, just like a star.

The GPS system has three parts. The space segment is the satellites. The control segment is the ground network that tracks and updates them. The user segment is every receiver on Earth.

The space segment has about 31 working satellites, more than the 24 needed, so the coverage stays strong even when a few are being serviced or fail. They are spread over six orbit planes, about four or five per ring.

The control segment works in a loop: monitor stations record each satellite's signal; the Master Control Station compares it with where the satellite should be and how its clock should be ticking; then ground antennas upload fresh orbit and clock data, which the satellite broadcasts to everyone.

New satellites are launched every year or two to replace old ones, which typically last 12–15 years. The newest generation, GPS III, sends a stronger civilian signal and a new one (L1C) that matches Europe's Galileo, so receivers can mix the two systems easily.

GPS satellites are so high that, from their point of view, the whole Earth is only about 28° across, about three fists side by side at arm's length.

Step 6 of 8 · The satellites

A clock with wings

Let's fly up to the satellite straight above your car and look at it up close. What's inside the box that makes all this work?

A GPS satellite is a van-sized box covered in gold foil, with solar wings for power, antennas pointing at Earth and atomic clocks inside.

Solar wings. Two long wings of solar panels turn sunlight into electricity, a few kilowatts, enough to run a home. Motors keep turning them to face the Sun as the satellite orbits. When Earth blocks the Sun, batteries take over. The gold foil on the body keeps the inside at a steady temperature. Like a solar-powered garden light that always turns its panel toward the Sun.
Space has no air to carry heat away, so the sunny side of the satellite would bake while the shady side froze. Shiny multi-layer foil reflects sunlight, and white radiator panels throw spare heat away as infrared light.
Steady temperature matters for timing. Clocks and electronics change their behaviour slightly as they warm up, so keeping them at an even temperature keeps the signal stable.
A GPS III satellite weighs about 3,900 kg at launch and is designed to work for 15 years. When its panels slowly wear out and its fuel runs low, it's moved to a higher 'graveyard' orbit, out of the way.
Antennas face Earth. The bottom of the satellite always points at Earth. It carries a ring of 12 spiral antennas that beam the navigation signal down in a wide cone, just wide enough to cover the whole planet edge to edge. Each spiral twists the radio wave in one direction, which helps receivers reject bounced signals. Like a shower head aimed at the ground, spraying signal evenly over the whole Earth.
The satellite uses small spinning wheels (reaction wheels) to stay pointed at the Earth's centre. Spin a wheel one way and the satellite turns gently the other way, with no fuel used.
Working together, the 12 spirals shape the beam so the signal is about equally strong at the edge of the Earth, which is further away, as directly below. That keeps receivers near the horizon from losing it.
Other antennas on board talk to ground control and, on newer satellites, to other satellites. They also carry sensors that detect nuclear explosions, a second job from GPS's military beginnings.
The clocks. Inside the box sit the real heart of GPS: atomic clocks. A GPS III satellite carries three, so if one fails another takes over. They're so steady that they drift by only a few billionths of a second a day, and ground control measures even that and tells receivers how much to correct. Like a spare tyre, but two of them, because a wrong clock means a wrong map.
Every part of the satellite exists to serve the clocks: the solar wings power them, the foil keeps them at a steady temperature, and the antennas broadcast their ticks.
Older GPS IIF satellites carried two rubidium clocks and one caesium clock. The newest GPS III satellites carry three improved rubidium clocks. Only one runs the signal at a time; the others wait as backups.
If a clock starts to drift oddly, ground control can switch to a backup. A clock 1 microsecond off would put every receiver using that satellite 300 m out, so it is watched constantly.

Mass: ≈3,900 kg
Atomic clocks: 3

Did you know? The very first GPS satellite was launched in 1978. The full system of 24 satellites wasn't complete until 1995.

A satellite is really a small, very reliable spacecraft that can't be repaired. Everything important is doubled or tripled, so that if one part fails, another can take over.

Its main jobs are to keep perfect time, to know where it is (from the data ground control uploads), and to broadcast both continuously to the whole planet, every second of every day for 15 years.

The signal is built directly from the atomic clock's ticks. The clock's steady 10.23 MHz beat is multiplied up to make the 1575.42 MHz radio wave and divided down to make the code. So the radio wave itself is a ticking clock, which is what lets receivers time it so precisely.

Power comes from the solar wings and batteries, attitude (which way it points) from reaction wheels and Earth sensors, and small thrusters adjust its orbit when ground control asks.

The 1575.42 MHz signal is exactly 154 times the clock's 10.23 MHz beat. The whole radio wave is a perfectly counted multiple of the atomic clock.

Step 7 of 8 · Inside the clock

Atoms keep the beat

Everything depends on those clocks. Let's slip inside one rubidium clock and see how atoms keep time.

A quartz crystal does the ticking, and a cloud of rubidium atoms checks it constantly, nudging it back the moment it drifts.

Quartz ticks. Every clock needs something that swings back and forth at a steady rate. Here it's a tiny quartz crystal on the circuit board: squeeze quartz and it makes a voltage, so it can be made to vibrate millions of times a second. It's a great ticker, but heat and age slowly change its rate. Like a pendulum swinging very fast, but one that slowly loses its rhythm on its own.
Quartz is the same crystal used in ordinary wristwatches. In a watch it vibrates 32,768 times a second; here it's cut to vibrate at millions of times a second. Quartz watches drift by about half a second a day. At that rate a GPS distance would be 300 m off within a fifth of a second.
So quartz alone isn't good enough. The trick of an atomic clock is not to replace the quartz, but to keep correcting it against something that never changes: an atom.
Rubidium atoms. In this glass bulb floats a gas of rubidium atoms. Every rubidium atom has a tiny inner flip it can make, and it only accepts microwaves of one exact frequency to make it: 6,834,682,610 wiggles a second. Every rubidium atom in the universe has the same flip, so it's a perfect, unchanging reference. Like a tuning fork that only rings at one exact note, and every copy rings the same.
Electrons in an atom spin like tiny magnets, and so does the atom's centre (the nucleus). They can line up two ways, with slightly different energies. Flipping between them needs exactly the right amount of energy, which means exactly the right microwave frequency.
The lamp on the left shines rubidium's own light (deep red, almost infrared; drawn violet here) through the bulb. That light 'pumps' the atoms into one of the two states, after which they stop absorbing it and the bulb becomes see-through.
When the microwaves in the copper box hit exactly 6.834… GHz, atoms flip back, start absorbing the light again, and the beam reaching the detector dims slightly. That dip is the signal that the frequency is spot on.
Feedback loop. Here's the loop: the quartz drives the microwaves. The light detector watches the beam. If the beam stops dimming, the microwaves have drifted off the atoms' frequency, so the electronics nudge the quartz back. This happens constantly, so the quartz ticks at the rate the atoms set, and the clock drifts only billionths of a second a day. Like steering a car: you keep making tiny corrections to stay in your lane.
The electronics wobble the microwave frequency slightly up and down on purpose. If the light dips more on one side, they know which way the quartz has drifted and push it back. Engineers call this a servo, or a feedback loop.
Caesium clocks, which define the official second, work in a similar way using caesium atoms at 9,192,631,770 Hz. Rubidium clocks are smaller, lighter and use less power, which is why they're favourites in satellites.
Even so, every atomic clock drifts a little. Ground control compares each satellite's clock with the ground clocks every day and broadcasts the tiny corrections, so receivers can remove even that.

Rubidium flip: ≈6.834 GHz
Drift: ns per day

Did you know? The official second is defined by atoms: it's exactly 9,192,631,770 flips of a caesium atom. Your watch, your phone and GPS all trace back to that number.

Every clock is two things: something that repeats at a steady rate, and a counter. A grandfather clock counts pendulum swings; a watch counts quartz vibrations. The better the 'repeater', the better the clock.

Atoms are ideal repeaters because every atom of an element is identical. A rubidium atom here behaves exactly like one in a lab on the other side of the world. Its inner flip happens at a frequency set by nature, which never wears out or warms up.

But atoms don't tick by themselves in a way you can count directly. So an atomic clock uses a normal quartz oscillator as the ticker, and the atoms as a referee that constantly tells the quartz whether it's fast or slow.

The referee works with light and microwaves: light pumps the atoms into one state, microwaves at the right frequency flip them back, and a photodetector notices the extra light absorbed. Locking the quartz to the strongest dip locks it to the atoms.

The best lab atomic clocks today would lose less than one second over the whole age of the universe.

Step 8 of 8 · Inside the clock

Einstein in your pocket

So the clocks are nearly perfect. But up in orbit, Einstein says time itself runs at a different speed. Let's see by how much.

Up in orbit, time runs a little faster than on the ground. GPS has to correct for Einstein's relativity, or your map would drift by kilometres a day.

Weaker gravity. Einstein discovered that gravity slows time: the stronger the gravity, the slower clocks tick. Picture gravity as a dent in space, deepest near the Earth. The satellite is far up, where the dent is shallow and gravity is weaker, so its clock runs fast, by about 45 millionths of a second a day. Like walking on a slope: the deeper you are in the dent, the slower time runs for you.
This is called gravitational time dilation, predicted by Einstein's general relativity in 1915. It's tiny here on Earth, but it's real and has been measured many times, even between clocks a few dozen centimetres apart in height.
From 20,200 km up, the effect adds up to +45.7 microseconds a day compared with clocks on the ground. On its own, that would shift positions by more than 13 km a day.
Speed. Einstein's other discovery pulls the opposite way: clocks that move fast tick slower. The satellite races along at nearly 14,000 km/h, so this effect slows its clock by about 7 millionths of a second a day. It's smaller than the gravity effect, so overall the satellite clock still runs fast. Like two runners in opposite directions: gravity's one is just faster, so it wins.
This is special relativity's time dilation, from 1905. It only gets big near the speed of light, but GPS timing is so precise that even 3.9 km/s shows up: about −7.1 microseconds a day.
Add the two: +45.7 − 7.1 ≈ +38.6 microseconds a day. So a GPS satellite clock, left alone, gains about 38 millionths of a second every day compared with one on the ground.
The fix. 38 millionths of a second sounds like nothing, but light covers about 11 km in that time. Ignored, every GPS position would drift by roughly 10 km a day. So before launch, engineers set each satellite clock slightly slow, so that once in orbit it ticks in step with clocks on the ground. Like setting your watch a little slow before a trip, because you know it runs fast there.
The clock beat should be 10.23 MHz. Instead it's set to 10.22999999543 MHz on the ground. In orbit, relativity speeds it up to exactly 10.23 MHz as seen from Earth.
There's a smaller extra effect because orbits aren't perfect circles: the satellite speeds up and dips slightly every orbit. Receivers correct that with a simple formula, worth up to about 45 billionths of a second.
GPS is one of the few everyday technologies that would visibly fail without Einstein's theories. Every time your blue dot is in the right place, relativity is being tested and passing.

Clock gain: +38 µs/day
Uncorrected drift: ≈10 km/day

Did you know? In 1977 a GPS test satellite flew its atomic clock uncorrected for 20 days, because some engineers doubted relativity would matter. It ran fast by almost exactly the amount Einstein predicted, and the correction was switched on.

We usually think time is the same for everyone. Einstein showed it isn't: time passes at different rates depending on how fast you move and how strong the gravity around you is. The effect is tiny in daily life, but atomic clocks can measure it.

For GPS, two effects compete. Weaker gravity high up makes the satellite clock run fast (+45.7 µs a day). Its high speed makes it run slow (−7.1 µs a day). Together it gains about +38.6 µs a day.

Since 1 µs of timing error means 300 m of distance error, 38 µs a day adds up to more than 10 km of error every day if nobody fixed it. The fix is simple: set the clocks slightly slow before launch, so they run at the right rate once in orbit.

All of this is done in the background. Your phone never notices, but every position it calculates depends on two of Einstein's theories being right, and on engineers who took them seriously.

Clocks on a mountain run faster than clocks at sea level. Over a lifetime, someone living 1 km up ages about a third of a millisecond more than someone at sea level.

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