How GPS actually works
Nothing in orbit knows where you are. Nothing you carry sends anything up. Your phone works out its position on Earth to within a few metres by listening to clocks — and by noticing, very precisely, how late each one is running.
It is a clock, not a map
The most common belief about GPS is that satellites track you. They do not. They have no receivers pointed at the ground, no record of anyone, and no idea that you exist. Each one does exactly one thing, continuously, to nobody in particular.
Position is worked out at your end
Your phone hears several of these announcements at once. Each one tells it where that satellite was and what time it was when the message left. By comparing that to the time the message arrived, the phone learns how long the signal spent travelling — and since radio travels at a known speed, that time is a distance.
Do this for four satellites and there is exactly one place in the universe you could be standing. All of the work happens in the receiver. The system is, in the most literal sense, a set of very accurate clocks that announce themselves.
Which is why it is private, and why it is not
A GPS receiver is passive. Using it tells nobody anything, which is genuinely unusual among modern technologies — a handheld unit on a mountain is invisible to everyone.
What is not private is what happens next. Your phone works out where it is and then hands that answer to whatever asked for it: the map app, the weather app, and often a dozen others. The tracking everyone worries about is real, but it is happening in software on the device, not in space.
The comparison worth holding on to
| Mobile network | GPS | |
|---|---|---|
| Direction | Two-way conversation | One-way broadcast |
| Who knows where you are | The network does | Only your own device |
| Capacity | Shared, and it runs out | Unlimited, by construction |
| Works with no subscription | Emergency calls only | Entirely |
The takeaway GPS does not find you. It publishes the information you need to find yourself, and then takes no further interest.
Distance becomes position
Knowing how far away something is does not tell you where you are. Knowing how far away three things are, and where each of them is, tells you exactly. This is the whole idea, and it is older than radio.
Measuring the distance
The satellite stamps every message with the exact moment it was sent. Your receiver notes the moment it arrived. The difference is the travel time — typically about 67 milliseconds from 20,000 km up — and multiplying that by the speed of light gives the distance.
This is why the accuracy of the whole system rests entirely on timekeeping. Light covers 30 centimetres in a nanosecond. Be out by a millionth of a second and you are out by 300 metres. Everything difficult about GPS follows from that one number.
Where the satellite says it is
A distance is useless without knowing the point it was measured from, so each satellite also broadcasts its own orbit — a precise mathematical description of where it will be, moment by moment, for the next few hours. This is called the ephemeris, it is updated from the ground regularly, and it is accurate to a metre or two.
Your receiver therefore has both halves of the problem: a set of known points in the sky, and its distance from each. What remains is arithmetic — four equations, solved several times a second.
The takeaway Three distances would be enough if your receiver had a perfect clock. It does not, and that single inconvenience shapes the rest of this guide.
The fourth satellite
To measure how long a signal took, you need to know what time it arrived — and the clock in your phone is a cheap quartz crystal that would be useless for this. The way round that problem is the cleverest idea in the whole system.
Guessing, and checking
Your receiver does not know its clock error, so it assumes one and sees what happens. If the assumption is wrong, the four distances describe a set of spheres that do not meet at any single point — they miss, all in the same direction and by the same amount, because one clock error affects every measurement identically.
That consistency is the gift. The receiver adjusts its assumed time, the spheres move towards each other, and it repeats until they agree. When they finally meet at one point, the receiver has learned two things at once: where it is, and precisely how wrong its own clock was.
The by-product that runs the world
Having corrected its clock against four atomic standards in orbit, every GPS receiver on Earth now holds the time to within tens of nanoseconds. For free. Anywhere outdoors.
That turns out to be at least as valuable as the navigation. Mobile networks use it to keep base stations in step — remember the scheduler that hands out half-millisecond slots. Electricity grids use it to keep generators in phase. Financial exchanges use it to timestamp trades in an agreed order. Broadcasters use it to keep transmitters synchronised. Large parts of modern infrastructure quietly depend on GPS and have nothing to do with knowing where anything is.
Three satellites will do, sometimes
If a receiver can eliminate one unknown by other means, three is enough. A ship knows it is at sea level, so altitude is not really unknown, and marine receivers exploit that. A device that is bolted down and has already learned the time can hold on to both and coast for a while. But for a phone in your hand, outdoors, on a hill, it is four.
The takeaway GPS does not require you to own an accurate clock. It requires you to own a consistent one for a fraction of a second — and then it gives you an accurate one as change.
Up there
Thirty-one satellites, in six tilted rings, twice as high as the space station is low and half as high as television satellites. The orbit was chosen so that from anywhere on Earth, at any moment, several of them are spread across your sky.
Why not geostationary
Television satellites hang still over the equator, which is ideal when you want a dish that never moves. It is the wrong answer here for two reasons. Geometry: satellites all in one direction give a poor fix, and from the UK the equator is well to the south — you want them spread right across the sky. And height: geostationary is further away, so the signal is weaker and the geometry worse still.
An orbit of just under twelve hours was chosen instead, tilted at 55 degrees so the ground tracks cover high latitudes properly. Each satellite passes over the same place twice a day, about four minutes earlier each time.
The clocks, and the people who watch them
Each satellite carries several atomic clocks — rubidium and caesium standards — of which one is in use at a time. They are extraordinarily stable and still not good enough to be left alone. A network of monitoring stations around the world tracks every satellite continuously, measures exactly how far each clock has drifted and where each satellite really is, and uploads corrections. Those corrections are then broadcast as part of the message, so your receiver can adjust for errors it has no way of detecting itself.
It was deliberately made worse for twenty-five years
Until May 2000, civilian GPS signals carried an intentional error called Selective Availability, which degraded position to around 100 metres. Military receivers had the key to remove it. Switching it off — a policy decision, made at the stroke of a pen — improved everyone's accuracy tenfold overnight and is the reason consumer satellite navigation became viable at all. Almost every use of GPS you can think of is younger than that decision.
The takeaway The constellation is a piece of public infrastructure paid for by one country's military and used, free, by everyone else's phones. There is no equivalent arrangement anywhere else in technology.
A signal below the noise
By the time a GPS signal reaches the ground it is weaker than the background hiss of the universe. Point an antenna at the sky and look at what arrives and there is nothing there — no signal, just noise. And yet it is read perfectly, by a chip that costs pennies.
All of them, on one frequency
Every GPS satellite transmits on exactly the same frequency — 1575.42 MHz — at exactly the same time. They do not interfere because each has its own pattern, and the patterns are chosen so that any two of them, compared against each other, produce nothing. Only the correct pattern produces a peak.
So the same search that measures the travel time also identifies which satellite you are listening to. A receiver looking for satellite 14 simply does not see the others at all.
Fifty bits a second
Underneath that pattern, the actual data crawls along at fifty bits per second — slower than a 1970s modem, and about eight hundred thousand times slower than home broadband. It is enough, because there is very little to say: its own precise orbit — the ephemeris from step 02 — what its clock is doing, and a rough directory of every other satellite, called the almanac.
But it sets a hard floor on how quickly a receiver can start from nothing. One satellite's ephemeris takes thirty seconds to arrive. The full almanac takes twelve and a half minutes. No amount of processing power makes that faster, which is the subject of the next step.
Why it stops at the door
A signal this faint has nothing to spare. A roof, a lorry, a dense tree canopy or a metallised window is enough to lose it — GPS effectively requires line of sight to the sky. That is why it works in a field and not in a car park, why it takes a moment to recover after a tunnel, and why indoor positioning has to be solved by completely different means.
The takeaway The received power is about a millionth of a billionth of a watt. Everything wonderful and everything fragile about GPS comes from that one number.
Getting a fix
Anyone who used a satellite navigation unit in 2005 remembers standing in a car park waiting. Your phone does the same job before you have finished unlocking it. Nothing about the satellites changed — what changed is that your phone cheats.
Two kinds of stored knowledge
The difference between those four bars is entirely about what the receiver already holds, and there are two separate things it can hold.
- The ephemeris is one satellite's precise orbit, and it is what the position calculation actually needs. It expires after about four hours, and there is a separate one for every satellite.
- The almanac is a rough directory of the whole constellation — good enough to work out which satellites should be above you and roughly where to look for them. It stays usable for weeks.
A hot start has both, so it is instant. A warm start has the almanac but a stale ephemeris, so it knows exactly where to look and still has to wait thirty seconds for the details. A cold start has neither, and must search blindly before it can even begin waiting.
The cheat, and why it is allowed
An ephemeris is a few hundred bytes. It is the same for everyone, it is published, and your phone has a perfectly good internet connection. So the phone downloads it — along with a rough idea of where it is, from the mobile network — and skips straight to measuring.
That is assisted GPS, and it is the single reason phones feel instant. It is also why a phone can be oddly slow to find itself abroad with data roaming turned off: deprived of its shortcut, it has to do it the old way, from space, at fifty bits a second.
What the receiver is actually doing while you wait
- Searching. For each satellite it might see, it tries every possible alignment of that satellite's pattern, and every plausible frequency shift caused by the satellite's motion. Modern chips run thousands of these searches in parallel, which is most of what a GPS chip is.
- Locking on. Once a peak is found it must be held, and tracked, as the satellite moves.
- Reading. Only now can it collect that satellite's ephemeris, at fifty bits a second.
- Solving. The arithmetic itself is trivial — microseconds. It is never the slow part.
Why a standalone receiver still makes you wait
A handheld unit on a hill, an older car satnav, a dashcam, a vehicle tracker or a marine plotter has no internet connection and no shortcut available. It can only get the ephemeris the original way: from space, at fifty bits per second, one satellite at a time. That is not a fault or an ageing chip — it is the same thirty-second wait that has always been there, and that a phone hides from you.
It is worse than that on the first power-on after a long gap. A receiver that has sat in a drawer for a few months has an expired almanac too, so it does not know which satellites to expect or where in the sky to look, and has to search the whole constellation from scratch. Move it several hundred miles while it was switched off — the drive home from the airport, a boat delivered on a lorry — and its stored position is wrong as well, so even the search pattern it starts from is useless. This is the genuine cold start, and it can take several minutes rather than thirty seconds.
Which is also why the old advice was sound: leave it switched on, stationary, with a clear view of the sky, and do not drive off while it is still thinking. It is collecting a very slow radio broadcast, and moving or losing sight of a satellite means starting that download again.
The Doppler problem nobody mentions
The satellites are moving at about 14,000 km/h, so their signals arrive shifted in frequency by several kilohertz, and by a different amount for each one depending on whether it is rising or setting. A receiver that does not know roughly where or when it is has to search that frequency range too, which multiplies the work. Being told your rough position first collapses the search — another reason assistance helps so much.
The takeaway A modern phone usually has a fix before you have looked at the screen, and it did that by asking the internet a question rather than by being better at radio.
Why it is a few metres out
A phone in the open is typically right to within three to five metres. Given that the measurement is a time-of-flight over 20,000 km, the interesting question is not why there is any error — it is which errors are left, and what they cost.
Geometry matters as much as measurement
How to do considerably better
| Method | Accuracy | What it needs |
|---|---|---|
| A phone, open sky | 3–5 m | Nothing |
| Two frequencies at once | 1–3 m | A modern receiver. The ionosphere bends two frequencies differently, so listening on both cancels most of it. |
| Satellite corrections (EGNOS in Europe) | 1–2 m | Nothing — the corrections are broadcast free, and most receivers already use them. |
| A nearby reference station | 1–2 cm | A second receiver on a known point, and a link to it. This is what surveyors and agricultural machinery use. |
The centimetre-level trick is worth understanding: two receivers close together experience almost exactly the same ionosphere, the same satellite errors and the same atmosphere. One of them sits on a point whose position is known precisely, so it can work out exactly how wrong it is being, right now — and simply tell the other one.
The blue circle on the map
That circle is not decoration. It is the receiver's own estimate of how uncertain it currently is, based on the geometry, the signal strengths and how much the sources disagree. When it swells as you walk between tall buildings, the phone is telling you something true and useful: it no longer trusts itself.
The takeaway Most remaining GPS error is not in the satellites or the receiver. It is in eighty kilometres of electrically charged atmosphere that the signal has to cross on the way down.
It is not just GPS
GPS is one system, built by one country, and it stopped being the only one years ago. Your phone has almost certainly been using three or four constellations at once for its entire life, and quietly calling the result “GPS” because that is the word everyone knows.
Why more is so much better
In the open, eight satellites is plenty and a ninth adds very little. Between buildings it is a different story: most of the sky is hidden, the few satellites you can see are bunched into a strip overhead, and the geometry is dreadful. Having thirty candidates instead of eight means there are usually four well-placed ones among them, and that is the difference between a usable fix and a blue dot skating across the wrong road.
They all work the same way, deliberately
All four use the same principle, similar orbits and overlapping frequencies, and they were designed to interoperate. A receiver chip supporting all of them is barely more expensive than one supporting a single system, which is why support became universal almost as soon as the other constellations were finished.
There are differences worth knowing. Galileo was built as a civilian system under civilian control, and broadcasts a signal that is harder to spoof. GLONASS satellites sit in orbits tilted more steeply, which helps at high latitudes — useful in Russia, and incidentally in Scotland. BeiDou includes satellites in higher, stationary-ish orbits over Asia for extra coverage there.
The reason four exist
It is not redundancy for its own sake. A navigation system that another government can degrade or switch off in your region is a strategic dependency, and Selective Availability in step 04 demonstrated that the ability is real. Galileo, GLONASS and BeiDou exist because their sponsors decided that relying on the United States Air Force for the time and for knowing where things are was not acceptable.
The pleasant side effect is that everyone's phone now gets four times the satellites, free.
The takeaway When your phone shows a location it is usually a blend of four independent constellations, plus corrections broadcast by a fifth system, plus Wi-Fi and motion sensors. “GPS” is doing a lot of work as a word.
When it goes wrong
GPS fails in a small number of very characteristic ways, and one of them is by far the most common: the signal arrives, but not by the route it claims to have taken.
Symptom to cause
| What you see | Almost certainly | Step |
|---|---|---|
| The dot drifts into buildings, or jumps streets | Reflections. Most of the sky is blocked and what does arrive has bounced off glass and stone. | 09 |
| Nothing at all indoors, in a tunnel, or a multi-storey | Expected. The signal is too weak to pass through a building. What you see afterwards is the phone estimating from its motion sensors. | 05 |
| Very slow to find itself after a flight | A cold start. Its stored orbit data is stale and its idea of where it is is a continent out. | 06 |
| Position is roughly right but the altitude is nonsense | Normal. Height is always the weakest axis, because there are no satellites below you to balance the ones above. | 02 |
| Accurate on foot, poor in one particular car | The windscreen. Heated and metallised glass is an effective radio shield, and some vehicles are far worse than others. | 05 |
| Fast but vague, before settling down | That first fix was not GPS. Phones estimate from nearby Wi-Fi networks and masts instantly, then refine it when the satellites arrive. | 06 |
| Everything in one area, for everyone, at once | Interference. Rare, but real — see below. | 09 |
The fix that is not a fix
When the satellites are unavailable, your phone does not stop. It carries on estimating from its accelerometer, gyroscope, compass and barometer, and from what it knows about the road you were on. A car navigation system does the same with wheel-speed sensors. This is why the arrow keeps moving sensibly through a tunnel and then snaps sideways as you emerge — one method handed over to the other and they disagreed.
Jamming and lying
A signal this weak is trivially easy to drown out. Small illegal jammers, sold to defeat vehicle tracking, will deny GPS to everything within a few hundred metres — which has caused real, documented disruption when one has been driven past an airport.
More troubling is spoofing: transmitting counterfeit signals so that receivers confidently compute a position that is completely wrong. It has become common near conflict zones, where aircraft and ships have reported being placed tens of kilometres from their actual location, and it is far harder to detect than jamming because nothing appears to be broken. The newer constellations broadcast authenticated signals specifically to address this, and the older ones cannot.
It is also why serious infrastructure no longer treats GPS as an unquestionable truth. The timing networks in step 03 increasingly keep their own atomic clocks running alongside, so that if the sky goes quiet — or starts telling lies — everything keeps working.
The takeaway Nine steps, 20,000 km, atomic clocks, relativity corrections and a signal quieter than noise — and the thing that will actually ruin your afternoon is a shop window on the other side of the street.