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The blue dot finds you in about two seconds, anywhere on earth. No satellite up there has ever detected your phone, received anything from it, or knows it exists — and it still works.
Open a map. A blue dot appears, and it is you, to within a few metres — in a forest, in a desert, in the middle of an ocean.
The usual explanation is that a satellite spotted you. Everyone says tracked by GPS, and the picture that comes with it is something up there looking down and finding your phone.
That is not what happens. It is not even close, and the truth is stranger and rather more reassuring.
A GPS satellite has no receiver aimed at the ground. It cannot detect your phone. It cannot count how many people are using it. Your phone never sends it anything, so there is nothing for it to detect in the first place.
All the radio goes one way: down. Every satellite transmits, continuously, to the whole hemisphere below it, whether one person is listening or a billion are. The system has never known who its users are, and it works exactly as well when nobody is switched on.
This is not a privacy feature bolted on afterwards. It is the only design that could possibly have worked. If every device had to talk back, the system would have to answer each one in turn, and it would have jammed solid decades ago. Broadcasting to everybody and answering nobody is what lets the entire planet use it at once, for free.
So if the satellite is not finding you, who is?
Your phone is. It does the whole job itself, out of nothing but a stopwatch and some arithmetic.
GPS is a broadcast, like a radio station. The satellites talk and never listen. Every bit of the finding happens inside the phone in your hand.
There are around 31 GPS satellites working at any time, each about 20,200 kilometres up, each going round the earth twice a day. And each one is broadcasting, over and over, a message that is almost boringly simple:
I am satellite number seven. Here is exactly where I am. And the time right now is exactly this.
That's it. No conversation, no personalisation. The same shout to everyone.
The magic is in that last part — the time — because these satellites carry atomic clocks, the most precise clocks humans build.
Your phone hears the message and compares the time inside it to the time on its own clock. The message took a while to get down here: radio travels at the speed of light, about 300,000 kilometres every second, so the trip from 20,200 km up takes about 67 thousandths of a second. Tiny — but measurable, and different for each satellite depending on how far away it is.
And a delay is a distance. If you know a message travelled for 67 thousandths of a second at the speed of light, you know it came from 20,200 kilometres away. That's the trick, and everything else is geometry.
It is worth sitting with how brutal the timing is. Light covers 300 metres in a millionth of a second. So if your phone's idea of the time is off by one millionth of a second, your blue dot lands 300 metres away — in the next street, or in the river.
A phone does not contain an atomic clock. The one in your pocket is a cheap quartz crystal, out by far more than a millionth of a second.
Hold on to that problem. It is the reason for the number four.
Your phone has just switched on and knows nothing. Press through as each satellite's shout arrives, and watch how much is left unknown after each one.
Three spheres already meet at a single point. So why does a phone need a fourth satellite before it will show you a dot?
It took about fifty years to get from a beeping metal sphere to a blue dot. Tap whichever you think happened next, and read what each one changed.
Tap whichever you think happened next
You walk across a field with your phone open on the map. What have the GPS satellites learned about you?
None of this means nobody knows where you are. Plenty of things do. It just means you were worried about the wrong part of the machine, and that matters, because worrying about the wrong part leaves the real one alone.
Once your phone has worked out its position, that answer is a piece of data sitting in your pocket — and whether it goes anywhere depends entirely on what your phone chooses to do with it. Every app you grant location access to can be handed it. Many send it onward. That is a decision made in software, on your device, with your permission, and it has nothing to do with any satellite.
Separately, your phone talks constantly to mobile masts, and it very much does transmit to those. The network can work out roughly where you are from which masts you are near, whether or not GPS is switched on at all, because that is a two-way conversation by design.
So the honest summary: the satellites are innocent, the network knows roughly, and the apps know exactly — but only the ones you let. The permissions screen on your phone is doing more real work than the entire constellation overhead.
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GPS satellites orbit at approximately 20,200 km altitude with a period of about 11 hours 58 minutes (two orbits per sidereal day). The constellation baseline is 24 satellites, with around 31 operational at a given time — GPS.gov. GPS is a one-way, transmit-only broadcast system: receivers do not transmit to the satellites, which is why it serves an unlimited number of simultaneous users and why the satellites cannot detect or count them — GPS.gov. Radio waves travel at the speed of light, about 299,792 km/s; the trip from 20,200 km therefore takes roughly 67 milliseconds, and a timing error of one microsecond corresponds to about 300 m of position error. A receiver solves for four unknowns — three position coordinates plus its own clock offset — which is why four satellites are required for a fix rather than three, and why a GPS receiver ends up as an extremely accurate clock. Timeline: Sputnik 1 launched 4 October 1957 and its Doppler-shifted radio signal led directly to the satellite-navigation concept; the US Navy's Transit system became operational in 1964; Navstar 1, the first GPS satellite, launched 22 February 1978; President Reagan announced civilian availability in September 1983 after the downing of Korean Air Lines Flight 007; Full Operational Capability was declared 17 July 1995; Selective Availability — the deliberate degradation of the civilian signal to roughly 100 m accuracy — was set to zero on 2 May 2000; the iPhone 3G (2008) brought a GPS receiver to a mainstream smartphone. Mobile phones do transmit to cellular base stations, which allows network-based location independent of GPS — a genuinely two-way system, unlike the satellite broadcast.