Your phone times radio signals from several satellites, turns each travel time into a distance, and works out the one point that fits all those distances.
GPS satellites do not know where you are. They only shout, again and again, 'this is my position and this is the exact time'. Your phone listens. It never transmits anything back. All the work happens inside your phone.
The US Space Force runs GPS, and the civil signal is free for anyone in the world to use.
Radio signals travel at the speed of light, about 300,000 km per second. If a signal left a satellite at a known time and reached your phone 0.07 seconds later, the satellite must be about 21,000 km away. That is roughly the right number, because GPS satellites fly about 20,200 km up.
One distance tells you only that you are somewhere on a huge sphere around that satellite. A second satellite narrows it to a circle where two spheres meet. A third leaves just two points, and one of them is far out in space. In practice your phone also needs a fourth satellite to correct its own clock, as explained in Why four satellites?.
GPS is built on 24 slots in six orbital planes, with four slots per plane. Each satellite goes round Earth twice a day. The layout is chosen so that at least four satellites are visible from virtually anywhere on the planet. In July 2023 GPS.gov listed 31 operational satellites, more than the 24 baseline slots.
India has its own regional system, NavIC, and many phones combine GPS, NavIC and other systems at once.
GPS.gov says GPS-enabled smartphones are typically accurate to within about 4.9 m under open sky. Accuracy gets worse near tall buildings, bridges and trees, which block or reflect the signals. That is why your blue dot wanders in a narrow city lane.
Note the satellites' altitudes: navigation satellites fly tens of thousands of kilometres up, far above the ISS. See navigation satellites over India
The measured quantity is the pseudorange. ESA's Navipedia writes it for satellite j as:
R_j = ρ_j + c × (dt_r − dt_j) + T_j + I_j + (small terms)
where ρ_j is the true geometric distance, c is the speed of light, dt_r is the receiver clock error, dt_j the satellite clock error (broadcast in the navigation message), and T_j and I_j are the delays caused by the troposphere and the ionosphere.
The true distance links to your unknown position (x, y, z):
ρ_j = √((x_j − x)² + (y_j − y)² + (z_j − z)²)
This is not linear, so the receiver starts from a guess (say, the centre of the Earth or its last fix), linearises the equations around it, solves, and repeats. With more than four satellites the system is over-determined and is solved by least squares, which also averages out noise.
Why does timing matter so much? Multiply any clock error by c. An error of 1 microsecond becomes 299,792,458 × 10⁻⁶ ≈ 300 m. An error of 10 nanoseconds is still 3 m. So everything depends on very precise clocks in the satellites and on solving for the receiver's own clock error.
A quick check on the travel time: a satellite straight overhead is about 20,200 km away, so the signal takes 20,200 / 299,792 ≈ 0.067 s to arrive. A satellite low on the horizon is farther away, so its signal takes longer.
No. GPS is one-way: satellites broadcast their position and time, and the receiver computes its own position.
Distance = speed × time = 300,000 km/s × 0.07 s ≈ 21,000 km.
Buildings block some satellites and reflect signals from others, so the receiver gets fewer and less reliable distance measurements.