Your phone is
listening to clocks.
GPS and Galileo do not send a location to your phone. They broadcast precisely timed, coded radio signals. Your phone turns their arrival times into a position.
Start with
the time.
A navigation satellite carries atomic clocks and broadcasts a signal whose code is tied to system time. The receiver also needs the satellite's orbit data, so it can know where that clock was when it transmitted.
The important question is simple: what time did this particular pattern leave that particular satellite?
A delay becomes
a distance.
The radio signal travels at the speed of light. A receiver compares its expected code with the code it received; the offset tells it the signal's travel time.
At GPS's nominal altitude, a signal directly below a satellite takes about 67.38 milliseconds to arrive. Multiply time by the speed of light and you have a range measurement.
First, find
the whisper.
At the antenna, the satellite signal is extremely weak. The receiver filters and digitises it, then searches for the distinctive ranging code used by each satellite.
No code lock, no measurement. Before it can time a signal, the phone must identify which satellite's pattern it has found in the noise.
Slide until
it clicks.
The receiver generates a local copy of a satellite's code and slides it against the incoming signal. When the patterns align, their correlation rises sharply: that peak is the timing mark.
This is why a code is useful. It lets a tiny signal be recognised and timed even when it arrives below the background noise level.
It is called a
pseudorange.
The first range is not quite geometric distance. Your phone's clock is not synchronised with GNSS system time, and atmosphere and hardware add small delays. The measured value is therefore a pseudorange.
That apparent defect is solvable. The receiver includes its own clock error as one more unknown in the calculation.
Four clues
make a fix.
Repeat the measurement with several satellites. In the basic solution there are four unknowns: east–west position, north–south position, height, and the receiver clock bias. Four independent pseudoranges can solve for them.
More than four is better. Extra satellites give the receiver redundancy and usually improve the geometry of the estimate.
The world bends
the answer.
Signals are delayed by the ionosphere and troposphere. A building can reflect a signal along a longer path, while satellite orbit and clock estimates have their own uncertainty. The receiver models and rejects what it can.
Two frequencies help. Because the ionosphere affects radio frequencies differently, compatible dual-frequency receivers can estimate much of that delay.
Choose the point
that fits best.
The receiver combines satellite positions, pseudoranges and correction models into a best-fit estimate. It repeats this process continuously; motion sensors can help bridge the gaps between radio measurements.
The output is PVT: position, velocity and time. A map is only one way an application uses that answer.
Time is the
measuring tape.
For each satellite, the receiver starts with a measured travel time and turns it into a pseudorange. The practical equation contains more terms than the neat picture of intersecting circles, which is exactly why a receiver keeps estimating and correcting.
“Four satellites” is not a mystical rule. It is the number needed for four unknowns in the simplest 3D solution. If a receiver already has a reliable clock or a known height, it can work with fewer constraints; in ordinary use it prefers as many good signals as it can get.
- SatellitesTransmit timing codes and navigation data; they do not calculate a phone's position.
- ReceiverCorrelates codes, measures their offsets, applies models and solves the local estimate.
- Map appUses the position estimate, often alongside Wi‑Fi, cellular and inertial data depending on the device and setting.
Listen.
Measure.
Solve.
That blue dot begins as a timing problem: coded signals from precise clocks in orbit, measured against a receiver's imperfect clock, corrected for a complicated trip through the atmosphere.