How GPS finds your position from timing

Each GPS satellite carries atomic clocks and constantly broadcasts its position and the exact time. A receiver measures how long each signal took to arrive and multiplies by the speed of light to get a distance. Distances to three satellites pin down a position, and a fourth satellite lets the receiver correct its own inexpensive clock.

Controls

Labels

Animation speed

Signal travel time

About 67 ms from a satellite straight overhead, up to about 86 ms near the horizon.

Receiver clock error

Use a fourth satellite to solve the clock

Step through it

Satellites broadcast timeA constellation of about 30 satellites orbits roughly 20,200 km up. Each sends a signal stamped with its position and the time from its atomic clocks.
Time becomes distanceRadio travels at the speed of light, about 300,000 km per second. A signal that took 72 milliseconds came from about 21,600 km away.
Three spheres meetEach distance puts the receiver somewhere on a huge sphere around one satellite. Three spheres intersect at just two points, and only one of them is on Earth's surface.
The clock problemA receiver's quartz clock is not atomic. If it is off by just 1 microsecond, every distance is off by about 300 meters.
The fourth satelliteWith a fourth signal, the receiver solves for its clock error along with its position, so the clock error drops out.

Distance equals speed of light times time

Range = c x travel time, with c = 299,792,458 meters per second. Light covers about 300 meters in one microsecond, so timing has to be precise to billionths of a second for meter level accuracy.

The receiver actually solves four equations with four unknowns: its three position coordinates and its clock offset. That is why at least four satellites must be in view for a full fix. The ring in the scene is a simplified picture of how a clock error spreads the answer out.

Try it: set the clock error to 0.1 microseconds and the range error is already 30 meters. Then turn on the fourth satellite and the clock term disappears.

What limits real accuracy

Once the clock is solved, the remaining errors come from the atmosphere slowing the signal, small errors in the broadcast orbits, signals bouncing off nearby objects and satellite geometry. The U.S. government reports civilian horizontal accuracy within about 8 meters 95 percent of the time, as a global average, and farm equipment adds correction signals to reach centimeters.

Where Avid uses this

Precision agriculture work like Avid's VerdantaIQ relies on accurate positioning to tie each sensor reading and field decision to the right spot in the field.

Sources

Introduction to GPS and other Global Navigation Satellite Systems (Goddard Space Flight Center)NASA Technical Reports Server
GPS Pseudorange Solution (CE 506 homework notes)Purdue University
GPS PerformanceGPS.gov (U.S. government)

How to cite this model

Avid Solutions International. (2026, November 11). How GPS finds your position from timing [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/gps-trilateration

Last updated November 11, 2026

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