The Global Positioning System (GPS) determines your location by measuring how long radio signals take to travel from satellites orbiting Earth to a receiver in your phone, car, or navigation device. But there is a surprising complication: time does not pass at exactly the same rate everywhere. Because GPS satellites move rapidly and experience weaker gravity than objects on Earth’s surface, their clocks run at a different rate from clocks on the ground.
This is where Albert Einstein’s theory of relativity becomes essential. GPS relies on corrections from both special relativity and general relativity to keep satellite time accurate. Without these corrections, the system would gradually produce increasingly inaccurate location readings, eventually becoming useless for precise navigation.
Understanding how GPS works reveals an extraordinary connection between modern technology and one of the most important discoveries in physics: time is not universal. Its passage depends on motion and gravity.
How GPS determines your location
GPS works through a process called trilateration. It calculates your position by measuring your distance from multiple satellites whose locations are already known.
A network of GPS satellites continuously orbits Earth, broadcasting radio signals containing information about their positions and the precise time each signal was transmitted. Your receiver picks up these signals and compares their transmission times with the times they arrive.
Because radio signals travel at approximately the speed of light, the time required for a signal to reach your receiver reveals how far away its satellite is.
For example, if a signal takes about 0.07 seconds to arrive, it has traveled approximately 21,000 kilometers. This measurement gives the receiver a distance from one satellite, but a single distance cannot identify one exact location. It could place the receiver anywhere on a large imaginary sphere surrounding that satellite.
A second satellite provides another distance measurement, narrowing the possibilities to a circle. A third measurement narrows them further. In principle, three satellites can determine a position in three dimensions when the receiver’s clock is perfectly synchronized with satellite clocks.
In practice, GPS receivers also have their own clock errors. They do not contain the highly precise atomic clocks carried by satellites. Consequently, a fourth satellite is normally needed to calculate the receiver’s clock offset along with its three-dimensional position.
The receiver combines these measurements to estimate its latitude, longitude, and altitude.
The entire process depends on accurately measuring extremely short periods of time. Since radio signals travel so quickly, even a tiny timing error can create a substantial distance error. An error of just one microsecond corresponds to approximately 300 meters of signal travel.
This sensitivity makes accurate timekeeping fundamental to GPS.
Why time passes differently in space
Before Einstein, Isaac Newton’s physics treated time as something universal. Under this traditional view, a second would pass at the same rate for everyone, regardless of where they were or how fast they were moving.
Einstein changed this understanding through his theories of relativity.
His special theory of relativity, published in 1905, established that time depends partly on an observer’s motion. His general theory of relativity, published in 1915, extended this idea to gravity, showing that gravitational fields also affect the passage of time.
These effects are known as time dilation.
GPS satellites experience both effects simultaneously. They move at high speeds relative to people on Earth’s surface, and they orbit at an altitude where Earth’s gravitational field is weaker than it is at the surface.
Each condition affects their clocks differently.
How special relativity affects GPS satellite clocks
Special relativity explains how motion affects the passage of time.
According to Einstein, a clock moving relative to an observer runs more slowly than a clock at rest in that observer’s reference frame. This effect is called velocity-based time dilation.
GPS satellites orbit Earth at approximately 14,000 kilometers per hour. Although this is extremely fast by everyday standards, it is much slower than the speed of light. Nevertheless, the motion produces a measurable difference in clock rates.
From the perspective of an observer on Earth’s surface, a satellite’s clock runs slightly slower because of its orbital motion.
For GPS, this effect causes satellite clocks to lose approximately 7 microseconds per day relative to clocks on Earth’s surface.
Seven microseconds might seem insignificant, but GPS depends on timing precision far beyond what ordinary human activities require. Because light travels approximately 300 meters in one microsecond, even tiny clock differences can translate into significant errors in calculated distances.
Special relativity therefore requires GPS engineers to account for the fact that moving satellite clocks do not tick at exactly the same rate as ground-based clocks.
However, motion is only half the story.
How general relativity affects GPS satellite clocks
General relativity describes gravity as a feature of the geometry of space and time. One of its important consequences is that clocks run at different rates depending on the strength of the gravitational field in which they are located.
Clocks closer to a massive object experience stronger gravity and run more slowly than clocks farther away, assuming the relevant clocks are compared under otherwise appropriate conditions.
GPS satellites orbit approximately 20,200 kilometers above Earth’s surface. At that altitude, Earth’s gravitational field is weaker than it is at ground level.
As a result, satellite clocks run faster than clocks on Earth’s surface because of gravitational time dilation.
This effect produces an increase of approximately 45 microseconds per day in the rate of GPS satellite clocks relative to ground-based clocks.
The difference is larger than the motion-related effect predicted by special relativity.
In other words, gravity makes satellite clocks gain time, while their orbital speed makes them lose time.
Both effects must be included to determine how quickly satellite clocks actually run relative to clocks on Earth.
How the two theories work together
The remarkable part of GPS is that its accuracy depends on combining the two relativistic effects correctly.
Special relativity predicts that satellite clocks lose approximately 7 microseconds per day because of their orbital speed.
General relativity predicts that they gain approximately 45 microseconds per day because they experience weaker gravity.
When these effects are combined, the net result is that GPS satellite clocks run approximately 38 microseconds faster per day than clocks on Earth’s surface.
45 − 7 = 38 microseconds per day
This difference may appear negligible, but its consequences accumulate continuously.
After one day, a timing discrepancy of 38 microseconds corresponds to roughly 11 kilometers of signal travel at the speed of light.
Without relativistic corrections, GPS distance calculations would therefore become increasingly inaccurate. The resulting errors would make reliable navigation impossible.
GPS engineers account for this difference by adjusting the satellite clocks before launch so that they operate at the correct effective rate once the satellites reach orbit.
The adjustment is incorporated into the system’s clock design and calibration. It is not necessary for a receiver in your phone to independently calculate Einstein’s equations every time you request directions.
Instead, the satellite timing system is designed with relativity already taken into account.
Why GPS satellites need extremely precise clocks
GPS satellites carry atomic clocks, which measure time using the predictable frequencies associated with atomic transitions.
Unlike ordinary mechanical or electronic clocks, atomic clocks can maintain exceptionally stable timing over long periods. This stability makes them suitable for satellite navigation, where even tiny timing errors can affect calculated positions.
However, an atomic clock is not automatically immune to relativistic effects. It measures time according to its own physical environment and motion.
A clock aboard a satellite experiences the same relativistic effects as any other clock traveling along that satellite’s path.
The distinction is important: atomic clocks provide exceptional precision, but relativity determines how their measured time compares with time measured by clocks elsewhere.
GPS engineers must account for several timing factors, including gravitational time dilation, orbital motion, and smaller effects caused by the changing positions and velocities of satellites.
Satellite signals also include timing information that allows receivers to account for variations in the satellite clocks. These corrections help maintain accurate synchronization throughout the system.
Without reliable satellite clocks and careful timing corrections, the distance calculations at the heart of GPS would quickly become unreliable.
Other relativistic effects that GPS must consider
The main relativistic corrections are the effects of orbital speed and gravitational potential, but practical GPS calculations also account for additional timing differences.
One example is the Sagnac effect, which arises because Earth rotates while satellite signals are traveling toward a receiver.
A GPS satellite and a receiver on the rotating Earth do not remain in exactly the same positions relative to an Earth-centered reference frame during signal transmission. This rotation affects the signal’s measured travel time.
The effect is not simply another form of gravitational or velocity-based time dilation. It is a consequence of using a rotating reference frame to describe signal propagation.
GPS calculations account for Earth’s rotation so that the receiver can correctly interpret the timing of incoming signals.
Satellite orbits are also not perfectly circular, and their speeds and gravitational environments change slightly as they move along their paths. These variations create small periodic differences in clock rates.
GPS systems use additional relativistic corrections to account for these effects.
Together, these adjustments allow the system to maintain the timing accuracy needed for reliable positioning.
What would happen if GPS ignored relativity?
GPS provides a practical demonstration of why relativity matters outside theoretical physics.
If engineers designed a navigation system using only classical ideas about time, they would assume that satellite clocks and ground clocks tick at identical rates.
That assumption would be wrong.
The combined relativistic difference would cause satellite clocks to gain approximately 38 microseconds per day relative to ground clocks. As the timing discrepancy accumulated, receivers would increasingly miscalculate the distances between themselves and the satellites.
Because positioning depends on combining several distance measurements, these errors would distort the calculated location.
The resulting inaccuracies would grow by kilometers per day if the clock-rate difference were left uncorrected.
GPS would not simply become a little less convenient. Its fundamental positioning calculations would fail to remain accurate.
Relativity is therefore not an optional refinement added to an otherwise complete navigation system. It is a necessary part of making satellite navigation work.
GPS is a practical test of Einstein’s ideas
GPS is often described as one of the clearest everyday applications of Einstein’s theories. It demonstrates that relativity is not limited to distant stars, black holes, or extremely high-energy experiments.
The effects of time dilation can be measured with precise clocks and must be included in technologies that depend on accurate timing.
GPS does not prove relativity by itself, but its successful operation relies on predictions that follow directly from Einstein’s theories.
The system also illustrates an important principle of modern science: scientific theories become especially powerful when their predictions can be translated into practical engineering solutions.
Every time a smartphone identifies a position, a vehicle follows a navigation route, or an aircraft uses satellite positioning, the calculations ultimately depend on an accurate understanding of how time behaves in different gravitational fields and states of motion.
Einstein showed that time is not an absolute background against which the universe operates. GPS turns that insight into a working technology, using the changing rates of satellite clocks to calculate distances and determine locations on Earth.