GPS is so woven into daily life that it's become invisible. Navigation, shipping logistics, financial transaction timestamps, power grid synchronization, earthquake monitoring — GPS is foundational infrastructure for the modern world, running quietly underneath dozens of systems most people never associate with satellite navigation.
Here's how it actually works — and why Albert Einstein is responsible for the fact that it works at all.
The geometry of satellites
The GPS constellation has a baseline design of 24 operational satellites in six orbital planes at about 20,200 km altitude. Buildings, terrain, interference, and indoor conditions can prevent a receiver from obtaining enough usable signals.
The principle is simple: each satellite broadcasts a signal containing two pieces of information — the satellite's precise location, and the exact time the signal was transmitted. Your receiver picks up this signal, compares the arrival time to its own clock, and calculates the travel distance using the speed of light. Radio signals travel at roughly 299,792 km per second, so measuring the travel time to within microseconds gives distance to within hundreds of meters.
With signals from three satellites, your receiver can narrow your position to two possible points (you're at the intersection of three spheres). A fourth satellite eliminates the ambiguity and also corrects for clock errors in the receiver.
Why precision matters so much
The speed of light is approximately 30 cm per nanosecond. A timing error of one microsecond (one millionth of a second) introduces a position error of 300 meters. GPS signals are timed to nanosecond accuracy using atomic clocks on each satellite — cesium and rubidium clocks accurate to about one second in 300,000 years.
Getting timing this precise requires correcting for effects predicted by relativity. There are two:
Special relativity: GPS satellites are moving at roughly 14,000 km/h relative to a receiver on Earth. According to special relativity, moving clocks run slow. The satellite clocks run slow by about 7 microseconds per day due to their velocity.
General relativity: GPS satellites are approximately 20,200 km above Earth's surface, where gravity is weaker. According to general relativity, clocks in weaker gravitational fields run faster. The satellite clocks run fast by about 45 microseconds per day due to reduced gravity.
The net effect: satellite clocks run approximately 38 microseconds fast per day relative to ground clocks. If uncorrected, this would accumulate to a position error of about 11 km per day. A GPS system without relativistic corrections would be useless within hours of deployment.
The GPS system corrects for this by running the satellite clocks slightly slow before launch — they're set to tick at a rate that, once in orbit and accounting for relativistic effects, matches ground clocks precisely.
Einstein published general relativity in 1915. GPS became operational in 1995. For 80 years, his equations waited for an application. When the application arrived, they were essential.
The accuracy you actually get
Consumer GPS can provide positions within a few meters under favorable conditions. Accuracy depends on satellite geometry, atmospheric effects, reflected signals, obstructions, and the receiver. Civilian signals are not currently degraded by Selective Availability: that policy ended in May 2000.
Real-world degradation comes from atmospheric effects: the ionosphere and troposphere slow signals slightly and variably. Differential GPS (DGPS) corrects for this using ground reference stations with known positions, achieving sub-meter accuracy. Real-Time Kinematic (RTK) systems used in surveying achieve centimeter accuracy by analyzing the phase of the carrier signal rather than just the timing of data pulses.
What depends on GPS that isn't navigation
Power grids use GPS timestamps to synchronize the alternating current phase across geographically distributed generators. Financial exchanges timestamp transactions to microsecond accuracy for regulatory compliance. Mobile phone networks synchronize base stations using GPS time. Internet infrastructure uses GPS for time synchronization.
GPS is not just a navigation system. It's a time distribution system that happens to also tell you where you are. The navigation is the application most people use; the timing infrastructure is what most of modern digital civilization quietly depends on.
A solar storm large enough to disrupt GPS satellites would not just disable navigation apps. It would desynchronize power grids, disrupt financial markets, and degrade mobile communications — a cascade of failures from 20,000 km above, measured in nanoseconds.