You open your maps app and within seconds it knows you are standing at a specific street corner, accurate to a few metres. This feels ordinary now, but the technology behind it involves satellites in orbit 20,200 kilometres above Earth, Einstein’s theories of relativity, and some of the most precise clocks ever built. Here is how GPS actually works.
The GPS Constellation
GPS — the Global Positioning System — is operated by the United States Space Force and consists of at least 24 operational satellites (currently around 31) arranged in 6 orbital planes, ensuring that at least 4 satellites are visible from any point on Earth at any time.
The satellites orbit at an altitude of 20,200 kilometres in Medium Earth Orbit, completing two orbits per day. They continuously broadcast radio signals containing two pieces of information: their precise location and the exact time the signal was sent.
Source: GPS.gov — Official US Government GPS information
The Core Principle: Trilateration
Your GPS receiver calculates its position using a process called trilateration (often confused with triangulation, which uses angles rather than distances).
Here is how it works:
- Your receiver picks up a signal from a satellite and calculates how long it took to arrive
- Since radio signals travel at the speed of light (~300,000 km/s), it can calculate the distance to that satellite: Distance = Speed × Time
- Knowing the satellite’s location and the distance, your receiver knows you are somewhere on a sphere of that radius around the satellite
- With a second satellite, the two spheres intersect in a circle — you are somewhere on that circle
- A third satellite narrows it to two points — one usually absurd (in space or underground)
- A fourth satellite eliminates the ambiguity and also corrects for errors in your receiver’s clock
The Clock Problem — And Einstein’s Solution
GPS requires extraordinarily precise timing. Light travels about 30 centimetres per nanosecond (billionth of a second). A timing error of just 1 microsecond (1 millionth of a second) would translate to a position error of 300 metres.
Each GPS satellite carries multiple atomic clocks accurate to nanoseconds. But here is where Einstein comes in — without corrections from both of his theories of relativity, GPS would drift by about 10 kilometres per day:
- Special relativity: The satellites are moving at ~14,000 km/h relative to the ground. According to special relativity, moving clocks run slower. This makes satellite clocks lose about 7 microseconds per day.
- General relativity: The satellites are further from Earth’s gravitational field than clocks on the ground. According to general relativity, clocks in weaker gravity run faster. This makes satellite clocks gain about 45 microseconds per day.
The net effect is that satellite clocks gain ~38 microseconds per day. This is corrected before launch by setting the satellite clocks to tick slightly slower than ground clocks. Without these relativistic corrections, GPS simply would not work.
Source: Ashby, Physics Today (2002)
Other Global Navigation Systems
GPS is American, but it is not the only system:
- GLONASS — Russian system, 24 satellites
- Galileo — European Union, 30 satellites, civilians get full accuracy
- BeiDou — Chinese system, 35+ satellites, global coverage since 2020
Modern smartphones use signals from multiple systems simultaneously, improving accuracy and reliability. This is why your phone’s location is described as using “GNSS” (Global Navigation Satellite System) rather than just GPS.
How Accurate Is GPS?
Standard civilian GPS is accurate to about 3–5 metres. Military GPS and high-precision civilian applications (surveying, aviation) can achieve centimetre-level accuracy using differential GPS or Real-Time Kinematic (RTK) corrections — comparing the satellite signal to a known fixed reference point on the ground.
What Could Disrupt GPS?
- Solar storms: Intense geomagnetic activity can disrupt signals
- Jamming: Radio interference can block signals (illegal in most countries)
- Spoofing: Fake GPS signals can deceive receivers — an emerging security concern for autonomous vehicles and shipping
- Urban canyons: Tall buildings can reflect signals, causing multipath errors
A technology that relies on atomic clocks, orbital mechanics, and Einstein’s relativity — running on satellites the size of a car, 20,000 kilometres above your head — is now so routine that we only notice it when it fails. That is the measure of how remarkable GPS truly is.
