We use GPS every day—whether to navigate through traffic, track food deliveries, or record a morning run. Yet few people stop to think about how this technology can pinpoint a location within just a few meters, even in the middle of a dense city. So how does GPS actually work, and why is a clear view of the sky so important?

What GPS is

GPS (Global Positioning System) is a global navigation system made up of a constellation of satellites orbiting the Earth. At any given time, around 31 satellites are operational, along with several backups. The system was originally developed for the U.S. military and later opened for civilian use.

Each satellite weighs between 1.3 and 1.8 tons, runs on solar power, and orbits at an altitude of roughly 20,000 kilometers. It completes two full orbits around Earth every 24 hours. The orbital layout ensures that at least four satellites are visible from any point on the planet at any time.

The principle: trilateration

A GPS receiver—built into your phone, car, or smartwatch—determines location using a method called trilateration, which measures distances to multiple satellites.

Imagine being lost somewhere in the United States. One person tells you that you are 625 miles from Boise, Idaho. That narrows your location down to a circle around the city. It’s accurate, but not specific enough.

A second person says you are 690 miles from Minneapolis. Now you have two circles that intersect at two points. A third distance measurement—say, from Tucson—leaves only one possible location: Denver, Colorado. That’s two-dimensional trilateration.

In reality, the process happens in three dimensions. Instead of circles, GPS uses spheres. The distance to one satellite defines a sphere of possible locations; a second satellite narrows it to a circular intersection; a third reduces it to two possible points. The Earth itself acts as a fourth constraint, eliminating the point that would be in space. That’s why a receiver ideally needs signals from at least four satellites to determine not only position but also altitude and improve accuracy.

How distance is measured

Distances are calculated based on signal travel time. Satellites transmit signals that move at the speed of light—about 300,000 km per second. The receiver measures how long the signal takes to arrive and multiplies that time by the speed of light to determine distance.

To make this possible, both satellites and receivers use a pseudorandom code—a long digital sequence. Each satellite and receiver generate the same code in sync. When the signal arrives, the satellite’s code is slightly delayed. That delay reveals how long the signal has traveled.

The clock problem and the clever solution

Accurate timing normally requires atomic clocks, which cost tens of thousands of dollars each. That makes it impossible to install them in every smartphone. Instead, satellites carry highly precise atomic clocks, while receivers use standard quartz clocks.

The system solves this elegantly: by comparing signals from at least four satellites, the receiver continuously corrects its own internal clock. In effect, it achieves atomic-level timing accuracy without having an atomic clock onboard.

Differential GPS (DGPS)

Standard GPS is already quite accurate, but its precision can be affected by atmospheric delays, signal reflections from buildings, and other interference.

Differential GPS (DGPS) improves accuracy by using a fixed ground station with precisely known coordinates. This station calculates the error in satellite signals and broadcasts correction data to nearby receivers, significantly improving accuracy.

What GNSS is

GPS is just one satellite navigation system. Others include Europe’s Galileo, China’s BeiDou, and Russia’s GLONASS. Together, they form GNSS (Global Navigation Satellite Systems). Modern smartphones typically use multiple systems at once, improving both accuracy and reliability, especially in challenging environments like urban canyons.

In brief

GPS is a network of satellites orbiting Earth that continuously transmit signals. Your device measures the time it takes for those signals to arrive from at least four satellites and uses trilateration to calculate your exact position. Despite atmospheric interference and timing challenges, the system remains remarkably precise thanks to synchronization, redundancy, and continuous correction. That’s why, as long as your device can see the sky, you’re very unlikely to get truly lost.