GPS, or the Global Positioning System, is a network of satellites that broadcasts precise timing signals to Earth, allowing any receiver (your phone, a car’s navigation unit, a handheld device) to calculate its exact location. The system is owned and operated by the United States government and is free for anyone in the world to use. It works by measuring how long radio signals take to travel from multiple satellites to your receiver, then using that timing data to pinpoint where you are to within roughly 8 meters horizontally on a global average.
The Three Parts of GPS
GPS has three segments that work together: the satellites in space, a network of ground stations that monitor and correct them, and the receivers people carry or install in vehicles and devices.
The space segment consists of roughly 32 operational satellites orbiting about 20,200 kilometers (12,550 miles) above Earth. They circle the planet twice per day, arranged so that at least four satellites are visible from virtually any point on Earth’s surface at any time. Each satellite carries multiple atomic clocks and continuously broadcasts radio signals toward the ground.
The control segment keeps those satellites accurate. A master control station at Schriever Air Force Base in Colorado manages the entire system, supported by monitor stations spread across the globe, from Hawaii and Cape Canaveral to Ascension Island in the Atlantic and Diego Garcia in the Indian Ocean (with additional stations added in 2005 in locations including Argentina, the United Kingdom, Ecuador, and Australia). Each monitor station tracks up to 11 satellites at a time and checks their altitude, position, speed, and overall health twice a day. Variations caused by the moon’s gravity, solar radiation pressure, and other forces are sent back to the master station, which computes corrections and uploads them to the satellites.
The user segment is everything on the receiving end: the GPS chip in your smartphone, a car’s built-in navigation unit, a handheld hiking device, or a surveyor’s professional receiver. These devices don’t transmit anything to the satellites. They simply listen to the signals and do the math locally.
How GPS Calculates Your Location
GPS works through a process called trilateration, which is often confused with triangulation. Triangulation measures angles between reference points. Trilateration measures distances, and that distinction matters for understanding the system.
Each satellite broadcasts a signal that travels at the speed of light, about 300,000 kilometers per second. Your receiver notes exactly when the signal arrives and compares that to the time stamp embedded in the signal itself. The difference tells the receiver how long the signal was in transit. Multiply that travel time by the speed of light, and you get the distance to that satellite. For example, a signal that takes 0.005 seconds to arrive means the satellite is about 1,500 kilometers away.
A single distance measurement tells you that you’re somewhere on a sphere of that radius centered on one satellite. A second satellite narrows your position to the circle where two spheres intersect. A third satellite reduces it to two possible points, and a fourth satellite (or basic logic about which point is on Earth’s surface) pins down your exact location. In practice, your receiver picks up signals from as many satellites as it can see, often six or more, and uses all of them to refine the fix and correct for timing errors.
Why Atomic Clocks and Relativity Matter
The entire system depends on extraordinarily precise timing. A timing error of just one millionth of a second translates to about 300 meters of position error, because the signals travel at the speed of light. That’s why each satellite carries atomic clocks, and it’s also why engineers had to account for Einstein’s theories of relativity when designing the system.
Two relativistic effects pull the satellite clocks in opposite directions. Because the satellites move fast through space, special relativity causes their clocks to tick slightly slower than clocks on the ground, falling behind by about 7 microseconds per day. But because they orbit high above Earth where gravity is weaker, general relativity causes their clocks to tick faster, gaining about 45 microseconds per day. The net result is that GPS satellite clocks run 38 microseconds per day faster than clocks on Earth’s surface. That sounds tiny, but without correction it would introduce roughly 10 kilometers of positioning error per day. The system compensates for this automatically.
How Accurate GPS Really Is
The civilian GPS signal, called the Standard Positioning Service, delivers a global average horizontal accuracy of 8 meters or better, 95% of the time. In the worst locations on Earth, that widens to about 15 meters horizontally. Vertical accuracy is less precise: 13 meters on average globally, up to 33 meters in worst-case spots. These figures represent the guaranteed performance floor. In open sky conditions with a modern receiver, real-world accuracy is often better.
Several factors degrade accuracy. Signals slow down and bend as they pass through the ionosphere and lower atmosphere. Buildings and canyon walls can reflect signals, creating “multipath” errors where the receiver picks up a bounced signal that traveled a longer path. Dense tree canopy and indoor environments weaken the signal further. Newer signals being added to the system are designed to address many of these problems.
New Signals Improving Performance
GPS is being modernized with additional civilian signals beyond the original L1 C/A signal that phones have used for decades. Two are especially significant.
L2C, which began launching on satellites in 2005, broadcasts at higher power than the original signal, making it easier to pick up under trees and even indoors. When a receiver can use both L1 and L2C together (called dual-frequency reception), it can correct for ionospheric distortion, a major source of error. This gives civilians accuracy comparable to what was once reserved for the military. As of mid-2023, 25 satellites were broadcasting L2C.
L5 is a third civilian signal designed for safety-critical applications like aviation. It began launching in 2010, was available on 18 satellites by mid-2023, and is expected to reach 24 satellites around 2027. When all three civilian frequencies are used together through a technique called trilaning, sub-meter accuracy becomes possible without any ground-based correction systems. That’s a dramatic leap from the 8-to-15 meter accuracy of single-frequency receivers.
GPS Does More Than Navigation
Location is actually built on top of a more fundamental output: time. GPS receivers can synchronize to satellite atomic clocks within 100 nanoseconds (100 billionths of a second), giving any device access to atomic-clock-level precision without owning an atomic clock.
This timing capability quietly underpins much of modern infrastructure. Wireless phone networks use GPS time to keep their base stations synchronized. Electrical power grids rely on it to coordinate generation and distribution. Financial institutions use GPS to timestamp transactions, creating consistent, traceable records across global networks. When people talk about GPS as critical infrastructure, timing is a big part of why. A widespread GPS outage wouldn’t just affect navigation; it would ripple through telecommunications, banking, and energy systems.
Other Countries’ Satellite Navigation Systems
GPS is the oldest and most widely used satellite navigation system, but it’s no longer the only one. Three other global systems now operate or are nearing full capability.
- GLONASS is Russia’s system, fully recovered and operational since 2011 with 24 satellites providing global coverage.
- Galileo is the European Union’s system, designed to offer more precise positioning with additional service tiers for different user needs.
- BeiDou is China’s system, which completed its global constellation in 2020 with a mix of 35 satellites in different orbit types.
Most modern smartphones and navigation devices receive signals from multiple constellations simultaneously. Using GPS alongside GLONASS, Galileo, or BeiDou increases the number of visible satellites at any given moment, which improves accuracy and reliability, especially in challenging environments like urban canyons or dense forests. The generic term for all these systems together is GNSS, or Global Navigation Satellite Systems, with GPS being the American component.