GPS, or the Global Positioning System, is a network of satellites that tells your phone, car, or any compatible device exactly where you are on Earth. It works by measuring the time it takes for signals from multiple satellites to reach your receiver, then using those tiny time differences to calculate your precise location. The system became publicly accessible in 1994 and now underpins everything from turn-by-turn navigation to food delivery apps.
Satellites, Ground Stations, and Your Device
GPS has three parts that work together: the satellites in space, a network of ground-based control stations, and the receiver in your pocket or on your dashboard.
The constellation consists of at least 24 satellites, each traveling in a circular orbit about 20,200 kilometers (12,550 miles) above Earth. They complete one full orbit every 12 hours, and they’re arranged so that at least four satellites are visible from virtually any point on the planet at any given time. The satellites themselves have evolved through several generations. Older models (called Block IIR-M) introduced jam-resistant military signals, while newer Block III satellites stand over 3.4 meters tall and deliver improved accuracy and signal reliability for both military and civilian users.
On the ground, a network of monitoring stations tracks each satellite’s exact position and the health of its onboard atomic clock. These stations send corrections back up to the satellites so they can broadcast the most accurate information possible. Your GPS receiver, the third piece, picks up those satellite signals and does the math to figure out where you are.
How Your Receiver Pinpoints Your Location
GPS receivers use a technique called trilateration, which is often confused with triangulation. Triangulation measures angles. GPS doesn’t use angles at all. Instead, it measures distances.
Each satellite continuously broadcasts a signal that includes two pieces of information: the satellite’s precise location in space and the exact time the signal was sent. Your receiver picks up that signal, notes when it arrived, and calculates how far the signal traveled based on the time difference. Since the signal moves at the speed of light, even tiny fractions of a second translate to meaningful distances.
One satellite’s signal tells you that you’re somewhere on the surface of a sphere centered on that satellite. A second satellite narrows it down to the circle where two spheres intersect. A third satellite reduces the possibilities to just two points. In practice, one of those two points is usually somewhere absurd (deep in space or inside the Earth), so three satellites can give you a rough fix. But a fourth satellite is needed to solve a critical problem: your receiver’s clock isn’t nearly as precise as the atomic clocks on the satellites. That fourth measurement lets the receiver calculate and correct for its own clock error, producing an accurate three-dimensional position: latitude, longitude, and altitude.
Why Atomic Clocks Matter So Much
The entire system depends on extraordinarily precise timing. Each GPS satellite carries atomic clocks accurate to billionths of a second, because even a tiny timing error creates a large position error. Light travels about 300,000 kilometers per second, so if a clock is off by just one microsecond (one millionth of a second), the calculated distance is wrong by about 300 meters. That’s why the ground control stations constantly monitor and correct the satellite clocks.
Your phone or car GPS receiver uses a much cheaper quartz clock. It can’t match atomic-clock precision on its own, but it doesn’t need to. By picking up signals from four or more satellites simultaneously, the receiver treats its own clock error as just another unknown to solve for. The math works out because four satellites provide four equations with four unknowns: your three position coordinates and the clock offset.
What Limits GPS Accuracy
A standard civilian GPS receiver is typically accurate to within a few meters, but several factors can degrade that. The biggest single source of error is the ionosphere, a layer of charged particles in the upper atmosphere that can slow and bend satellite signals. This alone can introduce errors of around 5 meters, and sometimes more during periods of high solar activity.
Other error sources add up:
- Satellite clock drift: approximately 2 meters of error
- Orbital position uncertainty: approximately 2.5 meters
- Multipath interference: about 1 meter, caused when signals bounce off buildings or other surfaces before reaching your receiver
- Tropospheric delays: about 0.5 meters, from moisture and temperature variations in the lower atmosphere
- Receiver noise: roughly 0.3 meters from the electronics themselves
These errors don’t simply stack on top of each other in the worst case every time, but they do combine to produce the few-meter accuracy most people experience. Modern receivers and newer satellite signals help reduce these errors, and techniques like using correction data from ground stations can push accuracy down to centimeter level for specialized applications.
How Civilian Accuracy Improved Overnight
GPS wasn’t always this accurate for the public. For years, the U.S. military intentionally degraded civilian GPS signals through a policy called Selective Availability, which introduced deliberate errors of tens of meters by dithering the satellite clocks. On May 2, 2000, Selective Availability was turned off. The clock variations, formerly on the order of tens of meters, dropped by orders of magnitude overnight. Civilian GPS accuracy jumped from roughly 100 meters to around 10 to 15 meters, and it has continued improving with newer satellites and signals since then.
GPS Signals and Frequencies
GPS satellites broadcast on multiple radio frequencies. The original civilian signal, called L1, is the one most consumer devices have used for decades. A newer frequency called L5 is now also available for civilian use and offers some technical advantages: its signal structure is ten times more detailed, which helps receivers lock onto it more precisely and resist interference better. Modern smartphones increasingly use both L1 and L5 signals together, which allows the receiver to compare how the two frequencies are affected by the ionosphere and cancel out much of that error automatically.
GPS Is Not the Only System
GPS is the American system, but it’s not the only satellite navigation network in orbit. Russia operates GLONASS, the European Union runs Galileo, and China has BeiDou. All four fall under the broader category of Global Navigation Satellite Systems, or GNSS. Most modern smartphones and navigation devices can receive signals from multiple constellations simultaneously, which means more satellites are available overhead at any given moment. More satellites translates directly to faster position fixes and better accuracy, especially in cities where tall buildings block parts of the sky.
The core principle is the same across all these systems: satellites broadcast timing signals, and your receiver uses the differences in arrival times to calculate where you are. GPS simply happened to be first and remains the most widely recognized name, which is why people use “GPS” as shorthand for satellite navigation in general.