How Does GPS Function?

We rely on GPS almost daily—whether it's for getting directions through navigation apps, tracking food deliveries, or mapping our jogging routes. But have you ever paused to think about how GPS actually works?

Nearly all smartphones are equipped with GPS technology, so let's break down what that means and why, with a GPS receiver and a clear sky above, you’ll never be lost again.

What Is GPS?

When people refer to "a GPS," they typically mean a GPS receiver. However, the Global Positioning System (GPS) itself is actually a network of Earth-orbiting satellites (31 of which are actively operational, and four are either in reserve or deemed "unhealthy").

Initially created and used by the U.S. military as a navigation tool for military operations, the GPS satellite system was eventually made available for public use.

Each of these solar-powered satellites, weighing between 3,000 and 4,000 pounds (1,361 to 1,814 kg), orbits the Earth from an altitude of approximately 12,427 miles (20,000 km), completing two full rotations per day. Their orbits are strategically positioned so that, at any given moment, at least four satellites are "visible" anywhere on Earth.

A GPS receiver taps into these satellites to pinpoint the exact location of the device's user.

How Does GPS Work?

GPS receivers determine your location by identifying four or more satellites, calculating the distance to each one, and then using that data to pinpoint where you are.

This process is based on a fundamental mathematical concept known as trilateration. While trilateration in three dimensions can be complex, we’ll first break it down with a simpler two-dimensional example.

2D Trilateration

Picture yourself lost somewhere in the United States with no idea where you are. You meet a helpful local who tells you, 'You're 625 miles away from Boise, Idaho.'

This is an interesting fact, but on its own, it doesn't provide much help. You could be anywhere along a circle with a radius of 625 miles around Boise, like this:

You ask someone else for your location, and she replies, "You're 690 miles away from Minneapolis, Minnesota." Now you’re making progress.

By combining this new info with the Boise data, you now have two intersecting circles. This means your location must be one of the two intersection points, as long as you're 625 miles from Boise and 690 miles from Minneapolis:

When a third person informs you that you're 615 miles from Tucson, Arizona, you can eliminate one of the possibilities because the third circle will only intersect with one of the previous two points. You now know precisely where you are: Denver, Colorado.

This technique is called 2D trilateration, as the points of intersection lie on a two-dimensional plane. Once height or altitude is added — enter the third dimension — we move to 3D trilateration.

3D Trilateration

At its core, three-dimensional trilateration is quite similar to its two-dimensional counterpart, though it can be harder to picture. Instead of just circles, imagine the radii expanding in all directions, forming a series of spheres.

If you're 10 miles from satellite A, you could be anywhere on the surface of a giant imaginary sphere with a 10-mile radius. Add in the fact that you're also 15 miles from satellite B, and you now have two spheres overlapping each other.

These two spheres intersect along a perfect circle. Once you include the distance to a third satellite, you introduce a third sphere, which intersects the circle at two points.

Earth itself can act as a fourth 'satellite' or sphere. Since only one of the two intersection points will lie on the Earth's surface, the other point in space can be eliminated. To enhance accuracy and provide precise altitude data, receivers typically rely on signals from four or more satellites.

How GPS Devices Calculate Your Location

For a GPS device to work accurately, it must have access to two crucial pieces of information:

GPS receivers determine both of these elements by analyzing high-frequency, low-power radio signals sent from orbiting satellites. More advanced models feature multiple receivers, enabling them to capture signals from several satellites at once.

Radio waves are a type of electromagnetic energy, which means they travel at the speed of light — approximately 186,000 miles per second (or 300,000 km per second in a vacuum). By timing how long it takes for the signal to reach the receiver, it can calculate the distance the GPS signal has traveled.

GPS Math: Using Timing to Calculate Distance

By this point, you can confidently explain that GPS operates through trilateration. But when asked how the GPS device determines the distance to the satellites, you can now answer: it’s all about timing.

At a specific time (say, midnight), the satellite sends out a long, digital pattern known as a pseudo-random code. The receiver starts playing the exact same pattern at the same moment. When the satellite’s signal reaches the receiver, there will be a slight delay, as the satellite’s transmission lags behind the receiver’s play of the pattern.

The delay in the signal's arrival corresponds to the time it took for the signal to travel. The receiver then multiplies this time by the speed of light to calculate how far the signal has traveled. If the signal traveled in a straight line, this gives the distance from the receiver to the satellite.

Maintaining Synchronicity

A key challenge is that this measurement only works if both the GPS device and the satellite’s clocks are perfectly synchronized to the nanosecond. Achieving this level of precision requires expensive atomic clocks, which cost anywhere from $50,000 to $100,000 each.

While we already fund the GPS satellites through our taxes, who would want to pay for a GPS receiver priced that high? Even Apple would struggle to sell iPhones at that cost.

The Global Positioning System has a clever workaround: each satellite carries a pricey atomic clock, but the receiver uses an ordinary quartz clock, which it continually adjusts.

In essence, the receiver tracks signals from four or more satellites and calculates its own time inaccuracies. By regularly resetting and comparing its internal clock with the GPS signals, even a humble smartphone can achieve atomic-clock precision—"for free."

Differential GPS

While GPS generally performs well, there are some imperfections. One limitation is that the system assumes radio signals travel through the atmosphere at a consistent speed—the speed of light.

However, satellite signals are constantly affected by interference. The Earth's atmosphere slows them down, and large structures like skyscrapers can alter their paths.

Differential GPS (DGPS) corrects these inaccuracies. The core idea is to measure GPS errors at a fixed receiver station with a known location. Since the station already knows its position, it can easily determine the error in its own signal.

The station then transmits a correction signal to all DGPS-enabled receivers in the vicinity, providing them with accurate adjustments for that area. This added correction significantly enhances the accuracy of DGPS receivers over regular ones.